THE INDUCTION OF THE PRIMITIVE NERVOUS SYSTEM 79 



This procedure results in a gastrula with two presumptive chordamesodermal 

 regions — a normal one over above the normal dorsal lip of the blastopore, 

 and a second one which has been transplanted. 



The host then develops into a neurula in which in addition to the normal 

 neural plate we find a secondary neural plate in the flank region. This 

 secondary neural plate is said to be induced in the presumptive epidermis 

 by the contact with the chordamesoderm which has been transplanted. The 

 inductive relationship shows up a little better if we allow this neurula to 

 develop into an embryo; we then find that out of the side of this embryo 

 there develops a second head, which contains brain, eye, and many other 

 structures (Fig. 36). The notochord of this secondary embryo is composed 

 in part of the regions 1 and 2, which were originally transplanted. Thus 1 

 and 2 have differentiated into notochord and in so doing they have induced 

 the presumptive epidermis to develop first into a secondary nervous system 

 and finally into a secondary embryo. The original transplant forms only a 

 small part of the tissues of the secondary embryo. 



Other regions around the dorsal lip will induce secondary embryos. The 

 region possessing this inducing power includes most of the material which 

 normally invaginates to form the roof of the archenteron. Because of its 

 property of organizing the ectoderm into nervous system, the region above 

 the dorsal lip is called the organizer of the amphibian egg. 



The concept of an organizer has two sides to it. In the first place, we must 

 consider the nature of the unorganized ectoderm, which needs some stimulus 

 in order to develop into a nervous system. Is it completely passive or does it 



Fig. 36. A salamander embryo with a secondary head induced by trans- 

 plantation of a dorsal lip. (Courtesy of Professor John A. Moore) 



