Cleavage:
Soon after fertilization, the zygote begins to divide via
cleavage. 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.
Salamander eggs (Phylum Chordata): Each egg has a
clearly-visible animal pole (dark) and vegetal pole (light).
Cleavage
Linear Mode
