The Lone Ranger
08-07-2008, 01:06 PM
An Introduction to Zoology
Chapter Six: Animal Development:
Theories Regarding Animal Development:
Historically, there have been two theories regarding how animals develop. The older theory is referred to as preformation. This theory has been disproved, and it has been shown that animals develop through what’s known as epigenesis. Even so, since it’s of historical interest, let’s consider the notion of preformation.
[b]Preformation:
Before the invention of microscopes, and especially before it was possible to directly view what happens during fertilization and early development, preformationist theories were frequently used to explain animal development. According to preformationism, either the egg or the sperm contained a complete, “pre-formed” individual inside of it, called a homunculus (from the Latin for “little man”). As such, development was simply a matter of the homunculus growing larger. Whether the homunculus was contained in the sperm or the egg was a matter of considerable debate.
The notion that the homunculus was contained in the sperm was known as spermatism or animalculism. (“Animalcule” means “little animal,” so “animalculism” was the notion that each spermatozoan contained a tiny, already-formed animal.) The notion that the homunculus was contained in the ovum was known as ovism.
Some vestiges of preformationist notions have survived even to the present day. For instance, even today, it’s not uncommon to refer to semen as a man’s “seed.” At one time, it was commonly believed that sperm were, in effect, seeds that “took root” in a woman’s womb and grew into a baby there. Of course, it was obvious that the woman contributed something to the baby’s makeup, so it was generally assumed that an individual’s growth and development would be influenced by the environment in which they occurred. That is, the “seed” of a given man would produce children that looked distinctly different if they happened to grow in the wombs of different women.
Similarly, we still use the term “fertilization,” which comes from the notion that the primary function of a man’s semen is to “fertilize” the woman’s womb, which is otherwise “barren.” According to this notion, the function of fertilization is not so much to contribute genetically to a future offspring, but rather to provide the “fertilizer” that makes it possible for the homunculus the woman is already carrying to start growing and thus become a baby.
http://www.freethought-forum.com/forum/gallery/files/5/0/homunculus.jpg
A 1694 diagram of a spermatozoan,
showing a pre-formed homunculus inside.
[b]Epigenesis:
The notion of preformation was strongly advocated by most seventeenth- and eighteenth-century naturalist-philosophers. Some, however, thought it more likely that development began not with an already-formed individual, but with undifferentiated material that gradually developed into an embryo. This theory is known as the theory of epigenesis.
Preformationist notions have a number of obvious problems, which is why no less a thinker than Leonardo da Vinci had argued that the “seed of the female was as potent as that of [the] male in generation” back in the 15th century. As da Vinci hinted, preformationist notions fail to provide an adequate explanation for the obvious fact that both parents contribute more or less equally to a child’s makeup. Another problem with preformationist notions is that they lead to an “infinite regress” problem. If each sperm (or ovum) contains a tiny, fully-formed individual, then its gonads must contain sperm (or ova) with even tinier, fully-formed individuals inside. And inside those individuals, there must be even tinier fully-formed individuals. And so on, and so on. Pretty soon, the whole notion begins to sound awfully silly.
As microscopes became better, it became increasingly clear that neither sperm nor ova contained tiny, fully-formed individuals. In 1759, the German embryologist Kaspar Friedrich Wolff demonstrated that the earliest developmental stages in a chicken involved no tiny, pre-formed chickens. Instead, as he showed, an undifferentiated zygote divided, producing cells that organized themselves into layers. These layers of cells thickened in some areas while thinning in others, then began to organize themselves into distinct segments. These cell layers folded in on themselves and continued to segment until a recognizable embryo was formed. Thus, Wolff conclusively demonstrated that animal development occurs through epigenesis, and not through preformation.
According to modern epigenetic theory, development involves two important and closely-related processes, differentiation and morphogenesis. Differentiation is the process by which cells form specific tissues (http://www.freethought-forum.com/forum/showthread.php?t=17155&garpg=6#content_start) and structures as the embryo develops. Morphogenesis is the process by which cells differentiate (become different), through changes in size and shape, and also the movement of differentiated cells to different parts of the developing embryo as the cells organize themselves into tissues and then organs (http://www.freethought-forum.com/forum/showthread.php?t=17155&garpg=6#content_start).
Development in Different Animal Taxa:
As discussed in a previous chapter, animals can be divided into three major groups according to how they develop. In the sponges (Phylum Porifera (http://www.freethought-forum.com/forum/showthread.php?t=18438&garpg=11#content_start)) and similar animals, there is no embryonic development at all. These animals have no true tissues, and it’s not an exaggeration to say that they’re basically just clumps of cells. These animals are sometimes referred to as the Parazoa (“beside-animals”), indicating that they split off from the main line of animal evolution (the Eumetazoans) very early.
http://www.freethought-forum.com/forum/gallery/files/5/0/parazoa_original.jpg
Development in a sponge: There is no gastrulation and, therefore, no embryonic tissues form. As
a result, the adult animal is basically a clump of largely undifferentiated cells with no true tissues or organs.
In the Eumetazoa (“true animals”), hox genes are present, which help to regulate developmental processes. All of the eumetazoans have embryonic development – that is, gastrulation occurs early in development, leading to the formation of embryonic germ tissues. These tissues ultimately form the tissues and organs of the adult animal’s body.
The Radiata (Phyla Cnidaria and Ctenophora) are diploblastic, meaning that only two germ tissues form during gastrulation – endoderm and ectoderm. Since mesodermal tissue does not occur in these animals, they have no mesodermal organs. Consequently, their bodies are relatively simple, though they do have true tissues, unlike parazoans.
