7 • THE NEURULA AND 

 SELF-DIFFERENTIATION 



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efore attempting to answer the many questions concerning the organ- 

 izer, let us recall the main arguments leading up to the concept of the organ- 

 izer. The early gastrula presumptive epidermis and presumptive neural plate 

 are able to form a large variety of structures, and they thus show little or no 

 determination. After gastrulation, however, the presumptive neural plate will 

 form only nervous system; it will not form other structures. We can say, 

 therefore, that the nervous system has become determined during gastrulation. 

 We have traced the stimulus for this determination to the chordamesoderm 

 adjacent to the lip of the blastopore. This region invaginates to form the 

 roof of the archenteron. As a result of this invagination, the roof of the 

 archenteron comes into direct contact with the presumptive neural plate, and 

 this contact stimulates the formation of the structurally denned neural plate. 

 The process of stimulation is called embryonic induction. The region of the 

 structure which stimulates is called the organizer. And since the organizer 

 acts by contact, the stimulus is presumably chemical in nature. That is, there 

 must be some interchange of substances between the organizer and the pre- 

 sumptive neural plate. Or perhaps the roof of the archenteron liberates some 

 substance which diffuses into the presumptive neural plate, thereby stimu- 

 lating it to develop into a nervous system. 



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