This is the inherent nature of the sexually reproductive multi-cellular animals that their
body originates from a large-sized, single celled (unicellular), nutrient-filled, diploid
and fertilized egg or zygote. In them, immediately after the activation of egg either
through the intervention of a spermatozoon (fertilization) or through some parthenogenetic
agent (parthenogensis), the activated egg is passed through a phase of repeated mitotic cell
divisions, which occur in rapid succession and produce an increasing number of cells. These
cells build up the body of the metazoan embryo and eventually of the adult. The spitting or
division of an activated egg by a series of mitotic cell divisions into a multitude of cells
which become the building units of future organisms, is called segmentation, cellulation or
cleavage. The cleavage occurs at the very beginning of embryological development and is
characterized by following events:
1. It divides the substances of the egg into a increasing number of cells of progressively
decreasing size. The divisions continue until the average cell sizes are those
characteristic of the differentiated cells of the parental organism. The number of
successsive divisions accordingly depends on the volume of the egg compared with the volume
of typical somatic cells and on the availability of intracellular or extracellular reserves,
such as yolk. The resultant cells of cleavage process are called blastomeres (Gr.
2. The general shape of the embryo does not change.
3. No growth occurs.
4. Chemical conversion of reserve food materials (yolk, glycogen, etc.) into active
cytoplasm containing pool of amono acids, monosaccharides, lipid molecules and variety of
enzymes, takes place. The active cytoplasm ultimately transforms into nuclear substances
(molecules of DNA, RNA and nucleoproteins) and those chemical molecules (e.g., proteins,
phospholipids, enzymes, etc.) which are needed as the building blocks in the formation of
microtubular fibres of mitotic spindle, and nuclear and plasma membranes. However, nuclear
gain is balanced by cytoplasmic decrease and qualitative changes in the chemical composition
of the egg are limited.
5. The constituent parts of ooplasm remain comparatively unaltered during cleavage, i.e.,
internal organization of ooplasm of a fertilized egg remains same in a cleaved egg.
6. The ratio of nucleus to cytoplasm remains very low at the beginning of cleavage, but, at
the end of cleavage (blastula stage) it is brought to the level found in ordinary somatic
7. The cleavage converts the egg into a compact mass of blastomeres called morula which
ultimately transforms into blastula, having uni-layered thick blastoderm around a central