SELF-DIFFERENTIATION OF THE PARTS OF THE NEURULA 



95 



The question arises as regards the time at which presumptive structures 

 acquire the property of self-differentiation. Do all structures suddenly become 

 determined at the neurula stage? Numerous transplantation experiments 

 have shown that different structures acquire the property of self-differentia- 

 tion at different times during development. Furthermore, this property is 

 not gained suddenly. A presumptive structure first self-differentiates by giv- 

 ing rise to an incomplete or abnormal structure. Then with time self- 

 differentiation results in more and more normal structure. 



We should, of course, like to know something of the nature of this self- 

 differentiation. Are all the parts of a structure such as the limb, for example, 

 determined within the area which is transplanted? Are all the muscles, bones, 

 and parts of the limb preformed in some chemical way? Any preformation 

 would have to be chemical in nature because there are no visible differences 

 within the limb disk that is transplanted. No visible differences serve to dis- 

 tinguish between the part forming the digits from that forming the radius, 

 the ulna, the humerus, and so on. And here we must face the same problem 

 that concerned us in our discussion of the organization of the sea urchin egg. 



Fig. 46. A section through the 

 pronephros of a frog embryo, show- 

 ing an eye which developed from 

 a transplant from a neurula. Note 

 that the presumptive eye forms a 

 pigment layer, a retinal layer, a lens, 

 and a transparent cornea. 



