372 



GERMINAL ORGANIZATION 



INDUCTION PHENOMENA 



when the donor was a young gastrula (Fig. 44, a). With the fragment presumed to be 

 prechordal (or mostly so), 10 of 35 grafts were assimilated, while most others produced 

 complex structures attributable to the head, the trunk or even the tail. In two cases, 

 however, the induction was exclusively of a prosencephalic nature. One of these complexes 

 was well isolated on the belly, and contained a small brain, two large nasal placodes, and 

 two large eyes. A third specimen showed only a rather complete head with a predominant 

 eye, plus one otocyst (Fig. 44, b, c, d). When the superior (larger) fragment was used as a 

 transplant, cases of assimilation were few, and most secondary systems produced a whole 

 embryo amputated at the level of its otocystes. In 3 out of 1 1 successful implantations, how- 

 ever, an anterior head was formed, sometimes more normal than the one produced by the 

 inferior graft of the same donor (Fig. 44, e, f). 



These data have a double meaning. No doubt a "differentiated" prechordal 

 territory exists not only at the end of gastrulation, but also during and even at 

 the beginning of this process. The fact that most grafts do not work so specifically 

 is due (j) to some encroaching on the next territory and (2) to a regulative 

 completing by the host. On the other hand, it is plain that sometimes the non- 

 prechordal presumptive territory becomes also capable, when grafted, of per- 

 forming the indtictive fimctions of the prechordal material by regtilation. This is 

 an important modification which is definitely provoked by this type of operation. 

 W^e shall meet it again, and try to discuss it in detail. 



For the moment, let us state that the paradoxical results of Mangold are at 

 least partially solved. It has been clearly established that the concept of a pre- 

 chordal territory, responsible for the induction of the acrencephalon, is valid. 



Fig. 42. Insertion of the prechordal endomesoblast of a newt neurula into the blastocoele 

 (close to the ventral wall of the cavity) of a gastrula. (a) Schema of the operation for 

 obtaining the grafts whole (A) or halved (D) or a middle (M) and two lateral (L) pieces. 

 (h) Secondary acromerite with eyes and balancers induced on the ventral part of the head 

 by an M implant, (c) Section of this specimen, showing a rather limited brain (below) and 

 two large optic cupules with lenses. Note the predominent utilization of the neuroepithelium 

 for the visual organs. An olfactory pit was also present. An example of definite although 

 somewhat limited acrogenesis. From Gallera, 1949. 



