404 GERMINAL ORGANIZATION INDUCTION PHENOMENA 4 



The same problem has been reexamined by Sala (1954, 1955, 1956); he did 

 not only use the anterior part of the notochord, but explored the whole median 

 strip of the archenteric roof, which was divided into 4 regions (Fig. 67, p. 403). 



TABLE 3 



INDUCTIONS PRODUCED BY THE SUCCESSIVE PARTS OF THE MEDIO -S A GITT AL 

 STRIP OF THE ARCHENTERON ROOF 



Part of Total Acrencephalic Deutencephalic Cranial Spinal Spinal Aielanocytes 



mediosagittal number structures structures ganglia cord ganglia or neural crest 



strip 



For sake of simplification, negative cases and some dubious diagnoses have been omitted; 

 the mesencephaHc structures have been left aside, considering them as having a transitional 

 value. The author's distinction between main series and transitional groups (plausible but 

 established a posteriori) has been suppressed. In each group, the numbers at the right of 

 the first qualitative quotation (heavy type) refer to observations made in the same sand- 

 wiches. Compiled from Sala, 1954. 



Fully established neurulae of axolotl were used, and the sandwich cultured as long as 

 3 weeks. The results given in several extensive tables have been summarized in Table 3. 

 In his conclusions, the author states that the prechordal explants only induce prosencephalic 

 structures, while the successive parts of the notochord induce respectively the anterior 

 half of the deutencephalon, then its posterior half, more posteriorly the spinal cord and 

 derivates of the neural crest, sometimes including the mesenchyme of the dorsal fin. The 

 inductions of maximal size are produced by the second fourth (anterior part of the noto- 

 chord), and decrease in size anteriorly and posteriorly. Taking into account the cephalo- 

 caudal stretching, Sala admits the existence of two factors which he calls activating and 

 caudogenic influences. The first one is assumed to diffuse more easily, while the other 

 would exert on the already activated ectoblast an action causing more and more caudal 

 structures to appear. The variation of these two factors is tentatively represented by two 

 discordant curves (Fig. 67). 



The following remark can be made when considering Sala's results, as they are 

 presented in Table 3. For the 2d fourth, one piece had an exclusively acren- 

 cephalic effect, while five pieces produced no more than deutencephalic structures. 

 These "exceptions" are difficult to explain by assuming the fragments to be 



