Cleavage refers to the early cell divisions that occur as a fertilized egg begins to develop into an embryo.
In eggs that contain no (mammals) or only moderate amounts (frog) of yolk, cytokinesis divides the cells completely. The figure shows the results of the first two cleavages in the frog embryo.
Figure 14.3.1: First two cleavages in a frog embryo
In eggs that contain a large amount of yolk, cytokinesis does not divide the egg completely.
Figure 14.3.2: Bird cleavage
The hen's egg consists of just a tiny patch of cytoplasm resting on the surface of a large ball of yolk (the "white" of the egg is noncellular accessory protein). When the first cleavages occur in the hen's egg, the cleavage furrows do not continue down through the mass of yolk. Therefore, each of the cells produced in the earliest stages is bound on the top and on the sides by a plasma membrane, but the bottom of the cell is in direct contact with yolk.
Figure 14.3.3: Zebrafish cleavage
This type of meroblastic cleavage is also found in the eggs of fish, reptiles, and 4 species of mammals — the monotremes. This photo, courtesy of H. W. Beames and Richard G. Kessel, shows the zebrafish (Danio) embryo at the 32-cell stage. Note that the cleavage furrows have not continued down through the yolk of the egg.
Insects use a different type of meroblastic cleavage.
Figure 14.3.4: Insect cleavage
The yolk of the eggs of insects is concentrated in the center of the egg. The daughter nuclei produced by mitosis of the zygote nucleus remain suspended within the single egg compartment. After several thousand nuclei have been produced, they migrate to the cytoplasm-rich margin of the egg. Only then does a plasma membrane form around each one.
What does cleavage accomplish in the development of the organism? First, it provides a stockpile of cells out of which the embryo will be constructed. Second, cleavage establishes a normal relationship between the nucleus and the volume of cytoplasm it regulates (and which in turn regulates it). Even small eggs are enormous when compared with other kinds of cells. The volume of the frog egg is about 1.6 million times larger than that of a normal frog cell. But it, too, contains only a single nucleus. During cleavage, thousands of new nuclei are produced by mitosis all of which finally end up in a cell of normal dimensions. The frog blastula, with its thousands of cells is no larger than the original fertilized egg.