52 GENETICS AND EUGENICS 



the stump of the old one. Such facts as these compelled 

 Weismann to assume that, in cases of leg regeneration, not 

 all the leg determiners pass out during development into the 

 leg, but a supply is also held in reserve in the adjacent parts 

 of the body; these being latent or inactive ordinarily, but 

 becoming active when the leg is removed. 



Experimental studies of regeneration made by Morgan, 

 Child, and others scarcely support Weismann's view. They 

 indicate that any undifferential cell of the body, if placed at 

 the stump of an amputated leg, might function in leg re- 

 generation, and so that specific leg regenerators do not exist. 

 It is true that, in many animals, particular groups of cells 

 have the ability to produce only a particular kind of struc- 

 ture, no matter where they are placed in the body, in a 

 transplantation experiment. But in such cases it is pretty 

 clear that we are dealing, not with the effects of specific deter- 

 miners, but with the consequences of cytoplasmic differentia- 

 tion which, in many cases at least, arose in the undivided ^gg 

 when no nuclear difference existed within the organism, since 

 it contained only a single nucleus. 



3. Polymorphism. In many species of animals and plants 

 the form of the adult differs fundamentally according to the 

 environment in which it is placed. In certain amphibious 

 plants (e. g., Ranunculus aquatilis) the plant when growing 

 in the air develops flat broad leaves, but when growing under 

 water develops leaves dissected into numerous hairlike ap- 

 pendages. Weismann supposed that in such cases there exist 

 alternative sets of determiners in the germ-plasm, one for the 

 land form of leaf, one for the water form, conditions of dry- 

 ness or dampness during development calling one or the other 

 set into activity. If intermediate conditions were shown to 

 produce intermediate effects, he would doubtless assume a 

 joint and partial activity of both sets. In animals more 

 complicated conditions of polymorphism occur. Many spe- 

 cies of butterfly have spring and summer generations of off- 

 spring (broods as they are called), quite different in appear- 

 ance, corresponding to different external conditions of tem- 



