vitellus

It has only been met with in the vitellus of some of the medium sized ova of a young female.

This vitellus or yolk undergoes a series of segmentation or dividings which are known as spontaneous segmentation.

In Ascidia canina these processes become as long as the whole diameter of the vitellus (Kupffer, No. 60).

It is formed from the vitellus at a much later period than the proper vitelline membrane.