Influence of Environment 227 



to differentiate. On the other hand it is said that a part of an 

 egg may give rise to a whole animal for the same reason that 

 parts of certain adult animals may do the same thing, viz., because 

 they have the power of regeneration. However there are many 

 animals which are incapable of regenerating 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 regulation, 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. 



Effects of Centrifugal Force. If the fertilized egg is whirled 

 rapidly on a centrifugal machine it may be subjected to a pressure 

 several thousand 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 rearrangement 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 ectoderm or skin on the inside, etc. (Fig. 80). 

 On the other hand in some mollusks and echinoderms the devel- 

 opment of centrifuged eggs is practically normal. In the for- 

 mer case the formative substances were dislocated ; in the latter 

 they were probably not. 



Double Monsters and Identical Twins. If the cleavage cells 

 are only partially separated they may produce animals which 

 are partially separated, such as Siamese twins, two-headed forms, 

 etc. (Figs. 75, 76). Or these double monsters may be produced 

 by division or budding of the embryo at a later stage of develop- 

 ment. In the human species, no less than in other animals, all 

 sorts of double monsters may be formed in this way by the par- 

 tial division of a single egg or embryo (Fig. 81). If the division 



