Ill 



PRIMARY INDUCTION 



EXPERIMENTAL DATA 



373 



Fig. 43. Comparison of inductive capacity of the anterior and posterior halves of the pre- 

 chordal endo-mesoblast taken from an advanced gastrula. Same technique as in Fig. 42; 

 (a) cutting the implants from a yolk plug stage; (b) induction of an anterior head by the 

 median part of the zone II; an eye and one balancer are present, (c) section through these 

 structures, showing from right to left the host gut, a large brain ampulla, a small thin- 

 walled otocyst, and a fine cyclopic eye with lens; an olfactory pit was also present. The 

 presence of the otocyst is probably due to some encroaching of the operation on the para- 

 chordal region. An example of definite acrogenesis, although accompanied by cyclopia. 



From Gallera, 1949. 



However, there remains the fact that the chordal parts of the archenteric roof 

 in neurulae can exert this same induction plus the production of hindbrain and 

 trunkal structures, etc. Gallera suggests that this depends on the excessive age 

 difference between the donor and the host in Mangold's experiment. Yet we 

 may remark that this appearance of prechordal abilities seems quite similar to the 

 sporadic observations made on the chordomesoblastic territory of the young 

 gastrula (Fig. 44, see also Figs. 52-3, pp. 384, 385). 



It may be also added here that in 1952 Gallera reexamined the effects of extirpating 

 the anterior part of the archenteric roof {i.e., prechordal territory, with some encroaching 

 on the chordal one) while leaving the neurectoblast intact. These operations were per- 

 formed on neurulae and gastrulae with a circular blastopore. They caused deficiencies 

 which conformed to expectation, such as tubular prosencephalon, synophthalmy and 

 cyclopy, and monorhyny, but the point must be stressed that for such an extensive defi- 

 ciency there was an appreciable degree of regulation. This could mean either that a 

 reconstitution of the prechordal inductor takes place at the expense of the adjacent chordo- 

 mesoblast, or that the neurectoblast early acquires an ability to autonomous progress. 

 The latter contention would agree with the results of some other "folding" experiments 

 (p. 387), but this neurectoblast auto-organization does not manifest itself in "ordinary" 

 transplantations nor in explantations (p. 397). It seems to me more likely that a relative 

 morphological and dynamic reconstitution of the prechordal plate occurs, with an increase 

 of morphogenetic potential. Thus, in these events disclosed by Mangold and by Gallera, 



Literature p. 483 



