Ill PRIMARY INDUCTION EXPERIMENTAL DATA 399 



Cr. n. 



Cr. i. exp. 



Taen. ect. 



Fig. 63. A case of direct neuralization in a heteroplastic transplantation. The middle part 

 of the archenteric roof was dissected from a T. cristatus neurula and wrapped in T. toeniatus 

 ectoblast. It provided a small neural tube (Cr. n. exp.), some myoblasts (Cr. m. c.) and 

 undifferentiated cells (Cr. i. exp.) Taen. ect., the enveloping ectoblast of T. toeniates. From 



Ter Horst, 1948. 



advanced stages, brain ampullae and optic vesicles were obtained. Clearly, this 

 method is less favorable than grafting on to the ventral region of host neurulae 

 (see Denis' data, p. 412); this can be easily understood owing to more normal 

 environmental conditions to which the incipient blood circulation soon contributes. 

 Better technical success was attained by Von Woellwarth (1952) with his pure 

 explantations (not sandwiches) of the newt brain territory plus some adjacent 

 epiblast, at 9 successive stages. 



The youngest one was a gastrula with a circular blastopore, the latest a neurula with 

 recently closed neural tube. There was no attempt toward a strict topographical limitation 

 of the neurectoblastic area ; on the contrary, a variable portion of the hind-brain was 

 always involved. The mesoblastic substratum was carefully avoided. Thus, the scope of 

 the work was nearly the same as Gallera's transplantations (p. 371): disclosing the con- 

 sequences of suspending the inductive action. A progression in the quality and complexity 

 of the structures formed was of course observed and accurately demonstrated. Whe shall 

 only refer to the results of the first stages investigated. Of 26 pieces isolated from gastrula's 

 with medium or small yolk plug, 19 formed only atypical indifferent epithelium, 5 some 

 mesenchyme, melanocytes and ganglia, 2 the same components plus brain parts, in i case 

 rudimentary, in the other barely recognizable as prosencephalic structures. At the slit 

 blastopore stage, half of the 20 explants were either undifferentiated or abortive, while 

 10 showed well-closed vesicles containing distinct but rather rudimentary prosencephalic 

 structures; about 4 of these also formed hind-brain parts. In view of the fact that at these 

 two stages contact had already been made with the anterior part of the middle layer, 

 it is intriguing that such a large proportion of the explants remained undifferentiated. 

 This negative result does not quite fit with Nieuwkoop's thesis of a primary and rather 

 intense activation. The positive aspect is that induction is not an "all or none" reaction 

 but passes through a neural crest level before full neuralization. 



Literature p. 483 



