386 



GERMINAL ORGANIZATION 



INDUCTION PHENOMENA 



Hd.1 



Hd.1 



end 



bl.2 



N.R.- 



.6^, 



-Ni. 



h 



-N,R. 



-N. R. 



Fig. 54. A graft of or- 

 ganizer with reversal 

 of the cephalo-caudal 

 axis. The young gas- 

 trula donor (b) has 

 received a combined 

 mark of Nile blue 

 (dots) and neutral red 

 (crosses) in the blasto- 

 poral region ; V = veg- 

 etative pole. The cen- 

 tral area of this patch 

 is grafted above the 

 ventro-marginal zone, 

 with reversal relative to 

 the animal-vegetative 

 axis (arrow), (c to f) 

 inspects of the grafted region at successive stages ; (g-h) dorsal views of the obtained embryo ; 

 (i-j) two sections through its cephalic part. In c to f, the upper part is stained with Nile 

 blue (Ni. B.), the inferior part with neutral red (N.R.); the arrowed line separates these 

 territories. The host is seen from its ventro-marginal aspect, i.e. the same orientation as in (a) . 

 The whole grafts sinks progressively, and is gradually surrounded by a sort of blasto- 

 poral lip(^/.2) formed by the host ectoblast (c, d). The endo-prechordal part of the implant 

 (stained in red) migrates straight toward the animal pole, while the blue part is covered 

 by the lateral lips (e, f). In g, the embryo at the 6th day (at ± 10 C) is orientated in the 

 same way; the hatching corresponds to red parts visible under the surface, the circles are the 

 blue material, also under the surface. 



In h, (8th day) the orientation has been reversed, the Nile blue was no longer perceptible 

 in vivo. The sections (i-j) show that the secondary head was formed of an isolated and rudi- 

 mentary fore-brain ampulla (/. br.) accompanied by a small endoblastic tube {e?id.) ; more 

 caudally, a hind-brain (h. br.) with a small otocyst appears, followed by a spinal cord. In 

 these structures and in the surrounding mesenchyme the stain is preserved, but the distinc- 

 tion between Nile blue and neutral red is not possible. Thus a complete, but hypomorphic 

 system has been formed. The depressive effect is related to a retardation of kinematics and, 

 correlatively, to a decrease in induction. 



Aud. -otocyst; bl.^-secondary blastopore (atypical); end.-endoblast; f.br. -fore-brain 

 (secondary) ; h. br.-hind-brain (secondary) ; h. br. '-primary hind-brain; Hd'-primary head; 

 Hd2-secondary head. From Benoit, 1952. 



