I. NEURULATION 117 



(Mangold, 1928). Next, Holtfreter (1934) showed that many 

 tissues of a wide variety of animals, both vertebrate and in- 

 vertebrate, have the power of induction. He studied not only 

 living tissues, but also dried, boiled, and minced material, and 

 these, too, appeared to have inductive capacities in many cases. 

 Moreover, not only neural plates, but also other organs, such 

 as olfactory pits, eyes, ear-vesicles, limbs, etc., were formed 

 by the host ectoderm under the influence of the inductors. It 

 was concluded from these experiments that the inducing agent, 

 or possibly agents, are very widespread in animal tissues, and 

 that the processes concerned are of a very general character, 

 and of low specificity. 



All interest now centred on the problem of the nature of 

 induction. Marx (1931) found that narcotized organisers did 

 not lose their power of induction. Spemann (1931) found the 

 same in ground organisers. Even dead tissue proved still to 

 possess inductive powers. Organisers that had previously been 

 dried, frozen, boiled, or killed with alcohol, still caused the 

 formation of neural plates in the overlying ectoderm (Holt- 

 freter, 1933). The remarkable discovery was made that even 

 embryonic tissues which had no power of induction when alive, 

 acquired this capacity after death. This was found, for example, 

 in the prospective ectoderm and endoderm of gastrulae. 



It was an obvious hypothesis at this stage to ascribe the 

 induction to a substance, given off by the inductor, which 

 causes the ectoderm cells to form neural tissue. This inducing 

 substance was called "evocator'' by Waddington, and "organ- 

 isiwe" by Dalcq. In normal development it would be given off 

 only by the archenteron roof. It might be present in other 

 parts of the embryo, but there it would be prevented from taking 

 effect in some way or other. It was assumed that many other 

 animal tissues also contained the substance. This was confirmed 

 by the successful preparation of extracts from animal tissues 

 which, absorbed by an agar-agar carrier, induced a neural 

 plate in the ectoderm of an amphibian gastrula. In principle, 

 the determination of the nature of these evocators had thereby 

 become possible. On this point, however, research has not led 

 to satisfactory results. It was possible, indeed, to determine the 



