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]. [40] _Phil. Trans._ 1875. 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. 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. [41] Fol, _Archives de Zoologie Expérimentale_, Vol. IV. 1875. 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.