194 PRINCIPLES OF EMBRYOLOGY 



German workers on Amphibia and the British on the chick were success- 

 ful in obtaining inductions by means of grafts which had been killed 

 (Bautzmann, Hokfreter, Spemann and Mangold 1932, Waddington 

 1933/)). Now it is fairly obvious that whatever a dead graft may be able 

 to induce, it can hardly produce something which could be regarded as 

 tending to complement the graft and convert it into a complete organic 

 unit, since no dead piece of tissue can possibly form part of a developing 

 embryonic structure. One could therefore consider the possibility that a 

 dead graft might be able to induce something, and even that different 

 regions of a dead organiser might tend to induce different parts of the 

 embryonic axis, but they could certainly not exhibit the whole complex- 

 ity of the inductive behaviour which is shov^oi, for instance, in the amal- 

 gamation of the grafted organiser and what it induces into a complete 

 embryonic axis. In other words, a dead graft might evocate, but it could 

 not individuate. In fact, from the investigation of the capacities of dead 

 organisers, one might hope to arrive at a much more profound analysis 

 of the induction process. Can we perhaps separate evocation again into 

 'evocation of some generalised sort of neural tissue' and 'evocation of a 

 defmite region of the nervous system' ? Or is the transmission of regional 

 character always bound up with the completion of a part-structure into 

 an organic whole, and thus necessarily an aspect of individuation ? As a 

 matter of fact, we are still not completely sure of the answer (p. 460). 



The first important advance beyond the bare fact of evocation by the 

 dead organiser was made by Hokfreter (1934^, b). He showed that 

 although, when a graft is made from a living egg, only the presumptive 

 axial mesoderm can induce, the properties of the dead material are rather 

 different; the whole presumptive ectoderm and mesoderm, after killing 

 by heat or organic solvents, will call forth the differentiation of new 

 neural tissue. Moreover, many adult tissues, such as liver or kidney, of the 

 most diverse species ranging through the whole animal kingdom, acted 

 as evocators, particularly when killed before being inserted into the blasto- 

 coel of a host egg. This appeared at first sight greatly to faciUtate the 

 attempts which several groups of workers were making to extract and 

 identify the evocator substance; instead of starting with dead organiser 

 material, which can only be obtained by dissection of the small gastrula, 

 one could start with large masses of liver and test the activity of various 

 fractions. But the different groups of investigators came to quite different 

 conclusions as to which fractions were the most active. Spemann and his 

 collaborators at first identified the evocator with glycogen; Fischer argued 

 that evocation could be brought about by the stimulus of various acids, 

 among which he mentioned the nucleic acids ; Needham and Waddington 



