Bilateral Cleavage

In some animals (for example tunicates and nematodes, although in different way), the arrangement of 4 blastomeres after second cleavage is almost radially symmetrical like in the radial kind of cleavage, but two of these are larger than to the other two blastomeres establishing a plane of bilateral symmetry in the developing embryo. During consequent cleavages the bilateral arrangement of blastomeres might be still more obvious. Further, relies on whether a particular cleavage furrow may divide the egg totally or partially, cleavage has been described as:

a) Holoblastic or complete: Each cleavage furrow divides the whole egg completely in the specific plane. It may be:

A) Equal holoblastic cleavage: It takes place in alecithal (eutherian mammals) or microlecithal (Amphioxus, echinoderms) eggs in which each cleavage furrow divides the egg so as to generate blastomeres of approximately equal size.

B) Unequal holoblastic cleavage: This occurs in mesolecithal and moderately telolecithal eggs (lower groups of bony fishes and amphibians), in which the yolk is mainly concentrated in the vegetal hemisphere. In these eggs the first and second cleavage divisions take place along Meridional planes generating 4 equal blastomeres. But, due to the yolk being concentrated in the vegetal region, the 3^rd cleavage furrow is latitudinal above the equator and closer to the animal pole. The furrow divides each of the 4 blastomeres totally but not equally into a small and larger daughter blastomeres. Hence the third cleavage is, unequal holoblastic producing 4 small blastomeres (micromeres) in the animal region and 4 large blastomeres (macromeres) in the vegetal region. Subsequently the micromeres consisting of relatively less yolk divide at a very much faster rate as compared to the large yolky macromeres.