![]() Though the abovementioned factors are not clearly understood, it is evident that three structures present within the cell: Cortical layer, Spindle structures and Chromosomes play the important part. (e) Formation of new cell membrane from the subcortical cytoplasm. (d) Contractile nature of the regions near the cortex and (c) Increase of tangential force activity in the cortex. ![]() Here division initiates from the animal pole and extends towards the vegetal pole, where the progress of cleavage slows down considerably. The amphibian egg contains yolk which is localised at the vegetal pole. Therefore the division is complete and early divisions occur at a very quicker rate. In the egg of Amphioxus, the yolk is thin and remains uniformly distributed. During division these bodies exert mechanical influences. The fertilized egg in most cases contains yolk, which are inert bodies. In many eggs, the cleavage is atypical which is neither typically holoblastic nor meroblastic, but assumes a transitional stage between the two. Usually the blastoderm is present in the animal pole and the vegetal pole becomes laden with yolk which remains in an tihcleaved state, i.e., the plane of division does not reach the periphery of blastoderm or blastodisc. When segmentation takes place only in a small portion of the egg resulting in the formation of blastoderm, it is called meroblastic cleavage.
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