The Bilateria are triploblastic, meaning that three germ tissues form during gastrulation. Since these animals have internal organs that form from mesodermal tissue, their bodies are much more complex than are those of the radiate animals. Depending on how their early development occurs, triploblastic animals are either protostomes (“mouth first”) or deuterostomes (“mouth second”).
[break=Stages in Embryonic Development]
[b]Stages in Embryonic Development:
As discussed in an earlier chapter, embryonic development involves five distinct stages: gametogenesis, fertilization, cleavage, gastrulation, and organogenesis. We’ll consider what happens in each of these stages.
[b]Gametogenesis:
Gametogenesis, of course, is the process by which the gametes (sex cells) are formed. In most animals, normal body cells are diploid, meaning that they have two sets of chromosomes – one set inherited from the mother and one set inherited from the father. These body cells reproduce through the process known as mitosis, which produces cells that are diploid and genetically-identical to the original cell.
The gametes are produced in organs known as the gonads. The male gonads are the testes (testicles) and they produce small, mobile gametes known as spermatozoa. The female gonads are the ovaries and they produce relatively large, non-mobile gametes known as ova. Within the gonads, a different form of cellular division occurs, known as meiosis. Meiosis results in cells that are not genetically identical to the original cell, and that have only half the normal number of chromosomes. These cells are therefore referred to as haploid, and they are the gametes.
After their production, the gametes are transported through tubes that make up the reproductive tract and, ultimately, to the outside of the body. Of course, in those species with internal fertilization, the female’s gametes are not expelled from the body, but are retained within the reproductive tract until after fertilization occurs.
The female’s ovum contains everything a cell needs in order to survive – except, of course, for half of its DNA. This is one reason why the ovum is typically far larger than is a spermatozoan; a spermatozoan is optimized for mobility and has lost absolutely everything that is not essential to the task of traveling to and fertilizing the ovum. To a first approximation, the father contributes only DNA to a zygote and the resulting embryo; everything else is contributed by the mother.
http://www.freethought-forum.com/forum/gallery/files/5/0/internalmale_original.jpg
Reproductive anatomy of the human male: Spermatozoa are produced within the testes and
stored in the epididymis. During ejaculation, spermatozoa travel out of the scrotum through
the vas deferens and into the abdomen. The seminal vesicles, prostate gland and
bulbourethral (Cowper’s) glands secrete fluids that mix with the spermatozoa to
form semen. The semen continues out of the body through the urethra.
http://www.freethought-forum.com/forum/gallery/files/5/0/female_tract.jpg
The reproductive tract of the human female: Ova are produced in the
ovaries. After ovulation (release of an ovum), the ovum is swept into a
Fallopian tube (oviduct). If fertilization occurs, the zygote will implant
in the lining of the uterus. Otherwise, the ovum continues through the
uterus and out of the body through the vagina.
Spermatozoa are specialized for movement; they seek out and swim toward ova. This doesn’t mean that the ovum is a passive participant in the process, however; it secretes chemicals to attract the spermatozoa.
A typical sperm cell has three main regions: a head, a midpiece, and a tail. The head is capped with an acrosome, which contains digestive enzymes. These chemicals partially dissolve the tissues that surround the ovum, allowing the spermatozoan to penetrate and fertilize the ovum. The head of a spermatozoan also contains the nucleus, which has one set of chromosomes. If a spermatozoan successfully unites with an ovum, only the head actually penetrates all the way into the ovum. The nucleus of the spermatozoan unites with the nucleus of the ovum, and a diploid zygote is formed.
The midpiece of a spermatozoan is packed full of organelles known as mitochondria. The mitochondria break down carbohydrate molecules to produce energy, which powers the spermatozoan as it swims toward the ovum.
The tail of a spermatozoan is a flagellum. Powered by the mitochondria in the midpiece, the tail rapidly beats back-and-forth, propelling the spermatozoan forward.
http://www.freethought-forum.com/forum/gallery/files/5/0/spermatozoan.jpg
Anatomy of a spermatozoan: A spermatozoan
is a very simple cell – basically, it’s just genetic
material with a motor attached.
http://www.freethought-forum.com/forum/gallery/files/5/0/ovum_original.jpg
A human egg: the ovum (oocyte) is surrounded by a clear layer of glycoproteins
known as the zona pellucida and by cells collectively known as the corona radiata.
[break=Fertilization]
[b]Fertilization:
Fertilization (also known as conception or syngamy) occurs when a spermatozoan and an ovum unite to form a diploid zygote. It isn’t quite as simple a process as you might think.
The terms “egg” and “ovum” are often used as if they’re interchangeable, but they aren’t, really. An “egg” consists of an ovum plus various supporting structures. In mammals, for instance, the ovum is surrounded by a clear layer of glycoproteins known as the zona pellucida. Surrounding the zona pellucida is a layer of cells known as the corona radiata. These structures that surround and support the ovum can make things difficult for spermatozoa, since a spermatozoan must penetrate them in order to fuse with the ovum.
That’s one of the reasons that many, many spermatozoa must be produced per ejaculation. First of all, since it lacks most of the organelles that a cell needs in order to survive for any length of time, a spermatozoan has a short life expectancy. Second, if fertilization is internal, the female’s reproductive tract (which is typically rather acidic) can be a rather hostile place from the perspective of spermatozoa. (By some estimates, more than half the spermatozoa deposited into a woman’s vagina when her partner ejaculates are killed within a minute.) Finally, it usually takes the combined actions of several spermatozoa to penetrate the corona radiata and zona pellucida.
If all goes well, however, a spermatozoan penetrates into the ovum and fertilization occurs.
