THE MOSAIC STAGE OF DIFFERENTIATION 239 



§9 



Turning now to the amphibian nervous system, a number of ex- 

 periments show that the neural fold region is first determined as a 

 field. 



In Triton, a dorsal half of an embryo, even at comparatively late 

 stages such as that of the early gastrula, will develop into a normally 

 proportioned little embryo. ^ In such cases the neural folds of the 

 miniature embryos are proportional to their reduced size, although 

 the material which the half contains is that from which a full-sized 

 neural plate would normally be formed.'^ If this experiment is 

 repeated at the late gastrula stage, the neural folds produced from 

 the half are full-sized, and therefore relatively too large. Irre- 

 versible determination of the neural fold region has therefore 

 taken place towards the close of the period of gastrulation (fig. 117). 



Experiments of another kind, likewise on Trito?i, lead to further 

 results. Gastrulae can be produced which are deficient in median 

 material lying in the plane of bilateral symmetry. This may be 

 eflFected either by cutting out a median disc of tissue and sticking 

 the two lateral portions together again, or by making paramedian 

 cuts in two embryos, on the right of the mid-line in one and on 

 the left in the other, and then exchanging halves and sticking 

 them together, so that one composite embryo so formed will be 

 deficient and the other overprovided as regards median material. 

 In the case of the deficient embryos, the region involved includes 

 part of the presumptive neural fold tissue. Normally, of course, 

 this region is broad anteriorly in the region of the brain, and 

 narrow posteriorly in the region of the spinal cord. The operation 

 to which the deficient embryos have been subjected results there- 



the lens is extirpated, a lens will be regenerated from the actual dorsal margin, 

 which was the presumptive ventral part of the eye-cup. But the original dorso- 

 ventral gradient of lens-forming potency persists, though masked by the in- 

 hibitory effect of the choroid fissure. For if sectors representing one-sixth of the 

 circumference of the iris of such eye-cups developed from rotated optic vesicles 

 be tested for lens-forming potencies, it is found {a) that the potencies of the 

 actual dorsal (presumptive ventral) sector are low, and (6) that there is little 

 difference between the potencies of this and other sectors. In other words, the 

 original dorso-ventral gradient of lens-forming potency and the ventro-dorsal 

 gradient of lens inhibition have here cancelled out. (Sato, 1933 ; see also Beck- 

 with, 1927.) 



1 Spemann, 1903. - Ruud and Spemann, 1923. 



