Amphibians 



251 



tures was assumed. However, Pasteels ('45), 

 who repeated these experiments on urodeles 

 and anurans, obtaind a series of continuous 

 rather than discontinuovis deficiencies both in 

 the mesoderm and in its inductions, which 

 he interpreted in terms of a cephalocaudal 

 gradient of susceptibility. 



The experiments of Dalcq ('46, '47) give 

 an indication of qualitative differences be- 

 tween the inductivity of the prechordal and 

 the chordal-parachordal mesoderm. He bi- 

 sected the young gastrula of the anuran, Dis- 

 coglossus, by a horizontal cut, rotated the 

 upper half 180 degrees and healed the halves 

 together. Two axial systems developed, one 

 at the original dorsal side and one on the 

 ventral side. Variations in the distance of 

 the plane of cutting from the blastopore re- 

 sulted in a series of incomplete axial sys- 

 tems. Isolated forebrains occurred in the 

 ventral systems, and they were invariably 

 correlated with prechordal mesoderm. 

 Whereas the autonomy of an archencephalic 

 inductor region was thus established, no evi- 

 dence was found for an independent deuter- 

 encephalic inductor. Hindbrain and spinal 

 cord were always induced together. The 

 author considers the hindbrain induction 

 merely as the result of a particiilarly strong 

 inductive capacity of the cranial end of the 

 chordal-parachordal mesoderm. Nieuwkoop 

 ('47, '50) transplanted upper blastoporal lips 

 from different gastrula stages of Triturus 

 into the ventral side of another gastrula and 

 likewise obtained secondary embryos show- 

 ing successive steps of brain deficiencies. The 

 boundary between the induced archencepha- 

 lon and deuterencephalon coincided invari- 

 ably with that of prechordal and chordal- 

 parachordal mesoderm. The detailed study of 

 the deficient brains suggested to the author 

 that the other subdivisions of the brain are 

 not induced by a mosaic of qualitatively dif- 

 ferent regions of the anterior archenteron 

 roof but that they reflect threshold values of 

 inductive potency within a cephalocaudal 

 gradient of neural inductivity in the archen- 

 teron roof. 



In the extensive experiments with adult 

 inductive tissues or their fractions it has been 

 found frequently that some of them induce 

 predominantly head structures, others trunk- 

 tail structures, or both (Chuang, '39, '40; 

 Toivonen, '40; and others, see p. 269). Iso- 

 lated ectoderm, when exposed to neuralizing 

 aqueous solutions formed exclusively anterior 

 head structures (Barth, '41; Shen, '42; Holt- 

 freter, '44b; Yamada, '50a). It may be 

 doubted whether these inductors have any 



similarity with the normal ones, but the 

 findings indicate again that the factors which 

 bring about an archencephalon differ from 

 those which induce the more caudal parts 

 of the nervous system. However, in all these 

 experiments, no specific inductor for the 

 deuterencephalon was found; the latter oc- 

 curred always combined with archencephalic 

 or with spinocaudal structures. 



It was pointed out above that the poste- 

 rior fifth of the medullary plate does not 

 form neural but mesodermal structures of 

 the posterior trunk and tail (see Fig. 87 e). 

 Hence one might expect that the caudal part 

 of the archenteron roof would have meso- 

 derm-inducing capacities. Spofford ('48) has 

 shown that this is, indeed, the case. He sub- 

 stituted vitally stained pieces of early gas- 

 trula ectoderm for caudal medullary plate 

 and found that the implants formed trunk 

 and tail somites and other mesodermal tis- 

 sues. 



Altogether, there is good evidence for a 

 distinction between a large anterior section 

 of the archenteron roof which induces the 

 various parts of the neural system and the 

 mesectoderm, and a short caudal section 

 which induces mesodermal tissues. The pre- 

 chordal mesoderm operates (in cooperation 

 with the tips of notochord and somites?) as 

 an archencephalic inductor, but no sharp 

 borderline seems to exist between the in- 

 ductors for hindbrain and for spinal cord. 

 The question of whether the inductive con- 

 ditions which specify the different brain 

 sections represent a discontinuous series of 

 qualitative differences or a continuous series 

 of merely quantitative differences of a single 

 agent is undecided. This problem has given 

 rise to extensive discussions (see for instance, 

 F. E. Lehmann, '38, '45, '48; Dalcq, '46; 

 Nieuwkoop, '47, '50; Waddington and Yao, 

 '50). We shall return to it later on. Such 

 variables as intensity of inductive effect and 

 period of contact between archenteron roof 

 and overlying ectoderm have to be taken into 

 account (Dalcq, '46). 



Whereas in normal development only the 

 median part of the mesoderm mantle sub- 

 jacent to the medullary plate actually exerts 

 an inductive influence, the more lateral meso- 

 derm also contains latent inductive capacities 

 (Holtfreter, '33c, '38a; Raven, '35; Wadding- 

 ton, '36a). When pieces of gastrula ectoderm 

 were placed over the dorsolateral mesoderm 

 of a neurula, at different levels along the 

 cephalocaudal axis, they were induced to 

 form a great diversity of accessory ecto- and 

 mesodermal structures, such as fragmentary 



