EMBRYONIC INDUCTORS AND ORGANIZERS 485 



both or either one of the two axes doubtless depends on incidental factors, 

 levels of activity of epiblast and entoderm, and perhaps their alterations 

 by operation ; it suggests varying relations of dominance of one or the other 



axis. 



Neural-plate induction in the chick has also been obtained by implanta- 

 tion of the anterior part of a two-somite rabbit embryo (Waddington, 

 1934), affording still another example of the nonspecific character of these 

 inductions. 



However, it appears from many experiments that neural tissue can de- 

 velop independently of an inductor in the chick. In various isolation ex- 

 periments forebrain has been found to develop independently of noto- 

 chord, and grafts of notochord have proved ineffective as inductors.^s 

 Moreover, explants of purely ectodermal pieces from anterior regions of 

 very early primitive-streak stages may give rise to neural tubes (Rudnick, 

 19386). 



MAMMALS 



The few data available show that neural induction can occur in a mam- 

 malian embryo but give no information as to the part it plays in normal 

 development. Chick primitive streak implanted in the rabbit embryo cul- 

 tivated in vitro may induce neural-plate formation, but thus far implanta- 

 tion of rabbit primitive streak in rabbit embryos has given only uncertain 

 results as regards induction, although the implants may differentiate into 

 neural tissue and notochord (Waddington, 1934, 19366, 1937). 



NEURAL induction: CONCLUSIONS AND QUESTIONS 



From the data it appears that many different substances can induce, 

 directly or indirectly, development of neural tissue in amphibians and that 

 under certain conditions neural tissue may develop without an underlying 

 inductor. It may perhaps be questioned whether there is a specific chem- 

 istry of induction in the sense that a particular substance or group of sub- 

 stances is the inducing factor. It also appears that the natural inductor 

 does not originate axiate embryonic pattern but is a part of it, and that the 

 pattern, either of the inductor or of the reacting ectoderm or both, rather 

 than the induction itself, is the real organizer. Induction in vertebrates 

 throws no light on the essential problem of organization, the problem of 

 the origin and nature of the pattern within which the inductor acts. 



The question whether neural induction is primarily an activation or a 

 specific action, or in normal development an orderly axiate series of spe- 



35 T. E. Hunt, 1931; Waddington, 1932, 1933; Wetzel, 1936. 



