i SEGMENTATION 27 



sionally one or other of them may become latitudinal. The various 

 meridional and vertical furrows gradually extend downwards towards 

 the lower pole of the egg in the order of their appearance, and during 

 the earlier stages the lower hemisphere of the egg possesses only 

 such furrows (Fig. 15, B 6). 



As the blastomeres go on segmenting there is produced eventually 

 a blastula with an upper hemisphere of small cells which appear 

 white because' of the finely subdivided condition of their yolk and a 

 lower hemisphere of larger more yolky elements (Fig. 15, B 7). 



Already at the stage when the egg is divided into four segments 

 a space develops between the blastomeres. As segmentation goes on 

 the micromeres tend to round themselves off, leaving wide chinks 

 between containing fluid. By the blastula stage the fluid has collected 

 together into a spacious segmentation cavity which is visible in the 

 whole egg as a dark shadow in its upper hemisphere. At first the cavity 

 is rounded and is roofed in by a single layer of cells but later it spreads 

 out, takes a planoconvex form and its roof comes to be composed of 

 two layers of closely apposed cells. 



Amphibia. — The Amphibia are in the matter of segmentation the 

 most interesting and important group of the vertebrata, for in no other 

 group does there exist so much variety in the proportional amount of 

 yolk present in the egg. Much work still remains to be done in 

 regard to this group in the way of detailed study of the process of 

 segmentation in its relation to the amount and concentration of the 

 yolk. 



As already indicated the extent of the influence which the yolk 

 exerts in retarding the living activities of the protoplasm, such as 

 growth and division, bears a rough relation to its proportional 

 amount. As regards the majority of cases this may be said to vary 

 directly with the size of the egg. The largest eggs are as a general 

 rule the most richly yolked. But the rule is by no means an invari- 

 able one that the influence on the segmentation is directly pro- 

 portional . to the total amount of yolk in the egg as a whole. For a 

 smaller egg, containing a smaller amount of yolk, may yet have that 

 yolk more concentrated in one region so as to produce there a more 

 intense retarding influence — as is the case naturally in many small 

 Teleostean eggs or as may be demonstrated experimentally by con- 

 centrating the yolk artificially through the action of centrifugal 

 force. 0. Hertwig was able by centrifugalizing frogs' eggs and so 

 causing the yolk to become concentrated in the abapical hemisphere, 

 to bring about a complete cessation of cleavage in that hemisphere 

 so that the egg thus assumed a meroblastic character. 



The variety in the size of the egg within the limits of the group 

 Amphibia has already been indicated by the table on page 2. 

 The process of segmentation agrees in the main with what has 

 been described for the frog but there is much variation in detail. 

 The variations have to do both with the position of the furrows and 

 with their appearance in point of time. One gets a good idea of the 