It’s important that only one spermatozoan penetrates into the ovum. If more than one penetrates, this is known as polyspermy. Since an ovum that is fertilized by more than one spermatozoan will have the wrong number of chromosomes, it is unlikely to survive.
As soon as a spermatozoan penetrates the ovum’s cell membrane, the membrane undergoes a rapid depolarization. This is known as activation of the ovum. Within a few seconds, rapid movement of calcium ions triggers “hardening” of the ovum’s cell membrane, making it impossible for any more spermatozoa to penetrate.
[break=Cleavage]
[b]Cleavage:
Soon after fertilization, the zygote begins to divide via cleavage (http://www.freethought-forum.com/forum/showthread.php?t=17155&garpg=11#content_start). Typically, the first cleavage occurs about 90 minutes after a spermatozoan first contacts the ovum. Exactly how cleavage occurs is influenced by how much yolk the egg contains.
Before cleavage begins, the zygote has a visible animal-vegetal axis. This axis is visible because yolk, nutrition for the developing embryo, is concentrated at one end of the zygote. The end where the yolk is concentrated is the vegetal pole of the zygote, and the opposite end is the animal pole. The amount and distribution of yolk influences cleavage because yolk, being non-living, interferes with cleavage.
http://www.freethought-forum.com/forum/gallery/files/5/0/salamander_eggs.jpg
Salamander eggs (Phylum Chordata): Each egg has a
clearly-visible animal pole (dark) and vegetal pole (light).
[break=Egg Types]
An egg with very little yolk that is distributed more or less evenly throughout the egg is known as an isolecithal egg (from the Greek “isos,” meaning “same” and “lekithos,” meaning “yolk”). Mammals typically have isolecithal eggs. Since the mother provides nutrients to the developing embryo through the placenta, most mammalian eggs require very little supporting yolk.
An egg with a moderate amount of yolk concentrated at the vegetal pole is a mesolecithal egg (from the Greek “mesos,” meaning “middle”). Amphibians such as frogs and salamanders typically have mesolecithal eggs.
An egg with a large amount of yolk that is densely concentrated at the vegetal pole is a telolecithal egg (from the Greek “telos,” meaning “end”). Telolecithal eggs are typical of birds and reptiles. Because so much yolk is present, the developing animal can remain inside the egg for an extended period of time, and so is hatched in a relatively well-developed state.
An egg with a relatively large amount of yolk that isn’t concentrated at one end but is instead found in the center of the egg is a centrolecithal egg. Many insects have centrolecithal eggs. As the embryo develops, it grows around the yolk, gradually absorbing it as it grows.
[break=Holoblastic Cleavage]
As mentioned, the amount of yolk in an egg and its distribution affect how cleavage occurs. In animals that have isolecithal eggs, because the cells have so little yolk, cleavage is holoblastic (from the Greek “holo,” meaning “whole” and “blastos,” meaning “germ”). In holoblastic cleavage, each cleavage extends all the way through the egg, completely dividing it.
http://www.freethought-forum.com/forum/gallery/files/5/0/holoblastic.jpg
A sea urchin (Phylum Echinodermata) has isolecithal eggs and
holoblastic cleavage. Each cleavage completely divides the egg.
[break=Meroblastic Cleavage]
In eggs with more yolk, cleavage cannot cut through the large mass of yolk, and so the egg does not completely divide with each cleavage. This is known as meroblastic cleavage (from the Greek “meros,” meaning “part”). In meroblastic cleavage, what ultimately happens is that a clump of dividing cells sits atop the undivided yolk.
In birds, reptiles, most fishes, some amphibians, cephalopod mollusks, and monotreme mammals (all of which have telolecithal eggs), so much yolk is present that cleavage can occur only in a narrow disk at the extreme animal end of the egg. As the embryo grows, blood vessels penetrate into the yolk. The yolk is gradually broken down and absorbed, but it never divides. In insects with centrolecithal eggs, cleavage occurs in a ring surrounding the central, uncleaved yolk. As the embryo develops, the yolk is gradually broken down and absorbed.
http://www.freethought-forum.com/forum/gallery/files/5/0/zebrafish.jpg
A zebrafish (Phylum Chordata) has telolecithal eggs and meroblastic cleavage.
Two cleavage divisions have occurred in this egg. The animal pole (left) has
divided into four cells, but the vegetal pole (right) has not divided at all.
[break=Blastula Formation]
Cleavage, of course, results in a solid ball of cells known as a morula. As cleavage continues, the morula hollows out to form a blastula. The blastula then folds inward to form a gastrula. This is gastrulation.
[break=Gastrulation]
[b]Gastrulation:
Gastrulation results in the formation of two embryonic germ layers, the inner endoderm and the outer ectoderm. The endoderm surrounds the archenteron, which will eventually become the animal’s digestive tube. The opening into the archenteron is the blastopore.
In diploblastic animals, the endoderm and the ectoderm are the only germ layers, and all adult tissues develop from those two embryonic tissues. In most diploblastic animals, there is only one opening into the digestive system, and so that opening functions both as the mouth and the anus.
In triploblastic animals, a third layer of tissue forms between the endoderm and the ectoderm, the mesoderm. Most of the internal organs form from mesodermal tissue, while the blastopore forms either the mouth or the anus, depending on whether the animal is a protostome or a deuterostome.
[b]Organogenesis:
Organogenesis is the formation of organs. As development continues, cells begin to differentiate, taking on different sizes and shapes. Differentiation leads to the formation of specialized cells that can perform specific functions. These cells become organized into tissues which, in turn, form organs.
The differentiation of unspecialized cells (stem cells) into tissues and then organs marks the transition from a gastrula to an embryo. Particularly in mammals, when the organ systems are more or less completely formed, the embryo is referred to as a fetus.
