6 • GASTRULATION IN RELATION 

 TO THE ORGANIZER 



L 



et us take stock of our analysis thus far. We have mapped out the fate 

 of the cells in the early gastrula. We have found that each region has a 

 definite presumptive value — that is, each region forms a definite structure 

 in the embryo during normal development. We further saw that during 

 gastrulation the region above the dorsal lip of the blastopore becomes the 

 roof of the archenteron, while the region below the dorsal lip forms the 

 floor of the archenteron. When gastrulation takes place normally, the 

 nervous system forms above the roof of the archenteron. When gastrulation 

 is prevented, no nervous system forms. And, therefore, some process during 

 gastrulation must stimulate the formation of the nervous system. We shall 

 now look into the nature of this stimulus. 



But first consider the gastrular ectoderm, which normally gives rise to 

 both the nervous system and the epidermis. Is this ectoderm predetermined 

 to form these structures, or is it labile, so that it may form other structures? 

 These possibilities can be tested by transplanting the ectoderm to new environ- 

 ments — environments different from its normal one. This procedure will tell 



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