Segmentation 



9 



The radial character is continued in some cases by the division 

 in vertical planes, of each of the four micromeres, and at a slower 

 rate, of the four macromeres, Fig. 5 A. But sometimes a dis- 

 tinctly bilateral phase is entered upon, Fig. 5 B. 



A B 



Fig. 5 A, Radial division; J5, Bilateral division. 



After this stage the segmentation proceeds less regularly and 

 the bilateral symmetry if established is soon lost. As early as 

 the eight segment stage a space is apparent between the segments 

 very much nearer the upper pole than the lower. This is the 

 first sign of the blastocoel or segmentation cavity. At about the 

 sixteen segment stage this cavity is quite enclosed, and we have 

 a condition comparable to that of Amphioxus, the chief difference 

 being that the segments are more uneven in size, fewer in number 

 and correspondingly larger, so that the blastocoel is correspondingly 

 smaller, Fig. 6 B. Segments are now formed by tangential 

 planes of division, so that the wall becomes first double and then 

 many-layered, and at the close of segmentation we have a blastula 

 very characteristic of Amphibians in which there is an upper pole 

 of very small black cells and a lower pole of considerably larger 

 and whiter cells, bounding between them the eccentrically placed 

 segmentation cavity, Fig. 6 D. 



There is a stage during the earlier phases to which I would 

 draw special attention. In Fig. 6 C it will be noticed that there 

 are very few division furrows extending to the lower pole, and no 

 horizontal ones. This is of course due to the fact that the 

 horizontal planes always result in the production of a small and 

 a large segment, owing to the greater amount of yolk in the lower 

 part. 



In Rana we have a good example of an egg which is holoblastic. 



