THE SEGMENTATION OF THE OVUM. 81 



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 envelope the former. Such a case as this has 

 been described in Aplysia by Lankester 1 . 



The types I have described serve to exemplify unequal segmen- 

 tation. 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 segmen- 

 tation as to the presence of a segmentation cavit}^ In some instance?, 

 e.g. the Frog, such a cavity is well developed. In. other cases it is 

 small, e.g. most Mollusca, while not (infrequently 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 2 , 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 vitelliue pole. 

 The protoplasmic pole is turned towards the polar bodies. In the third 

 segmentation, which takes place along au 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. 



1 Phil. Tnins. 1875. 



2 Fol, Archives de Zoologie Experimeii-tfiJe, Vol. rv. 1875. 



B. E. <; 