[b]Amniotes and the Four Extraembryonic Membranes:
Because their eggs aren’t surrounded by protective membranes that would prevent desiccation, early vertebrates (fishes and amphibians) had no choice but to lay their eggs in water, or at least wet environments. Eventually, however, the amphibians gave rise to descendents known as the amniotes. In amniotes (reptiles, birds, and mammals), the egg is surrounded by four extraembryonic membranes that provide nutrition and protection for the developing embryo. In some amniotes, the embryo remains inside the mother’s body to develop, but in most, the mother secretes a protective shell around the egg and then expels it from her body.
The four extraembryonic membranes that surround the amniotic egg are the yolk sac, the chorion, the amnion, and the allantois. The key membrane is the amnion, because it is more or less water-impermeable, which means that the egg does not have to be deposited into a wet environment.
The amniotic egg is water-impermeable but allows free exchange of oxygen and carbon dioxide between the developing embryo and the outside environment. It also contains yolk to nourish the developing embryo. As such, it’s an effective life-support system that allows amniotes to lay their eggs on land, making them much more independent of water than are their amphibian ancestors.
http://www.freethought-forum.com/forum/gallery/files/5/0/amniotic_egg.jpg
The amniotic egg.
[b]The Yolk Sac:
As you might imagine, the yolk sac surrounds the yolk in a developing embryo. It develops as an outgrowth of the embryo’s gut.
As the embryo grows, blood vessels from the embryo penetrate into the yolk. Meanwhile, the tissues of the yolk sac secrete digestive enzymes that break down the yolk, allowing it to be absorbed into the blood and transported back to the embryo.
[b]The Chorion:
The chorion is the outermost of the four extraembryonic membranes, and its primary role is in gas exchange. You can easily find the chorion in a chicken egg; it’s the thin membrane that lies just under the shell.
In amniotes that lay eggs, oxygen diffuses into the egg across the amnion as carbon dioxide diffuses across it in the opposite direction and to the outside. As the embryo inside the egg grows and its oxygen demands increase, the chorion partially fuses with the allantois to form the chorioallantoic membrane. The chorioallantoic membrane has a rich supply of blood vessels, allowing for rapid exchange of oxygen and CO2 between the embryo and the outside environment.
In placental mammals, extensions of the chorion known as chorionic villi penetrate into the lining of the mother’s uterus. This brings embryonic and maternal blood vessels into close proximity, allowing for efficient exchange of nutrients and gases between the mother and the developing embryo.
[b]The Amnion:
The amnion is a more or less waterproof membrane that surrounds the developing embryo and prevents it from drying out. A layer of fluid known as the amniotic fluid fills the space between the amnion and the embryo. Not only does the amniotic fluid prevent the embryo from dying of dehydration, it also helps to cushion and protect the embryo.
In placental mammals, uterine contractions during labor cause the amnion to rupture, releasing the amniotic fluid. When this happens in a human, we say that her “water breaks.”
[b]The Allantois:
The allantois is similar to the yolk sac in that it originates as an outgrowth of the embryonic gut. Its primary function is to store metabolic wastes generated by the developing embryo. In many amniotes, it’s also important in gas exchange, as it partially fuses with the chorion to produce the chorioallantoic membrane. The base of the allantois develops into the urinary bladder.
In those amniotes that remain inside the egg while developing, the importance of the allantois cannot be overestimated. Not only is it important in gas exchange, but it stores and isolates poisonous metabolic waste products that would otherwise kill the developing embryo. (The shell that surrounds the egg prevents the embryo from expelling wastes.)
In placental mammals, the allantois plays a much less important role, since the embryo’s metabolic wastes are transported away by the mother’s blood.
[b]Mammalian Development:
The placental mammals (Phylum Chordata, Class Mammalia, Subclass Theria, Infraclass Eutheria) or eutherians, embryos are retained within the mother’s body until they reach a relatively advanced stage of development. In many eutherians, newborns can walk and run within minutes of birth. This is in contrast to the monotremes, which lay eggs, and the marsupials, which give birth to underdeveloped young that then crawl into a pouch (marsupium) on the mother’s body, where they continue their development.
Eutherians can give birth to such highly-developed young because the developing embryo/fetus is nourished by a placenta. The placenta develops from both embryonic and maternal tissues, and is connected to the developing embryo/fetus by the umbilical cord.
The umbilical cord develops from the yolk sac and the allantois and contains blood vessels that transport fetal blood to and from the placenta. In the placenta, blood vessels from the mother and fetus come into close proximity, but there is normally no mixing of fetal and maternal blood. In the placenta, metabolic wastes diffuse from the blood of the fetus into the mother’s blood and are transported away. Meanwhile, oxygen and nutrients diffuse from the mother’s blood into the blood of the fetus. In this way, the fetus is completely supported by its mother until it is sufficiently developed that it can survive outside her body.
The placenta, which allows eutherian mammals to give birth to highly-developed young, is one of the major reasons that eutherians can be found in such a wide variety of habitats. Eutherian mammals can be found on all continents and in the oceans. Eutherian mammals give birth in deserts, on Arctic ice floes where temperatures are well below freezing, and even under water.
http://www.freethought-forum.com/forum/gallery/files/5/0/placenta.jpg
In eutherian mammals, the placenta lines the uterus of a pregnant
female and is connected to the developing fetus by an umbilical cord.
The placenta allows the mother’s blood to supply oxygen and nutrients
to the fetus’ blood and also removes metabolic wastes from the fetus’ blood.
[break=What Comes Next]
In our next chapter, we’ll take a brief look at the members of the Kingdom Protista. Animals evolved from protozoan protists, so this discussion will lead us into the origins of the Kingdom Animalia.
