Ill EXPERIMENTAL MODIFICATIONS 419 



Waddington (1952a) has attempted to find out how far reactions evoked by 

 a dead inductor are related to its origin, or different, or simply chaotic. He used 

 newt stages fi"om neurula to tail bud stage. The embryos were immersed not more 

 than 5 sec. in 90' C water; then, the anterior and posterior parts of the axis were 

 used in sandwiches, the ectobiast being taken from gastrulae with sickle-shaped 

 blastopores. There was some regional effect in the sense that eyes were induced 

 only by the anterior pieces, in 30% of the cases. However, close attention was 

 devoted to earlier stages, where the situation appeared less clear. 



"In general, the earlier the stage, the more chaotic the induced tissues are likely to be* 

 We frequently find the whole interior of the explant full of a complex of neural vesicles 

 and tubes, with larger plate-like areas lying against the more extended surfaces of the 

 dead implant. At very early stages one can sometimes find a symmetrically folded neural 

 groove, but in the immediately succeeding stages it is rare to find any formation which 

 bears much resemblance to the normal neural system of similar age" (Waddington, 1952, 

 p. 492). This is an important statement. It leads the author to assume that "if some 

 region of the mass happens to have a shape roughly like that of some part of the neural 

 system, forces arise within it which cause it to approach that part in its development. 

 This would amount to the spontaneous self-individuation of particular organs, arising on 

 the basis of chance approximations to some normal arrangement". Waddington also 

 stresses the fact that in these forebrain-like inductions no "mesoderm" is formed, while 

 it is well-represented in cases where a trunk neural tube appears. He insists on the "meso- 

 dermal-to-neural ratio" and we have seen already how well-founded this idea is. However, 

 I doubt that we may feel satisfied with an explanation based only on resemblance attained 

 by chance. It has already been emphasized that the difference between two types of 

 primary induction is of a more fundamental nature; this will become still more evident 

 in the discussion of chemically-modified inductors and of xeno-inductors. 



As far as I know, only one author has examined the effect of intraliminal tempera- 

 tures^. Takaya (1955b) did this on fragments cut above the blastoporal lip at 

 five successive stages, and subjected them to temperatures of 30° to 38° C for one 

 hour. The ability to cause notogenesis was not suppressed. There was however 

 some shifting of the induction toward the acrogenetic type. 



It is also necessary to recall that, according to Penners (1936), warming the 

 ventro-marginal zone of an egg while cooling the dorsal part may cause the 

 formation of an embryo on the ventral side. This isolated experiment would be 

 worth confirmation and a more detailed analysis. 



{b) Biochemical modification of inductors and reactors 



A considerable number of definite chemical variations and different alterations 

 of the standard medium have been tried on several species. W^e shall consider 

 here only the instances which bear on the primary induction problem; all of 

 them will concern amphibians. 



The ionic equilibrium of such a medium as Holtfreter's solution has been rec- 

 ognized to be of importance, especially in certain urodeles groups, Ambystoma 

 and Triturus. Barth (1941) first observed the aberrant neuralization of isolated 

 Ambystoma ectobiast^. Holtfreter (ig44a; 1945, ig47a) submitted this puzzling 



^ In the molluscan egg Linmea, an elaborate study of the morphogenetic effects of heat 

 shock has been performed (Raven, de Roon and Stadhouders, 1955; see also Raven, 1958). 

 A similar process may be observed by culturing ectobiast isolated from Rana pipiens gas- 

 trulae; the differentiation of muscular and neural cells is observed (Barth and Barth, 1958). 



Literature p. 483 



