414 



GASTRULATION 



SECONDARY NEURALIZATION 



EPIDERMAL TRANSPLANT 

 INTO NEURAL PLATE ^^sff^. 





NEURAL Plat 

 INTO EPIDER 



LATE TRANSPLANT 

 M A L AREA 



Fig. 196. Ectodermal potencies of the amphibian gastrula. (A and B from Spemann. 

 '38, after Fischer; C and D from Spemann. '38, after Spemann, '18.) (A) Induction of 

 a secondary neural plate in the axoloti gastrula by five per cent oleic acid, emulsified in 

 agar-agar. (B) Induction of secondary neural plate by nucleic acid from calf thymus. 

 (C) Formation of neural plate tissue from presumptive epidermal cells transplanted into 

 neural plate region. (D) Reverse transplant, presumptive neural plate becomes epi- 

 dermal tissue. 



(d) Still more lateral and extending ventraliy are the lateral plate and ventral 

 mesoderm (figs. 191F-I; 198A-C; 221). 



The third or inner germ layer of entoderm is encased within the mesodermal 

 or middle germ layer. The entodermal layer is an oval-shaped structure con- 

 taining a small archenteric cavity filled with fluid. Its ventral portion is heavily 

 laden with yolk substance. Also, the future trunk portion of the archenteric 

 roof is incomplete, the narrow notochordal band forming a part of its mid- 

 dorsal area (figs. 19 IF; 194B; 219D). Within each of these germ layers are 

 to be found restricted areas destined to be particular organs. Each layer may 

 be regarded, therefore, as a general mosaic of organ-forming tendencies. 



4) Position Occupied by the Pre-chordal Plate Material. Another feature 

 of the late gastrula remains to be emphasized, namely, the pre-chordal plate 

 composed of entodermal and mesodermal cells integrated with the anterior 

 end of the notochord. During gastrulation the pre-chordal plate invaginates 

 with the entoderm and comes to occupy the roof of the foregut, just anterior 

 to the rod-like notochord (fig. 191D-F). In this position it lies below the 

 anterior part of the neural plate area; it functions strongly in the induction 

 and formation of the cephalic structures, including the brain as indicated above. 

 Because of this inductive ability, it is regarded as a principal part of the head 