Chapter Six: Animal Development:
Theories Regarding Animal Development:
Historically, there have been two theories regarding how animals develop. The older theory is referred to as preformation. This theory has been disproved, and it has been shown that animals develop through what’s known as epigenesis. Even so, since it’s of historical interest, let’s consider the notion of preformation.
[b]Preformation:
Before the invention of microscopes, and especially before it was possible to directly view what happens during fertilization and early development, preformationist theories were frequently used to explain animal development. According to preformationism, either the egg or the sperm contained a complete, “pre-formed” individual inside of it, called a homunculus (from the Latin for “little man”). As such, development was simply a matter of the homunculus growing larger. Whether the homunculus was contained in the sperm or the egg was a matter of considerable debate.
The notion that the homunculus was contained in the sperm was known as spermatism or animalculism. (“Animalcule” means “little animal,” so “animalculism” was the notion that each spermatozoan contained a tiny, already-formed animal.) The notion that the homunculus was contained in the ovum was known as ovism.
Some vestiges of preformationist notions have survived even to the present day. For instance, even today, it’s not uncommon to refer to semen as a man’s “seed.” At one time, it was commonly believed that sperm were, in effect, seeds that “took root” in a woman’s womb and grew into a baby there. Of course, it was obvious that the woman contributed something to the baby’s makeup, so it was generally assumed that an individual’s growth and development would be influenced by the environment in which they occurred. That is, the “seed” of a given man would produce children that looked distinctly different if they happened to grow in the wombs of different women.
Similarly, we still use the term “fertilization,” which comes from the notion that the primary function of a man’s semen is to “fertilize” the woman’s womb, which is otherwise “barren.” According to this notion, the function of fertilization is not so much to contribute genetically to a future offspring, but rather to provide the “fertilizer” that makes it possible for the homunculus the woman is already carrying to start growing and thus become a baby.
http://www.freethought-forum.com/forum/gallery/files/5/0/homunculus.jpg
A 1694 diagram of a spermatozoan,
showing a pre-formed homunculus inside.
[b]Epigenesis:
The notion of preformation was strongly advocated by most seventeenth- and eighteenth-century naturalist-philosophers. Some, however, thought it more likely that development began not with an already-formed individual, but with undifferentiated material that gradually developed into an embryo. This theory is known as the theory of epigenesis.
Preformationist notions have a number of obvious problems, which is why no less a thinker than Leonardo da Vinci had argued that the “seed of the female was as potent as that of [the] male in generation” back in the 15th century. As da Vinci hinted, preformationist notions fail to provide an adequate explanation for the obvious fact that both parents contribute more or less equally to a child’s makeup. Another problem with preformationist notions is that they lead to an “infinite regress” problem. If each sperm (or ovum) contains a tiny, fully-formed individual, then its gonads must contain sperm (or ova) with even tinier, fully-formed individuals inside. And inside those individuals, there must be even tinier fully-formed individuals. And so on, and so on. Pretty soon, the whole notion begins to sound awfully silly.
As microscopes became better, it became increasingly clear that neither sperm nor ova contained tiny, fully-formed individuals. In 1759, the German embryologist Kaspar Friedrich Wolff demonstrated that the earliest developmental stages in a chicken involved no tiny, pre-formed chickens. Instead, as he showed, an undifferentiated zygote divided, producing cells that organized themselves into layers. These layers of cells thickened in some areas while thinning in others, then began to organize themselves into distinct segments. These cell layers folded in on themselves and continued to segment until a recognizable embryo was formed. Thus, Wolff conclusively demonstrated that animal development occurs through epigenesis, and not through preformation.
According to modern epigenetic theory, development involves two important and closely-related processes, differentiation and morphogenesis. Differentiation is the process by which cells form specific tissues (http://www.freethought-forum.com/forum/showthread.php?t=17155&garpg=6#content_start) and structures as the embryo develops. Morphogenesis is the process by which cells differentiate (become different), through changes in size and shape, and also the movement of differentiated cells to different parts of the developing embryo as the cells organize themselves into tissues and then organs (http://www.freethought-forum.com/forum/showthread.php?t=17155&garpg=6#content_start).
Development in Different Animal Taxa:
As discussed in a previous chapter, animals can be divided into three major groups according to how they develop. In the sponges (Phylum Porifera (http://www.freethought-forum.com/forum/showthread.php?t=18438&garpg=11#content_start)) and similar animals, there is no embryonic development at all. These animals have no true tissues, and it’s not an exaggeration to say that they’re basically just clumps of cells. These animals are sometimes referred to as the Parazoa (“beside-animals”), indicating that they split off from the main line of animal evolution (the Eumetazoans) very early.
http://www.freethought-forum.com/forum/gallery/files/5/0/parazoa_original.jpg
Development in a sponge: There is no gastrulation and, therefore, no embryonic tissues form. As
a result, the adult animal is basically a clump of largely undifferentiated cells with no true tissues or organs.
In the Eumetazoa (“true animals”), hox genes are present, which help to regulate developmental processes. All of the eumetazoans have embryonic development – that is, gastrulation occurs early in development, leading to the formation of embryonic germ tissues. These tissues ultimately form the tissues and organs of the adult animal’s body.
The Radiata (Phyla Cnidaria and Ctenophora) are diploblastic, meaning that only two germ tissues form during gastrulation – endoderm and ectoderm. Since mesodermal tissue does not occur in these animals, they have no mesodermal organs. Consequently, their bodies are relatively simple, though they do have true tissues, unlike parazoans.
