Patters of Cleavage

In most of the animal groups with spherical or almost spherical egg and little or moderate amount of yolk (micro-or mesolecithal eggs), the first and second divisions result in four blastomeres of almost equal size. Because of greater concentration of yolk platelets in the vegetal hemisphere, the third cleavage divides the 4 blastomeres in latitudinal plane giving rise to 8 cells arranged in two tiers of 4 blastomeres each including one tier of 4 small blastomeres (micromeres) in the animal hemisphere and the second tier of 4 large blastomeres (macromeres) in the vegetal hemisphere. The arrangement of blastomeres in these two tiers is very distinct and on this basis, the cleavage may be of:

• Radial Type
• Spiral Type