THE SEGMENTATION OF THE OVUM. 99 



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 conspi- 

 cuous 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 segmenta- 

 tion, and eventually completely envelop the former. Such a 

 case as this has been described in Aplysia by Lankester 1 . 



The types I have described serve to exemplify unequal seg- 

 mentation. 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 seg- 

 mentation 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 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 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 



1 Phil. Trans. 1875. 



- Fol, Archives dc Zoologie Experimentale, Vol. iv. 1875. 



72 



