Amphibians 



247 



which themselves tended to become elongate 

 structm-es. Adopting the terms of Spemann 

 ('31) one may thvis distinguish between head 

 and trunk-tail organizers. (Their regionally 

 specific effects in a host are illustrated in 

 Figs. 92a, b.) The latter extends far into the 

 lateral marginal zone, but fades out toward 

 the ventral sector. The ventral entoderm and 

 the ectoderm of the gastrula showed no in- 

 ductive capacity. This deserves special men- 

 tioning because in later stages certain deriva- 

 tives of these two germ layers do possess 

 inductive faculty. 



It is interesting to note that small explants 

 from the upper blastoporal lip induced out 

 of their own material ectodermal structures 

 which showed again regional specificity. In 

 these instances head structures, such as single 

 eyes, olfactory pits, otocysts or balancers, 

 could appear independently of each other, 

 and from different, though somewhat over- 

 lapping, districts of the marginal zone (Fig. 

 84^). This seems to indicate that even at the 

 gastrula stage the head organizer is not ac- 

 tually an equipotential entity but is subdi- 

 vided into specialized inductors although 

 distinct boundaries between them do not 

 seem to exist. 



THE ORGANIZATION OF THE MESODERM 



MANTLE AND THE TAIL BUD IN THE 



NEURULA 



MESODERM MANTLE 



The following chapters are devoted to the 

 organization of the neurula and, in particu- 

 lar, to the mesoderm mantle and the medul- 

 lary plate. These primordia represent sig- 

 nificant intermediate stages in a gradually 

 increasing stabilization of differentiations. 

 Of particular interest are the interactions be- 

 tween these two structures and the complex 

 relationships between the different compo- 

 nents of the mesoderm. For orientation see 

 Figure 85. 



Notochord. During neurulation, the dorsal 

 median strip of the mesoderm mantle be- 

 comes separated from the lateral parts. Its 

 differentiation tendency for notochord is es- 

 tablished from early neurula stages on 

 (Bautzmann, '28; Yamada, '37; Chuang, '47; 

 and others). 



Somite Area. Prospective somite material of 

 the early neurula of Triturus alpestris and 

 T. pyrrhogaster, when reared within epider- 

 mal jackets or when transplanted to the mid- 

 ventral body region, does not form skeletal 

 mviscle but, unexpectedly, well differentiated 

 pronephric tubules in a high percentage of 

 cases (Yamada, '37, '39b; see Fig. 86). Ap- 



parently, the somite material requires for its 

 normal differentiation the intervention of 

 extrinsic factors. The notochord contains 

 these factors, since if it is explanted in com- 

 bination with somite material, the latter 

 forms typical musculature (Yamada, '39b; 

 Muchmore, '51; see Fig. 86). The notochord, 

 however, holds no monopoly in this respect. 

 For instance, either the transplantation of 

 prospective somites to older hosts (Yamada, 

 '39a), or the explantation of large pieces, con- 

 taining prospective somite and adjacent 



Fig. 85. Urodele neurula; the mesoderm mantle 

 is exposed on the right side of the embryo. (Orig- 

 inal.) 



pronephros areas (Muchmore, '51), has the 

 same effect. Moreover, typical somites are dif- 

 ferentiated after the complete extirpation of 

 the notochord in the early neurula (Kitchin, 

 '49; Horstadius, '44; Nieuwkoop, '46; Much- 

 more, '51). In normal development, muscle 

 differentiation is probably brought about by 

 the synergistic action of various neighboring 

 tissues. 



In addition to pronephric tubules and 

 mvisculature, notochord and even neural tis- 

 sue can be differentiated from prospective 

 somites (Muchmore, '51; Lopashov, '35b). 



Pronephros Area. Explants of this region 

 when taken from Triturus neurulae can 

 differentiate into typical pronephric tubules 

 (Yamada, '40), but when isolated from 

 Amblystoma punctatum they give rise to 

 few and poorly developed tubules (Much- 

 more, '51). This area is capable also of form- 

 ing musculature when isolated in combina- 

 tion with notochord or when grafted to the 

 somite level (Yamada, '37, '40) but it forms 

 ventral mesoderm and blood islands when 

 grafted to ventral regions (Yamada, '37; see 

 Fig. 86). 



Lateral plate and blood islands differentiate 

 in explants largely according to their pros- 



