Great variations are presented by the ova with unequal segmentation as to the presence of a segmentation cavity. In some instances, e.g. the Frog, such a cavity is well developed. In other cases it is small, e.g. most Mollusca, while not unfrequently it is altogether absent.
Before leaving this important type of segmentation, it will be well to enter with slightly greater detail into some of the more typical as well as some of the special forms which it presents.
As an example of the typical Molluscan type the normal Heteropod segmentation, accurately described by Fol[41], may be selected.
The ovum divides into two and then four equal segments in the usual vertical planes. Each segment has a protoplasmic and a vitelline pole. The protoplasmic pole is turned towards the polar bodies. In the third segmentation, which takes place along an equatorial plane, four small protoplasmic cells or segments are segmented or rather budded off from the four large segments, so that there are four small segments in one plane and four large below these. In the fourth segmentation the four large segments alone are active and give rise to four small and four large cells; so that there are formed in all eight small and four large cells. The four small cells of the third generation next divide, forming in all twelve small cells and four large. The small cells of the fourth generation then divide, and subsequently the four large cells give rise to four new small ones, so that there are twenty small cells and four large. The small cells form a cap embracing the upper pole of the large segments. It may be noted that from the third stage onwards the cells increase in arithmetical progression—a characteristic feature of the typical gasteropod segmentation.
In the later stages of segmentation the large cells cease to give rise to smaller ones in the same manner as before. One of them divides first into two unequal parts, of which the smaller becomes pushed in towards the centre of the egg. The larger cell then divides again into two, and the three cells so formed occupy the centre of a shallow depression. The remaining larger cells divide in the same way, and give rise to smaller cells which line a pit which becomes formed on one side of the ovum. The original smaller cells continue in the meantime to divide and so form a layer enclosing the larger, leaving exposed however the opening of the pit lined by the latest products of the larger cells.
The eggs of Anodon and Unio serve as excellent examples of the type in which the ovum has a uniform structure before the commencement of segmentation, but in which a separation into a protoplasmic and a nutritive portion becomes obvious during segmentation.
In Anodon[42] the egg is at first uniformly granular, but after impregnation it throws out on one side a protuberance nearly free from granules ([fig. 42], 1)
Fig. 42. Segmentation of Anodon piscinalis. (Copied from Flemming.) r. polar cells. v. vitelline sphere. 1. Commencing division into two segments; one mainly formed of protoplasm, the other of yolk. 2. Stage with four segments. 3. Formation of blastosphere, and segmentation cavity. 4. Definite segmentation of the yolk sphere.