The Bilateria are triploblastic, meaning that three germ tissues form during gastrulation. Since these animals have internal organs that form from mesodermal tissue, their bodies are much more complex than are those of the radiate animals. Depending on how their early development occurs, triploblastic animals are either protostomes (“mouth first”) or deuterostomes (“mouth second”).
[break=Stages in Embryonic Development]
[b]Stages in Embryonic Development:
As discussed in an earlier chapter, embryonic development involves five distinct stages: gametogenesis, fertilization, cleavage, gastrulation, and organogenesis. We’ll consider what happens in each of these stages.
[b]Gametogenesis:
Gametogenesis, of course, is the process by which the gametes (sex cells) are formed. In most animals, normal body cells are diploid, meaning that they have two sets of chromosomes – one set inherited from the mother and one set inherited from the father. These body cells reproduce through the process known as mitosis, which produces cells that are diploid and genetically-identical to the original cell.
The gametes are produced in organs known as the gonads. The male gonads are the testes (testicles) and they produce small, mobile gametes known as spermatozoa. The female gonads are the ovaries and they produce relatively large, non-mobile gametes known as ova. Within the gonads, a different form of cellular division occurs, known as meiosis. Meiosis results in cells that are not genetically identical to the original cell, and that have only half the normal number of chromosomes. These cells are therefore referred to as haploid, and they are the gametes.
After their production, the gametes are transported through tubes that make up the reproductive tract and, ultimately, to the outside of the body. Of course, in those species with internal fertilization, the female’s gametes are not expelled from the body, but are retained within the reproductive tract until after fertilization occurs.
The female’s ovum contains everything a cell needs in order to survive – except, of course, for half of its DNA. This is one reason why the ovum is typically far larger than is a spermatozoan; a spermatozoan is optimized for mobility and has lost absolutely everything that is not essential to the task of traveling to and fertilizing the ovum. To a first approximation, the father contributes only DNA to a zygote and the resulting embryo; everything else is contributed by the mother.
http://www.freethought-forum.com/forum/gallery/files/5/0/internalmale_original.jpg
Reproductive anatomy of the human male: Spermatozoa are produced within the testes and
stored in the epididymis. During ejaculation, spermatozoa travel out of the scrotum through
the vas deferens and into the abdomen. The seminal vesicles, prostate gland and
bulbourethral (Cowper’s) glands secrete fluids that mix with the spermatozoa to
form semen. The semen continues out of the body through the urethra.
http://www.freethought-forum.com/forum/gallery/files/5/0/female_tract.jpg
The reproductive tract of the human female: Ova are produced in the
ovaries. After ovulation (release of an ovum), the ovum is swept into a
Fallopian tube (oviduct). If fertilization occurs, the zygote will implant
in the lining of the uterus. Otherwise, the ovum continues through the
uterus and out of the body through the vagina.
Spermatozoa are specialized for movement; they seek out and swim toward ova. This doesn’t mean that the ovum is a passive participant in the process, however; it secretes chemicals to attract the spermatozoa.
A typical sperm cell has three main regions: a head, a midpiece, and a tail. The head is capped with an acrosome, which contains digestive enzymes. These chemicals partially dissolve the tissues that surround the ovum, allowing the spermatozoan to penetrate and fertilize the ovum. The head of a spermatozoan also contains the nucleus, which has one set of chromosomes. If a spermatozoan successfully unites with an ovum, only the head actually penetrates all the way into the ovum. The nucleus of the spermatozoan unites with the nucleus of the ovum, and a diploid zygote is formed.
The midpiece of a spermatozoan is packed full of organelles known as mitochondria. The mitochondria break down carbohydrate molecules to produce energy, which powers the spermatozoan as it swims toward the ovum.
The tail of a spermatozoan is a flagellum. Powered by the mitochondria in the midpiece, the tail rapidly beats back-and-forth, propelling the spermatozoan forward.
http://www.freethought-forum.com/forum/gallery/files/5/0/spermatozoan.jpg
Anatomy of a spermatozoan: A spermatozoan
is a very simple cell – basically, it’s just genetic
material with a motor attached.
http://www.freethought-forum.com/forum/gallery/files/5/0/ovum_original.jpg
A human egg: the ovum (oocyte) is surrounded by a clear layer of glycoproteins
known as the zona pellucida and by cells collectively known as the corona radiata.
[break=Fertilization]
[b]Fertilization:
Fertilization (also known as conception or syngamy) occurs when a spermatozoan and an ovum unite to form a diploid zygote. It isn’t quite as simple a process as you might think.
The terms “egg” and “ovum” are often used as if they’re interchangeable, but they aren’t, really. An “egg” consists of an ovum plus various supporting structures. In mammals, for instance, the ovum is surrounded by a clear layer of glycoproteins known as the zona pellucida. Surrounding the zona pellucida is a layer of cells known as the corona radiata. These structures that surround and support the ovum can make things difficult for spermatozoa, since a spermatozoan must penetrate them in order to fuse with the ovum.
That’s one of the reasons that many, many spermatozoa must be produced per ejaculation. First of all, since it lacks most of the organelles that a cell needs in order to survive for any length of time, a spermatozoan has a short life expectancy. Second, if fertilization is internal, the female’s reproductive tract (which is typically rather acidic) can be a rather hostile place from the perspective of spermatozoa. (By some estimates, more than half the spermatozoa deposited into a woman’s vagina when her partner ejaculates are killed within a minute.) Finally, it usually takes the combined actions of several spermatozoa to penetrate the corona radiata and zona pellucida.
If all goes well, however, a spermatozoan penetrates into the ovum and fertilization occurs.
It’s important that only one spermatozoan penetrates into the ovum. If more than one penetrates, this is known as polyspermy. Since an ovum that is fertilized by more than one spermatozoan will have the wrong number of chromosomes, it is unlikely to survive.
