the vegetal section remains more or less undivided; it is merely
consumed as food by the forming cells. The larger the accumulation of
food, the more restricted is the process of segmentation. It may,
however, continue for some time (even after the gastrulation is more
or less complete) in the sense that the vegetal cell-nuclei
distributed in the deutoplasm slowly increase by cleavage; as each of
them is surrounded by a small quantity of protoplasm, it may
afterwards appropriate a portion of the food-yelk, and thus form a
real "yelk-cell" (merocyte). When this vegetal cell-formation
continues for a long time, after the two primary germinal layers have
been formed, it takes the name of the "after-segmentation."

The meroblastic ova are only found in the larger and more highly
developed animals, and only in those whose embryo needs a longer time
and richer nourishment within the foetal membranes. According as the
yelk-food accumulates at the centre or at the side of the ovum, we
distinguish two groups of dividing ova, periblastic and discoblastic.
In the periblastic the food-yelk is in the centre, enclosed inside the
ovum (hence they are also called "centrolecithal" ova): the formative
yelk surrounds the food-yelk, and so suffers itself a superficial
cleavage. This is found among the articulates (crabs, spiders,
insects, etc.). In the discoblastic ova the food-yelk gathers at one
side, at the vegetal or lower pole of the vertical axis, while the
nucleus of the ovum and the great bulk of the formative yelk lie at
the upper or animal pole (hence these ova are also called
"telolecithal"). In these cases the cleavage of the ovum begins at the
upper pole, and leads to the formation of a dorsal discoid embryo.
This is the case with all meroblastic vertebrates, most fishes, the
reptiles and birds, and the oviparous mammals (the monotremes).