428 'S- J- Holmes 



istic of its distal end we should bear in mind the possibility that 

 the differentiation which first appears is not necessarily that which 

 first occurs. What we know of developmental processes renders 

 it very probable that a great deal of differentiation is going on in 

 the rudiment of the limb before it is manifested by any external 

 signs. Between maturation and cleavage an ovum may show no 

 outward sign of differentiation, but experiment shows that this 

 period is one in which developmental processes are rapidly tak- 

 ing place. Before any external features are produced in the devel- 

 opment of a limb, the main outlines of its differentiation may have 

 been established through influences proceeding from its basal 

 part, after which the tip might differentiate more rapidly than the 

 intervening portion, and the other visible features of structure 

 appear successively toward the base. I do not suggest this merely 

 to save my contention by a retreat into the invisible, but there are 

 certain considerations that make such an interpretation more or 

 less probable. In many cases the visible differentiation is cen- 

 trifugal rather than centripetal. The tail of a tadpole cannot be 

 said to differentiate from the tip toward the body as it regenerates. 

 Zeleny has shown that in the early regeneration, as in the embry- 

 onic development of the antenna? of Mancasellus the formation of 

 segments proceeds at first from the base to the tip; later new seg- 

 ments are formed in the reverse direction. 



But granting that, in many cases, differentiation actually begins 

 at the extremity and works toward the base of the regenerating 

 organ, the process is not inconsistent with the point of view here 

 set forth. We may suppose that the influence of the environment 

 causes the extremity of an organ to begin to differentiate like that 

 of the missing part. That is only one step. We have then to 

 account for the numerous coordinated differentiations that take 

 place as the part develops toward the base. In my illustrations 

 of the course of differentiation under the guidance of social pres- 

 sure, I have taken the old part as a starting point, but if we have an 

 undifferentiated mass of cells, it is conceivable that, if, for any 

 reason, differentiation should start at the distal extremity of the 

 mass, it might work back under the guidance of social pressure 

 toward the base. The distal differentiation would have to get 



