240 



THE MOSAIC STAGE OF DIFFERENTIATION 



fore in partial or complete removal of the presumptive neural 

 fold tissue from the trunk region, and in removal of only a 

 portion of it from the region of the head. Such embryos develop 

 complete brains with properly proportioned eyes, but the spinal 



Fig. 117 



Loss of power of regulation in the development of dorsal halves of newt embryos. 

 A, The constriction and isolation of a dorsal half at the early gastrula stage leads 

 to' the development of (B) a diminutive embryo with neural folds of proportion- 

 ately reduced size. C, Normal neurula for comparison with B. D,The constric- 

 tion and isolation of a dorsal half at the late gastrula stage leads to the development 

 of (E) a diminutive embryo with disproportionately large neural folds, unable to 

 close. (From Ruud and Spemann, Arch. Entwmech. lii, 1923.) 



cord may be absent. The important point to notice is that the 

 neural tube was already determined at the time of the experiment, 

 for the complete removal of its rudiment in the trunk region will 

 resuh in absence of spinal cord altogether. But the neural tube 

 was only determined as a whole and not in its detailed constituents, 



