How does the amount of yolk affect cleavage?
1 Answer
Immediately after its fertilization the ovum enters upon a series of mitotic divisions which occur in close succession. This series of divisions constitutes the process of segmentation or cleavage.
In birds and other animals there is yolk to consider. Yolk is stored in an egg cell as deutoplasm is non-living and inert.
The deutoplasm has no part in mitosis except it mechanically influences the activities of the protoplasm of the cell.
It is obvious that any considerable amount of yolk will retard the division, or prevent the complete division, of the fertilized ovum.
The amount and distribution of the yolk will therefore determine the type of segmentation.
The cleavage can be holoblastic (total or entire cleavage) or meroblastic (partial cleavage).
Since mammals implant into the uterine wall they do not need much yolk (a food source). Their cleavage is holoblastic or equal.
Others (fish and frogs) which have to live off their yolk, have a cleavage which is meroblastic or unequal.
