THE vertebrates: the amphibia and birds 



149 



determine the position of the crescent ; the cases in which they fail are 

 those in which they do not overcome the existing predisposition. Again, 

 in normal development there seems to be a tendency for the sperm to 

 enter the egg in a predetermined plane, so that its influence on the position 

 of the crescent usually reinforces a prior condition. Finally, when newly 



c 



Figure 9.2 



A newt's egg, in its jelly capsule, is constricted into a dumb-bell shape soon 

 after fertilisation. A, cleavage occurs first in the portion containing the 

 nucleus; B, after some time a nucleus passes through the stalk, after which 

 cleavage begins in the other (left) portion also ; C, both parts may give rise 

 to a complete embryo. (From Schleip 1929, after Spemann.) 



fertilised eggs are constricted into two halves with a hair loop, we find 

 that certain halves, although they contain the nucleus, develop into 

 featureless lumps exactly similar to those, derived from a later stage,]which 

 contain no grey crescent material; and this indicates that, even immedi- 

 ately after fertiHsation, the crescent material is to some extent localised 

 so that egg fragments may contain none, or too little, of it. 



