The Formative Stimulus the Determining Factor 29 



of the lens. For the cells of the iris cannot preserve 

 within them potentially any trace of a formative capacity, 

 or of a germinal "anlage," or of any "determinant" which 

 provokes the formation of the lens, seeing that in normal 

 development the latter takes its origin from another 

 tissue. 



In these examples, both in the post generation of 

 Roux's half embryos and in the regeneration of the lens 

 in the triton, the cells which serve as constructive 

 material appear then to be absolutely incapable of any 

 auto-transformation and ready on the contrary to differ- 

 entiate themselves and to dispose themselves indifferently 

 in any manner whatever, according to the formative 

 stimulus to which they happen to be exposed. 



At this point the fundamental biological question pre- 

 sents itself: What is the nature of these formative 

 stimuli, of this continued action which the formative part 

 exercises upon the part being formed? 



The attempt to build up a hypothesis relating to so 

 important a question is the object of the studies presented 

 in the second part of this chapter. 



2. Hypothesis of the Nature of the Formative Stimulus 



If, in our study of the nature of the formative stimulus 

 in the development of organisms, we start with the 

 primitive pluricellular form, consisting simply of aggre- 

 gations of cells that are all alike, we observe that during 

 some stages of their ontogeny the essential nature and 

 the behaviour of these cells is clearly determined by 

 phenomena of nervous nature in the widest sense of the 

 word. For example, phenomena of this nature exist 

 undoubtedly in the little mononuclear amoebae into which 

 the spores of the myxomycetes become changed, also in 



