402 



GERMINAL ORGANIZATION 



INDUCTION PHENOMENA 



neur. pl.H 



muscle c 

 (implj 



otyp. epid. 



neur. pi. X 



Fig. 65. Transverse section (schematic) through the 7 days newt explant. The two neural 



plates (neur. pi. I, \l) are still open. The notochord (not.) is accompanied by a few muscle 



cells {muscle c.) from the implant (impl.) and small necrotic elements. Atyp. ep.- atypical 



epidermis. From same paper as Fig. 64. 



An unexpected effect has been disclosed as a result of inserting the cephalic 

 part of the notochord in a sandwich made of early gastrula ectoblast. The complex 

 remains apparently quiescent for 2-3 days; then, an induced neural plate becomes 

 apparent on both sides. The surface soon smoothes out, and the central part 

 swells (Fig. 64). Full differentiation takes a long time: 2 weeks in the axolotl, 

 4 weeks for the newt material. In the latter, one or two plates are still present 

 after 7 days (Fig. 65). When they are closed, thin brain ampullae are formed in 

 which the authors (Nieuwkoop and Nigtevecht, 1954) recognize a rhomben- 

 cephalon and sometimes prosencephalic parts, including retinal and tapetal 

 regions. Near the surface of these ampullae, small otocysts, nasal placodes and 

 balancers may be formed. The proposed interpretation is that the implant first 

 exerts a general activating action which spreads as far as the periphery of the 

 sandwich. Secondarily, this field is modified by a "transformative action" which 

 causes the material surrounding the chordal implant to become brain, while 

 the periphery only attains the level of mesectoblast and, farther outside, may 

 either remain as indifferent epiblast or produce nasal placodes (Fig. 66). 



