Influence of Yolk on Cleavage

Though biological significance of yolk is to provide nourishment to the developing embryo, it is not part of the active cytoplasm. Yolk is dead and inert component not participating in the cellular activities, but, it influences cleavage in, the following ways:

1. With gradual increase in the amount of stored yolk, the total amount of the active cytoplasm tends to decrease.
2. Cell division is the activity of only the nucleus and cytoplasm. With increase in the yolk amount the formation of spindles, cell membranes and cleavage furrows takes place in the active cytoplasm which is restricted to relatively smaller areas of the zygote and its daughter blastomeres.
3. The speed of cleavage is inversely proportional to the amount of yolk present. In the telolecithal eggs, blastomeres nearer to the animal pole divide at a faster rate than the blastomeres located towards the vegetal pole because the passive behaviour of the inert yolk in the yolky parts of the zygote and its daughter blastomeres obstructs the formation of cleavage furrows.

Therefore, the nature of various metabolic activities of the egg and the blastomeres derived from it depends upon the amount and placement of yolk mass.