120 INDUCTION AND ORGANISATION 



Whatever their nature may in the end prove to be, we must 

 in any case assume that the evocators originate in the cellular 

 metabolism of the archenteron roof (possibly also in that of many 

 other animal tissues), and that they are taken up by the over- 

 lying ectoderm, and influence its chemodifferentiation so that 

 it becomes able to form a neural plate. For this purpose, the 

 ectoderm cells must be sensitive to the activity of these partic- 

 ular substances. We have seen that this is the case only during 

 a short period. Therefore, the production of the evocator by 

 the organiser, and the specific sensitivity of the reaction system 

 must CQincide. Presumably the sensitivity is due to the fact 

 that at exactly the right moment the evocator fits the cellular 

 mechanism of the reaction system, as a key fits the lock. It 

 may be, for instance, that the evocator can saturate unsaturated 

 bonds which would not be present at any other stage, or that 

 it can catalyse certain reactions which take place at this stage 

 in the cell's development, and which are impossible at all 

 other times. It is the mutual adjustment of the chemical activity 

 of the cells of organiser and reaction system, that ensures the 

 normal course of development. 



We have already mentioned that the activity of a trans- 

 planted organiser is not restricted to the ectoderm of the host, 

 but that the adjacent mesoderm is influenced as well. Evidently, 

 the induction spreads from cell to cell in the latter (Fig. 42). 

 This causes the cells of the host's mesoderm to differentiate 

 into notochord and somite tissue. The inductor, therefore, makes 

 this tissue similar to itself, hence the name "assimilative in- 

 duction'' for this process. As there are a number of differences 

 between this type of induction and contact induction, we shall 

 now discuss progressive induction. 



We have already seen that the grafted organiser does not 

 always form the same components of the secondary embryo. 

 Often the materials of graft and host co-operate in the forma- 

 tion of its organs, giving them a "chimaerical" composition, 

 which varies from case to case, depending upon the experi- 

 mental conditions. This is clearly illustrated by a number of 

 experiments by B. Mayer (1935). He performed heteroplastic 



