Influence of Environment 



219 



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 

 suhstances 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. No). 

 < )n 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, 



Fig. 80. Two Larvae ok Styela which were centrifuged in the 4-cell 

 stage thereby changing the position of various organ-forming substances. 

 Nervous system (11s), eyes (E), notochord (ch) and muscles iius) have 

 been displaced, and the larva has been turned inside out, the endoderm 

 (end) being outside and the ectoderm (ret) inside. 



