II. THE PERIOD OF ORGAN DEVELOPMENT 145 



It is highly probable that this contact induction is brought 

 about by material influences, i.e. that the cells secrete sub- 

 stances which diffuse into the environment, and which are 

 taken up by, or at least act on, the neighbouring cells. It must 

 be admitted that for most inductions this has not been proved 

 with the same certainty as for neural plate induction. In many 

 cases, however, there are plenty of indications which support 

 this assumption. The fact that dead tissues, and tissue extracts, 

 often can induce not only neural tissue but many other organs 

 as well, is one strong argument in its favour. 



We must now turn our attention to the problem of how far 

 the specificity of the effect depends on the nature of the in- 

 duction, i.e. whether each organ is induced by its "own" 

 evocator. Several possibilities must be considered here. One 

 and the same evocator might be operating in all cases. The 

 divergent development of the cell groups, e.g. into organs so 

 different as an eye-lens, an olfactory pit, or an auditory organ, 

 would then be due to differences in the reactivity of these cell 

 groups themselves. The influence of induction would then be 

 of a purely activating nature; it would not cause the intrinsic 

 differences between the parts of the germ, but only make them 

 manifest. We must not forget here that such a specific re- 

 activity of the cell groups must owe its origin to previous 

 chemodifferentiation. This, in its turn, would require an 

 explanation. 



Another attempt at an explanation also takes one evocator 

 as its starting point, but attributes great importance to varia- 

 tions in the concentration of this substance. The occurrence 

 of specifically different organ-forming potencies would, in this 

 view, be a consequence of concentration differences, to which 

 the reaction system would be extremely sensitive. Which of two 

 possible courses development would take, would depend on 

 whether or not the evocator concentration exceeded a certain 

 "threshold". We have already seen (p. 126) how Dalcq ex- 

 plained the properties of the primary organisation-field of the 

 germ by means of this hypothesis, and it may well be asked 

 how far such an explanation might apply to the phenomena 

 of organogenesis as well. 



Raven - Outline Physiologie 10 



