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 
