GASTRULATION AND THE FORMATION OF THE NERVOUS SYSTEM 



67 



UNDIFFERENTIATED 

 ECTODERM 



PRONEPHROS 

 NOTOCHORD 

 5 

 SOMITES 



Fig. 29. Results of exogastrulation. Top: Section ° 

 of the early gastrula with the regions above and z_, 

 below the dorsal lip designated by numbers and § c 

 letters. Right: The presumptive notochord, somites, 

 and endoderm fail to move into the interior of the 

 gastrula, with the result that the ectoderm fails to 

 form a nervous system. 



From all these experiments and observations we may conclude, first of all, 

 that the region of the gray crescent, which contains the factors for normal 

 development, becomes the dorsal lip of the blastopore in the early gastrula. 

 And thus the gray crescent determines where gastrulation will occur. Second, 

 gastrulation is necessary for the formation of the nervous system. That is 

 to say, the dorsal lip which is derived from the gray crescent must come in 

 contact with the ectoderm in order for the nervous system to form. 



The foregoing relationship is an example of another principle of embry- 

 onic development. When two groups of cells are so related that the differen- 

 tiation of group A is dependent upon group B, then A is said to exhibit 

 dependent differentiation. If B is able to differentiate without the presence 

 of A, then B shows independent differentiation. In these terms, the differen- 

 tiation of the ectoderm of the gastrula is the dependent type, whereas that 

 of the mesoderm and the endoderm is independent. Development, therefore, 

 is not a matter of the independent differentiation of the parts of the egg. 

 There is a great deal of interaction among the parts, and in this interaction 

 some parts play a very dominant role as compared with others. 



