2$2 RE GENERA TION 



In the light of the questions discussed in the preceding pages, we 

 may now attempt to follow out in a more connected way some of the 

 modern views and hypotheses dealing with the problem of develop- 

 ment. 



Hertwig, as we have seen, has opposed the Roux-Weismann 

 hypothesis, and has formulated a view of his own. According to 

 Hertwig, the cleavage divides the egg into equivalent parts, an idea 

 very similar to that of Pfltiger. The cells he regards as units, and 

 the development as the result of the interaction of the cells, a 

 process that in a way Roux had also assumed to take place between 

 the different parts of the later embryo. Thus, while Hertwig's hy- 

 pothesis contains little that is really new, it has selected portions from 

 several already existing hypotheses, and united them into a consistent 

 whole. It has been objected to Hertwig's view that the interaction 

 of equivalent cells could never account for the introduction of new 

 processes in the development ; but if we grant that the cells are 

 never entirely equivalent, whatever their potence may be, this objection 

 can, I think, be met. Hertwig's chief service has been his destructive 

 criticism of the Roux-Weismann idea of qualitative nuclear division. 



Hertwig maintains that each stage in the development is the cause 

 of the next stage, and states that a description of the series of stages 

 through which the embryo passes gives a causal knowledge of the 

 phenomena of development. He claims that beyond this descriptive 

 knowledge we cannot hope to penetrate. Both Roux and Driesch 

 have taken issue with Hertwig, and have pointed out that while each 

 stage in the development contains within itself the causes of the suc- 

 ceeding stage, yet we gain no idea as to these causes from a simple 

 description of two consecutive stages themselves. To state that the 

 fertilized egg is the cause of the cleavage gives us no idea of what 

 sort of a process the cleavage is, or how it arises, or what determines 

 the sequence of the divisions, etc. The blastula, for instance, con- 

 tains the factors that produce the gastrula; but to state that, in a 

 physical sense, the blastula is the cause of the gastrula is an erroneous 

 interpretation of what is meant by causal knowledge. If Hertwig's 

 idea were correct, there would be as many causes in each embryo as 

 there are stages in its development, and as many causes in the whole 

 range of embryology as there are forms that develop multiplied by the 

 number of stages in each embryo. What we should seek to discover 

 is the particular cause that brings about each kind of process. If 

 we could discover the cause in one single case, it is highly probable 

 that it would be found to extend to a large number of other cases. 



Driesch formulated an hypothesis of development in his Ana- 

 lytische Theorie, but has modified and changed it in several later 

 contributions. It is difficult to give in a few words the subtile analysis 



