138 



organisers: inducers of differentiation 



positions where they are not underlain by mesoderm. If, on the 

 other hand, they are situated in regions where mesoderm does 

 underHe them, they become typical neural folds (see fig. 64). 



Fig. 64 



Effects of a lateral temperature differential on development in Urodeles. A-F, 

 Triturus torosus, treatment by temperature-gradient in blastula stage, with 5° C. 

 temperature difference between the two sides of each egg. A, B, Warmed on left ; 

 A, dorsal view ; B, anterior view ; the warmed neural fold is much larger and more 

 differentiated. C-E, Dorsal views. F, Lateral view, showing secondary neural 

 structures on the previously warmed side, either connected with the main neural 

 folds, or F, isolated from them. In C, D, E, the secondary formations appear to 

 be underlain by mesoderm, and have differentiated into structures of neural fold 

 type. In F they are not underlain, and do not show typical morphogenesis. 

 (From Gilchrist, Quart. Rev. Biol, iv, 1929.) 



Another method of testing this labile determination is by inter- 

 plantation, i.e. the grafting of portions of blastulae (i.e. portions of 

 tissue taken from an embryo before the invagination of the 

 organiser) into the eye-sockets or coelomic cavities of other larvae.^ 

 The differentiation of various structures can be obtained in this 

 way (fig. 148, p. 316). The fact that interests us here is that neural 

 tube may be differentiated in these circumstances from tissue which 



^ Diirken, 1926. 



