The majority of the smaller segments in the segmented Frog’s ovum are destined to form into the epiblast, and the larger segments become hypoblast and mesoblast.
With a few exceptions (the Rabbit, Lymnæus, etc.) the majority of the smaller segments always become epiblast and of the larger segments hypoblast.
The Frog’s ovum serves as a good medium type for unequally segmenting ova. There are many cases however in which a regular segmentation is far more closely approached, and others in which it is less so.
One familiar instance in which a regular segmentation is nearly approached is afforded by the Rabbit’s ovum, which has indeed usually been regarded as offering an example of a regular segmentation.
The ovum of the Rabbit[39] becomes first divided into two sub-equal spheres. The larger and more transparent of the two may, from its eventual fate, be called the epiblastic sphere, and the other the hypoblastic. The two spheres are divided into four, and then by an equatorial furrow into eight—four epiblastic and four hypoblastic. One of the latter assumes a central position. The four epiblastic spheres now divide before the four hypoblastic. There is thus introduced a stage with twelve spheres. It is followed by one with sixteen, and that by one with twenty-four. During the stages with sixteen spheres and onwards the epiblastic spheres gradually envelop the hypoblastic, which remain exposed on the surface at one point only. There is no segmentation cavity.
In Pedicellina, one of the entoproctous Polyzoa, there is a sub-regular segmentation, where however the two primary spheres can be distinguished much in the same way as in the case of the Rabbit.
A very characteristic type of unequal segmentation is that presented by the majority of Gasteropods and Pteropods and probably also of some Lamellibranchiata. It is also found in some Turbellarians, in Bonellia, some Annelids, etc. In many instances it offers a good example of the type where in the course of segmentation the protoplasm becomes aggregated at one pole of the ovum, or of its segments, to become separated off as a clear sphere.
The first four segments formed by two vertical furrows at right angles are equal, but from these there are budded off four smaller segments, which in subsequent stages divide rapidly, receiving however, a continual accession of segments budded off from the larger spheres. The four larger spheres remain conspicuous till near the close of the segmentation. The process of budding, by which the smaller spheres become separated from the larger, consists in a larger sphere throwing out a prominence, which then becomes constricted off from it.
In the extreme forms of this unequal segmentation we find at the end of the second cleavage two larger spheres filled with yolk material and two smaller clear spheres; and in the later stages, though the large spheres continue to bud off small spheres, only the two smaller ones undergo a regular segmentation, and eventually completely envelop the former. Such a case as this has been described in Aplysia by Lankester[40].
The types I have described serve to exemplify unequal segmentation. The Rabbit’s ovum stands at one end of the series, that of Aplysia at the other; and the Frog’s ovum between the two.