384 



GERMINAL ORGANIZATION 



INDUCTION PHENOMENA 



the Discoglossus as in the frog egg, the acquisition of the prechordal prerogatives 

 is on the way before this presumptive territory migrates around the blastoporal 

 Up. The appearance of two acrencephaUc regions becomes easy to explain, 

 without postulating a division of the pre-existing prechordal area. If the section 

 cuts through the chordomesoblast higher than the prechordal material, the upper 

 part would become able to form by its own activity a new prechordal region 

 when transferred to the ventral side. The lower the section, the more easy will 

 be this regulation and the more typical the new prechordal territory produced, 

 and vice versa. This would explain why the secondary (ventral) acrencephalon is 

 as symmetrical as the dorsal one, for its inductors do not partake in the system 

 of predisposed areas. 



A similar interpretation seems to be valid for the induction of a forebrain in 

 the half-einbryos obtained by sagittal ligature of newt blastula or young gastrula 

 (cf. p. 346). It is a striking fact that in these half-embryos the forebrain appears 

 symmetrical and forms two optic vesicles. This can be readily understood if the 

 prechordal material has been formed from the chordomesoblast and does not 

 proceed, as generally assumed, from a pre-existing prechordal "plate". 



Another aspect of my enquiry concerning this essential prechordal material 

 consisted in repeating the classical grafts of the blastopore lip. The main difference 

 with Spemann's original work was that vital staining was used in an especially 



Fig. 52. A graft of a dorso-marginal area, with the control of vital staining, (a) The young 

 gastrula with the graft (dots) inserted in its ventro-marginal zone; the graft has been cut 

 out of the stained area (dots) of the (b) embryo; the empty place has been filled with the 

 corresponding material of a third gastrula (c). The distribution of the stain in the fixed 

 (b) embryo will check the presumptive value of the grafted area. (a>,a=,a3) Successive stages 

 of the host embryo. In (ai ) , the orientation is the same as in (a) , the mark has been invaginated 

 and a neural plate is forming. In (a^) , where the orientation is reversed, the secondary system 

 visibly forms a head. In (a^), a considerable stretching has occurred, from the point a, the 

 stained cells have been drawn to the right of the primary axis, in p. The degree of cephaliza- 

 tion is seen in Fig. 53 (From Dalcq and Lallier, 1948). 



