146 INDUCTION AND ORGANISATION 



It is true that in certain cases differences in the concentration 

 of an evocator could give rise to specifically different patterns 

 of development of cell groups, in the way indicated by Dalcq. 

 It is probable, therefore, that such differences may play a role 

 in the normal induction processes. Yet it is not very likely that 

 this alone could account for the strong increase in specificity 

 of the parts of the germ occurring in this phase of development, 

 which transforms the embryo into a system with a very high 

 degree of multiplicity. Moreover, several investigations, in 

 particular those of Chuang (1938-40) and Toivonen (1940-45) 

 give strong support to the view that there are several 

 qualitatively different evocators. Both investigators proved that 

 living and dead tissues of different origins induce organ 

 complexes of different composition, if grafted into the blasto- 

 coel of Triton. In some cases, the rostral organs of the head 

 are formed, such as fore-brain, eyes, and olfactory pits. In 

 other cases, organs of the posterior region of the head are 

 produced, such as the caudal parts of the brain and ear-vesicles, 

 or organs of the trunk, such as spinal cord, muscle tissue, 

 kidney, etc. Heat treatment of the inductors sometimes results 

 in a change in the nature of the induced organs. All this tends 

 to show that these "abnormal inductors" contain chemically 

 different substances, and that this is the reason why the germ 

 reacts with the formation of different organs. Hence, it is 

 very likely that qualitatively different evocators play a role 

 in normal development as well. In that case the specificity of 

 the induced organs can be ascribed at least in part to differ- 

 ences in the evocators. 



Taking all the investigations in this field as a whole, it 

 seems probable that at early stages (gastrulation and early 

 neurulation), a quantitative difference in evocator concentration 

 plays the main role, whereas in the course of further develop- 

 ment more and more qualitative differences arise in the 

 chemical constitution of the evocators for the various organs. 



Special attention must in this connection be paid to the 

 situation in the head-region. We have seen above th-^t we know, 

 for several organs of the head, e.g. lens, olfactory pits, auditory 

 organs, mouth, that their determination takes place in two 



