Influence of Environment 



215 



Fig. 76. Double Embryos of Frog Developed from Eggs Inverted when 

 in the 2-cell Stage. A, twins with heads turned in opposite directions. 

 B, twins united back to back. C, twins united by their ventral sides. 

 D, double headed tadpole. (From Wilson after O. Schultze.) 



There has heen much discussion among biologists as to the 

 meaning of these results. On the one hand it has been said that 

 the totipotence of any one of the first four cleavage cells proves 

 that all of these cells are alike and that they have not yet begun 

 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 



