(diagrammatic median section). (From Hatschek.) g primitive gut, u
primitive mouth, p peristomal pole-cells, i entoderm, e ectoderm, d
dorsal side, v ventral side.)

In these "telolecithal" ova, or ova with the yelk at one end (for
instance, in the cyclostoma and amphibia), the gastrulation then
usually takes place in such a way that in the cleavage of the
impregnated ovum the animal (usually the upper) half splits up more
quickly than the vegetal (lower). The contractions of the active
protoplasm, which effect this continual cleavage of the cells, meet a
greater resistance in the lower vegetal half from the passive
deutoplasm than in the upper animal half. Hence we find in the latter
more but smaller, and in the former fewer but larger, cells. The
animal cells produce the external, and the vegetal cells the internal,
germinal layer.

Although this unequal segmentation of the cyclostoma, ganoids, and
amphibia seems at first sight to differ from the original equal
segmentation (for instance, in the monoxenia, Figure 1.29), they both
have this in common, that the cleavage process throughout affects the
WHOLE cell; hence Remak called it TOTAL segmentation, and the ova in
question holoblastic, or "whole-cleaving." It is otherwise with the
second chief group of ova, which he distinguished from these as
meroblastic, or "partially-cleaving ": to this class belong the
familiar large eggs of birds and reptiles, and of most fishes. The
inert mass of the passive food-yelk is so large in these cases that
the protoplasmic contractions of the active yelk cannot effect any
further cleavage. In consequence, there is only a partial
segmentation. While the protoplasm in the animal section of the ovum
continues briskly to divide, multiplying the nuclei, the deutoplasm in