INFLUENCE OF ENVIRONMENT 321 



many animals which are incapable of regener- 

 ating lost parts of their bodies, and similarly 

 there are cases in which part of an egg cannot 

 give rise to a whole animal. The evidence 

 available at present favors the view that in 

 cases where one of the cleavage cells is capable 

 of giving rise to a whole animal there is a 

 greater capacity of regeneration or regula- 

 tion, and possibly also a lower degree of initial 

 differentiation, than in those cases in which 

 part of an egg is capable of producing only 

 part of an animal. 



If the fertilized egg is whirled rapidly on a 

 centrifugal machine it may be subjected to a 

 pressure several hundred times that of gravity. 

 Under such conditions the heavier particles 

 are thrown to one side of the egg and the 

 entire substance of the egg becomes stratified 

 into layers or zones. In the ascidian egg, 

 where the different kinds of protoplasm give 

 rise to different tissues and organs, this rear- 

 rangement of the egg substances may lead to a 

 marked dislocation of organs ; the animal may 

 be turned inside out, having the endoderm on 

 the outside and its skin and ectoderm on the 

 inside, etc. (Fig. 75). 



