Ill 



PRIMARY INDUCTION 



EXPERIMENTAL DATA 



397 



Fig. 62. A graphic summary of the resuhs obtained from the explantation of Urodele dorso- 

 marginal material. For the meaning of the map areas, see Fig. 49, p. 381. In (a) the dots 

 indicate the areas which yielded brain parts, and the hnes, the areas which produced 

 tail and fin structures. Thus, a rough distinction between cephalo-and ourogenesis (Fig. 39, 

 p. 368). In (b), similar observations for olfactory placodes (empty circles,) optic vesicles 

 (dotted circles) and otic placodes (dots) are given. This distribution corresponds to the 

 distinction between aero- and deutogenesis. From Holtfreter, 1938 b. 



lature are regularly obtained, and often also pronephric tubules. The mesoblastic 

 structures are frequently accompanied by a neural axis, which may show a brain- 

 like swelling at one end^ It seems that such an ectoblast formed by cells of the 

 marginal territory is more easily induced than the normal ectoblast. In cases 

 where some cephalization takes place, the relation to the intrinsic axes (or 

 gradients) of the explant has not been strictly ascertained. However, a strong 

 probability that such a relation exists is suggested by the fact that the explants 

 which yield olfactory placodes and optic and otic vesicles, are obtained from the 

 region where the existence of the corresponding inductors is assumed (Fig. 62). 



Owing to technical difficulties involved in obtaining typical explantations, 

 many authors have prefered to wrap the fragments in a piece of ectoblast taken 

 from a young gastrula. This envelope also serves as a detector of the inducing 

 ability possessed by the explant. A further modification has been to insert the 

 investigated piece of embryonic tissue between two ectoblastic sheets (sandwich 

 technique). 



The development of the newt explants wrapped in young ectoblast of anurans 

 has already been examined by Holtfreter, in 1936. The xenoplastic method 

 allowed him to discern which structure originated from the explant and which 

 from the enveloping tissue. A clear-cut difference was observed between cases in 

 which the explant was a young blastoporal lip, and those in which it was the 

 hind part of the neurula archenteric roof. In this last instance, trunk-tail com- 



^ When spinal cord or brain parts appear in such explants, they do not result necessarily 

 from a process exactly similar to the normal course of events. Except when relatively large 

 amounts of material are available, no neural plate is observed, and a fortiori no closing 

 in a neural tube. Concurrently with the distribution of the inner material into a noto- 

 chordal and a myogenic anlage, a certain amount of inner cells of the ectoblastic covering 

 apparently detach themselves and form a mass or cord which becomes neuralized. 



Literature p. 483 