As soon as a spermatozoan penetrates the ovum’s cell membrane, the membrane undergoes a rapid depolarization. This is known as activation of the ovum. Within a few seconds, rapid movement of calcium ions triggers “hardening” of the ovum’s cell membrane, making it impossible for any more spermatozoa to penetrate.
[break=Cleavage]
[b]Cleavage:
Soon after fertilization, the zygote begins to divide via cleavage (http://www.freethought-forum.com/forum/showthread.php?t=17155&garpg=11#content_start). Typically, the first cleavage occurs about 90 minutes after a spermatozoan first contacts the ovum. Exactly how cleavage occurs is influenced by how much yolk the egg contains.
Before cleavage begins, the zygote has a visible animal-vegetal axis. This axis is visible because yolk, nutrition for the developing embryo, is concentrated at one end of the zygote. The end where the yolk is concentrated is the vegetal pole of the zygote, and the opposite end is the animal pole. The amount and distribution of yolk influences cleavage because yolk, being non-living, interferes with cleavage.
http://www.freethought-forum.com/forum/gallery/files/5/0/salamander_eggs.jpg
Salamander eggs (Phylum Chordata): Each egg has a
clearly-visible animal pole (dark) and vegetal pole (light).
[break=Egg Types]
An egg with very little yolk that is distributed more or less evenly throughout the egg is known as an isolecithal egg (from the Greek “isos,” meaning “same” and “lekithos,” meaning “yolk”). Mammals typically have isolecithal eggs. Since the mother provides nutrients to the developing embryo through the placenta, most mammalian eggs require very little supporting yolk.
An egg with a moderate amount of yolk concentrated at the vegetal pole is a mesolecithal egg (from the Greek “mesos,” meaning “middle”). Amphibians such as frogs and salamanders typically have mesolecithal eggs.
An egg with a large amount of yolk that is densely concentrated at the vegetal pole is a telolecithal egg (from the Greek “telos,” meaning “end”). Telolecithal eggs are typical of birds and reptiles. Because so much yolk is present, the developing animal can remain inside the egg for an extended period of time, and so is hatched in a relatively well-developed state.
An egg with a relatively large amount of yolk that isn’t concentrated at one end but is instead found in the center of the egg is a centrolecithal egg. Many insects have centrolecithal eggs. As the embryo develops, it grows around the yolk, gradually absorbing it as it grows.
[break=Holoblastic Cleavage]
As mentioned, the amount of yolk in an egg and its distribution affect how cleavage occurs. In animals that have isolecithal eggs, because the cells have so little yolk, cleavage is holoblastic (from the Greek “holo,” meaning “whole” and “blastos,” meaning “germ”). In holoblastic cleavage, each cleavage extends all the way through the egg, completely dividing it.
http://www.freethought-forum.com/forum/gallery/files/5/0/holoblastic.jpg
A sea urchin (Phylum Echinodermata) has isolecithal eggs and
holoblastic cleavage. Each cleavage completely divides the egg.
[break=Meroblastic Cleavage]
In eggs with more yolk, cleavage cannot cut through the large mass of yolk, and so the egg does not completely divide with each cleavage. This is known as meroblastic cleavage (from the Greek “meros,” meaning “part”). In meroblastic cleavage, what ultimately happens is that a clump of dividing cells sits atop the undivided yolk.
In birds, reptiles, most fishes, some amphibians, cephalopod mollusks, and monotreme mammals (all of which have telolecithal eggs), so much yolk is present that cleavage can occur only in a narrow disk at the extreme animal end of the egg. As the embryo grows, blood vessels penetrate into the yolk. The yolk is gradually broken down and absorbed, but it never divides. In insects with centrolecithal eggs, cleavage occurs in a ring surrounding the central, uncleaved yolk. As the embryo develops, the yolk is gradually broken down and absorbed.
http://www.freethought-forum.com/forum/gallery/files/5/0/zebrafish.jpg
A zebrafish (Phylum Chordata) has telolecithal eggs and meroblastic cleavage.
Two cleavage divisions have occurred in this egg. The animal pole (left) has
divided into four cells, but the vegetal pole (right) has not divided at all.
[break=Blastula Formation]
Cleavage, of course, results in a solid ball of cells known as a morula. As cleavage continues, the morula hollows out to form a blastula. The blastula then folds inward to form a gastrula. This is gastrulation.
[break=Gastrulation]
[b]Gastrulation:
Gastrulation results in the formation of two embryonic germ layers, the inner endoderm and the outer ectoderm. The endoderm surrounds the archenteron, which will eventually become the animal’s digestive tube. The opening into the archenteron is the blastopore.
In diploblastic animals, the endoderm and the ectoderm are the only germ layers, and all adult tissues develop from those two embryonic tissues. In most diploblastic animals, there is only one opening into the digestive system, and so that opening functions both as the mouth and the anus.
In triploblastic animals, a third layer of tissue forms between the endoderm and the ectoderm, the mesoderm. Most of the internal organs form from mesodermal tissue, while the blastopore forms either the mouth or the anus, depending on whether the animal is a protostome or a deuterostome.
[b]Organogenesis:
Organogenesis is the formation of organs. As development continues, cells begin to differentiate, taking on different sizes and shapes. Differentiation leads to the formation of specialized cells that can perform specific functions. These cells become organized into tissues which, in turn, form organs.
The differentiation of unspecialized cells (stem cells) into tissues and then organs marks the transition from a gastrula to an embryo. Particularly in mammals, when the organ systems are more or less completely formed, the embryo is referred to as a fetus.
