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.
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.
Meroblastic Cleavage
Linear Mode
