H. WADDINGTON ' H. V. BR0NDSTED 



mechanisms combined, one for preserving identity and the other for ensuring diver- 

 sity. By such a mechanism some of the experimental results obtained in planarians 

 by Professor Brondsted, such as double gradients, space-time-graded inhibition, and 

 blockage, might conceivably be explained. It might also offer a new view point on 

 the problem of unspecific inductions, as the unspecific stimulus would simply have 

 the effect of removing or destroying the inhibitor. These views are put forward in a 

 purely tentative way, and no doubt many alternative explanations could be sought. 



C. H. Waddington. I agree with Dr. Rinaldini that it is quite possible that differentiat- 

 ing cells produce diffusible substances which inhibit neighbouring cells from entering 

 the same path of differentiation. A hypothesis of this kind has recently been proposed 

 by Dr. Merrill Rose. However, although possible, it is not in my opinion very prob- 

 able that such inhibitory substances play an important part in early determination, 

 though they may more probably do so in the growth of already differentiated tissues 

 at later stages. In the early stages of development cells differentiating in one way may 

 stimulate their neighbours to differentiate in the same way (cf. 'homoiogenetic 

 induction'). It may be mentioned that only if the substance P acts autocatalytically, 

 to encourage the production of more P, does one obtain an exaggeration of an initial 

 difference in conditions; this would not happen on ordinary mass action principles. 



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