EMBRYONIC INDUCTORS AND ORGANIZERS 503 



tive effects are not strictly regionally specific but show gradations depend- 

 ing on gradient-levels in the reacting ectoderm as well as in the inductor 

 region. The head-inductor may induce trunk; the trunk-inductor, head; 

 and between a normal head and a trunk without head there is a graded 

 series of defective heads, as in the planarian, though not of the same 

 types. Moreover, the great number and variety of inductors of neural 

 tissue in amphibians does not support the view that the inductor is a 

 specific chemical substance with a specific effect; in fact, this possibihty 

 seems to be excluded. If neural induction is primarily a localized, non- 

 specific activation of ectoderm, effectiveness of many different inductors 

 is to be expected and seems to have been demonstrated. 



In so far as an inductor is a nonspecific activator, it is not directly an 

 organizer or organization center. The natural inductor is itself a product 

 of pattern and organization, and as inductor it alters pattern relations in 

 other parts; but the pattern thus altered is the real organizer or reorgan- 

 izer. The inductor comes nearer to being a real organizer when it deter- 

 mines an entirely new pattern, as in hydroids and planarians, where the 

 region activated by section at the proximal end of an isolated piece or a 

 small graft determines a new polarity at right angles to, or opposed to, 

 the original polarity. In these cases the inductor apparently acts solely 

 as activated region without pattern and determines a gradient pattern on 

 a smaller or larger scale. Even here, however, the activated region or 

 graft is only indirectly organizer through the gradient pattern. When an 

 implanted piece of the dorsal lip induces in a vertebrate an axis in a dif- 

 ferent direction from, perhaps opposed to, the host axis, this is not en- 

 tirely new but is apparently an imposition on the ectoderm of the axiate 

 pattern of the inductor, and this pattern is the organizer. 



Even if the earliest inductions in development are nonspecific activa- 

 tions or begin as such, it does not follow that all inductions have this char- 

 acter. With progress of differentiation induction may become more spe- 

 cific, both as regards inducing factor and reacting system. Certain hor- 

 mones are evidently inducing factors. They are specific substances pro- 

 duced by specifically differentiated organs and producing certain effects 

 on other specifically differentiated organs, but even in some of these cases 

 the question whether the effect is primarily an activation of the reacting 

 organ or direct determination of specific differentiation is open. In any 

 case, these specific inductors of later stages are scarcely to be regarded 

 as organizers. Their effects result from presence of a specific organization, 

 not from its absence. 



