THE INDUCTION OF THE PRIMITIVE NERVOUS SYSTEM 



77 



Fig. 34. The presumptive eye at the neurula stage forms only an eye when 

 transplanted. The presumptive eye is cut out of the neural plate of a neurula 

 and is transplanted to position 3 in an older embryo. It develops into an eye, 

 whereas in Figure 32 the presumptive eye of the early gastrula formed noto- 

 chord, somites, pronephros, and spinal cord. 



tern only. What influences or stimulates the potency for eye and represses all 

 the other potencies we have found ? 



Inasmuch as the development of the nervous system depends upon the 

 process of gastrulation, and since in normal gastrulation it is the dorsal lip 

 of the blastopore which comes in contact with the ectoderm, we look to 

 this dorsal lip as the possible stimulus for the formation of the nervous 

 system. If we suspect the dorsal lip of bringing about this transformation 

 of the ectoderm, then placing the dorsal lip in contact with any part of the 

 ectoderm of the early gastrula ought to influence that ectoderm to form 

 nervous tissue. 



This sort of experiment is shown in Figure 35. The region which normally 

 comes in contact with the early nervous system is the region above the dorsal 

 lip which we have numbered 1 through 5 in a previous analysis of gastrula- 

 tion. This region gives rise to notochord and mesoderm. For this reason it 

 is called presumptive chordamesoderm. 



Regions 1 and 2 of the chordamesoderm are cut out and transplanted 

 into another early gastrula in the region of the presumptive epidermis. 

 These transplants are made by cutting out a square of tissue with a glass 

 needle and placing the tissue in the blastocoele of the host gastrula. The 

 transplant heals into position in contact with the presumptive epidermis. 



