TYPES OF CLEAVAGE -
Depending upon the distribution & amount of yolk, cleavage is of following types -
1. HOLOBLASTIC EQUAL CLEAVAGE (holos, whole ; blastos, germs) -
In this type of cleavage the cleavage furrow take place in the whole egg.
It occurs in alecithal, mesolecithal & isolecithal eggs.
Forming blastomere are equal in size.
2. HOLOBLASTIC UNEQUAL CLEAVAGE-
In this type of cleavage forming blastomeres are unequal. Small blastomeres are called micromeres.
Large blastomeres are called macromeres. It take place in the mesolecithal, telolecithal.
3. MEROBLASTIC OR DISCOIDAL CLEAVAGE (Gr. meros, part)-
It take place in macrolecithal or heavily telolecithal egg. The cleavage take place only in the yolk free animal pole.
Mainly it take place the germinal disc or blasto disc. The yolk remains uncleaved.
4. SUPERFICIAL CLEAVAGE -
This cleavage occur in centrolecithal eggs. It take place in the surface layer of the egg.
The central yolky part remains uncleaved. The peripheral ooplasm become multi nucleated.
On the basis of future of blatomeres, the cleavage is of 2 types -
1. DETERMINATE CLEAVAGE -
It is also called as mosaic cleavage. In this cleavage a specific blastomere forms a specific part of the embryo.
Example - Tunicata, Cephalopoda, Nematoda.
2. INDETERMINATE CLEAVAGE -
The fate of the blastomeres is not definite. If the two blastomeres after Ist cleavage get separated, both the cells form two different embryos. Similarly, if two fertilized eggs are fused, a single embryo is formed.
Example - Echinodermata, Hemichrodata & Vertebrates.
Note :- The cleavage results in the formation of multicellular & solid ball-like morula.
The cleavage is controlled by protein synthesis which inturn is controlled by informosomes because during cleavage mRNA are not formed.