Amphibians 



249 



tions of a single agent. The overlap of the 

 pronephros over the somite field would ac- 

 count for the formation of tubules in isolates 

 of the somite area, and a mediad extension 

 of the pronephros field would explain the 

 tubule formation from ventrolateral meso- 

 derm. The somite field seems to require 

 extrinsic agents for its materialization to a 

 higher degree than the other mesodermal 

 pi'imordia. 



POSTERIOR TRUNK AND TAIL 

 MESODERM 



Vogt ('26b, '29) has shown that in urodeles, 

 at the stage of the closed blastopore, only the 



tion and elongation of the tail bud begins 

 (for details see particidarly Chuang, '47). 



According to the studies of Pasteels ('39b, 

 '43), the different structures of the posterior 

 trunk and tail originate from localized tis- 

 sue primordia which undergo morphogenetic 

 movements considered to be a continuation 

 of the gastrulation movements. They are not 

 the result of differential growth processes, 

 since there was no indication of a high or 

 differential mitotic activity. Therefore, am- 

 phibian development does not conform to the 

 concept of Holmdahl (since '25; see '39), 

 who ascribes the organogenesis of the caudal 

 end to an outgrowth from an indifferent 

 growth center, and not to the segregation of 



Fig. 87. Maps and morphogenetic movements of posterior trunk and tail in urodeles. a, Morphogenetic 

 movements during neurulation; b, vital staining of three areas in the posterior medullary plate; c, the fate 

 of the three marks, d. Topography of prospective tail structures in the neural groove stage; e, same in 

 neurula with rising folds. The numbers in d and e indicate somite numbers. (After Chuang, '47.) 



first six to eight somites, the adjacent lateral 

 mesoderm, and the greater part of the noto- 

 chord are invaginated. The posterior trunk 

 and tail somites are still on the surface oc- 

 cupying the posterior fifth of the medullary 

 plate. Detailed maps of these regions were 

 constructed on the basis of vital staining and 

 transplantation experiments (Vogt, '26b, '29; 

 Bijtel, '31; Nakamura, '38; Pasteels, '39b, '42; 

 Spofford, '45; Chuang, '47; see Fig. 87). Dur- 

 ing neurulation, the tail material moves in- 

 ward by way of complicated invagination 

 and folding processes, whereupon the forma- 



germ layers characteristic of the more ante- 

 rior regions. A distinction between "pri- 

 mary" and "secondary" body formation is 

 not warranted in amphibians. 



Holmdahl's notion finds still less support in 

 the results from defect, transplantation and 

 isolation experiments. They have shown that, 

 from early neurula stages on, the different 

 primordia of the tail region are even more 

 rigidly determined and less capable of regu- 

 lation than the more anterior parts of the 

 mesoderm mantle (Bytinski-Salz, '31, '36; 

 Mangold, '32, '33b; Bijtel, '36; Von Aufsess, 