[b]Amniotes and the Four Extraembryonic Membranes:
Because their eggs aren’t surrounded by protective membranes that would prevent desiccation, early vertebrates (fishes and amphibians) had no choice but to lay their eggs in water, or at least wet environments. Eventually, however, the amphibians gave rise to descendents known as the amniotes. In amniotes (reptiles, birds, and mammals), the egg is surrounded by four extraembryonic membranes that provide nutrition and protection for the developing embryo. In some amniotes, the embryo remains inside the mother’s body to develop, but in most, the mother secretes a protective shell around the egg and then expels it from her body.
The four extraembryonic membranes that surround the amniotic egg are the yolk sac, the chorion, the amnion, and the allantois. The key membrane is the amnion, because it is more or less water-impermeable, which means that the egg does not have to be deposited into a wet environment.
The amniotic egg is water-impermeable but allows free exchange of oxygen and carbon dioxide between the developing embryo and the outside environment. It also contains yolk to nourish the developing embryo. As such, it’s an effective life-support system that allows amniotes to lay their eggs on land, making them much more independent of water than are their amphibian ancestors.
http://www.freethought-forum.com/forum/gallery/files/5/0/amniotic_egg.jpg
The amniotic egg.
[b]The Yolk Sac:
As you might imagine, the yolk sac surrounds the yolk in a developing embryo. It develops as an outgrowth of the embryo’s gut.
As the embryo grows, blood vessels from the embryo penetrate into the yolk. Meanwhile, the tissues of the yolk sac secrete digestive enzymes that break down the yolk, allowing it to be absorbed into the blood and transported back to the embryo.
[b]The Chorion:
The chorion is the outermost of the four extraembryonic membranes, and its primary role is in gas exchange. You can easily find the chorion in a chicken egg; it’s the thin membrane that lies just under the shell.
In amniotes that lay eggs, oxygen diffuses into the egg across the amnion as carbon dioxide diffuses across it in the opposite direction and to the outside. As the embryo inside the egg grows and its oxygen demands increase, the chorion partially fuses with the allantois to form the chorioallantoic membrane. The chorioallantoic membrane has a rich supply of blood vessels, allowing for rapid exchange of oxygen and CO2 between the embryo and the outside environment.
In placental mammals, extensions of the chorion known as chorionic villi penetrate into the lining of the mother’s uterus. This brings embryonic and maternal blood vessels into close proximity, allowing for efficient exchange of nutrients and gases between the mother and the developing embryo.
[b]The Amnion:
The amnion is a more or less waterproof membrane that surrounds the developing embryo and prevents it from drying out. A layer of fluid known as the amniotic fluid fills the space between the amnion and the embryo. Not only does the amniotic fluid prevent the embryo from dying of dehydration, it also helps to cushion and protect the embryo.
In placental mammals, uterine contractions during labor cause the amnion to rupture, releasing the amniotic fluid. When this happens in a human, we say that her “water breaks.”
[b]The Allantois:
The allantois is similar to the yolk sac in that it originates as an outgrowth of the embryonic gut. Its primary function is to store metabolic wastes generated by the developing embryo. In many amniotes, it’s also important in gas exchange, as it partially fuses with the chorion to produce the chorioallantoic membrane. The base of the allantois develops into the urinary bladder.
In those amniotes that remain inside the egg while developing, the importance of the allantois cannot be overestimated. Not only is it important in gas exchange, but it stores and isolates poisonous metabolic waste products that would otherwise kill the developing embryo. (The shell that surrounds the egg prevents the embryo from expelling wastes.)
In placental mammals, the allantois plays a much less important role, since the embryo’s metabolic wastes are transported away by the mother’s blood.
[b]Mammalian Development:
The placental mammals (Phylum Chordata, Class Mammalia, Subclass Theria, Infraclass Eutheria) or eutherians, embryos are retained within the mother’s body until they reach a relatively advanced stage of development. In many eutherians, newborns can walk and run within minutes of birth. This is in contrast to the monotremes, which lay eggs, and the marsupials, which give birth to underdeveloped young that then crawl into a pouch (marsupium) on the mother’s body, where they continue their development.
Eutherians can give birth to such highly-developed young because the developing embryo/fetus is nourished by a placenta. The placenta develops from both embryonic and maternal tissues, and is connected to the developing embryo/fetus by the umbilical cord.
The umbilical cord develops from the yolk sac and the allantois and contains blood vessels that transport fetal blood to and from the placenta. In the placenta, blood vessels from the mother and fetus come into close proximity, but there is normally no mixing of fetal and maternal blood. In the placenta, metabolic wastes diffuse from the blood of the fetus into the mother’s blood and are transported away. Meanwhile, oxygen and nutrients diffuse from the mother’s blood into the blood of the fetus. In this way, the fetus is completely supported by its mother until it is sufficiently developed that it can survive outside her body.
The placenta, which allows eutherian mammals to give birth to highly-developed young, is one of the major reasons that eutherians can be found in such a wide variety of habitats. Eutherian mammals can be found on all continents and in the oceans. Eutherian mammals give birth in deserts, on Arctic ice floes where temperatures are well below freezing, and even under water.
http://www.freethought-forum.com/forum/gallery/files/5/0/placenta.jpg
In eutherian mammals, the placenta lines the uterus of a pregnant
female and is connected to the developing fetus by an umbilical cord.
The placenta allows the mother’s blood to supply oxygen and nutrients
to the fetus’ blood and also removes metabolic wastes from the fetus’ blood.
[break=What Comes Next]
In our next chapter, we’ll take a brief look at the members of the Kingdom Protista. Animals evolved from protozoan protists, so this discussion will lead us into the origins of the Kingdom Animalia.