Amphibians 



235 



ventral mesoderm proceeds around the entire 

 rim of the blastopore. Before and during in- 

 vagination, the mesoderm material undergoes 

 a conspicuous convergence towards the me- 

 dian plane, and at the same time an axial 

 stretching to form the substratum for the 

 medullary plate. At the end of gastrulation, 

 the medullary plate arises as the result of a 

 condensation of the dorsal ectoderm into a 

 columnar epithelium which then converges 

 mediad and folds inward to form the neural 

 tube. Concurrently, the ventral ectoderm 

 continues to expand and flatten (Gillette, '44). 

 The mechanisms underlying the morpho- 

 genetic movements cannot be discussed here 



Horstadius and Sellman ('45) and Niu ('47) 

 worked out localizations in the neural crest. 

 The mesoderm mantle was mapped by 

 Yamada ('37) and the entoderm by Balinsky 

 ('47). Concerning the tail, see p. 247. 



It is important to realize that these maps 

 indicate merely topographic relations. They 

 represent the "prospective significance" (ac- 

 tual fate) of a given area, in normal devel- 

 opment, but do not indicate its state of deter- 

 mination nor its inherent potentialities. 

 These can be evaluated only by transplanta- 

 tion, explantation, and other devices. On the 

 basis of such experiments, Holtfreter ('36; 

 see Fig. 80) has constructed maps represent 



Fig. n . Maps of the prospective regions of the early gastrula of the axolotl (after Pasteels, '42). a. Lateral 



view; h, dorsal view. 



(see Holtfreter, '43b, '44a; Lewis, '47, '49). 



Once the gastrulation movements were 

 known in detail, it became possible to trace 

 the origin of the different organs back to 

 earlier stages. As a result of his systematic 

 staining experiments on urodeles and anu- 

 rans, Vogt was in a position to construct the 

 so-called fate maps which represent the 

 projection of the main organ primordia onto 

 the surface of the late blastula or early gas- 

 trula. These maps have been an invaluable 

 aid to all subsequent experimental work on 

 pre-neurula stages. Nakamura ('38) and 

 Pasteels ('42) have made significant addi- 

 tions and corrections, and the map of Pasteels 

 for Amblystoma mexicanum (Fig. 77) may 

 now be considered as representative of 

 urodeles in general. 



No complete maps exist for neurula stages. 

 However, information is available for special 

 areas: Goerttler ('25) and Schechtman ('32) 

 mapped the ectoderm in general, Roehlich 

 ('31) and Carpenter ('37) the ectodermal 

 derivatives of the head, Manchot ('29) and 

 Woerdeman ('29) the prospective eye areas; 



ing regulative and differentiation potencies 

 and compared them with the fate maps. 



SIGNIFICANCE OF GASTRULATION 



MOVEMENTS AND INDUCTION AS 



ORGANIZING FACTORS 



TOTAL EXOGASTRULATION 



The importance of the morphogenetic 

 movements as an organizing principle is well 

 demonstrated in embryos whose gastrulation 

 movements have become inhibited or disori- 

 ented (Holtfreter, '33a,d). When a blastula 

 of the axolotl or of other urodeles, after the 

 removal of the supporting membranes, is 

 placed into a slightly hypertonic physio- 

 logical salt solution, the entomesoderm, in- 

 stead of moving inward beneath the ecto- 

 derm, moves in an opposite direction, away 

 from the ectoderm (Fig. 78^). Even in its 

 "exogastrulated" position the marginal zone 

 proceeds to execute the characteristic gas- 

 trulation movements of dorsal convergence 

 and anteroposterior elongation, becoming at 

 the same time embedded in the mass of ento- 



