EMBRYONIC INDUCTORS AND ORGANIZERS 475 



haps steepen the primary ectodermal gradient? Assuming that the in- 

 ductor does act in this way, it is only indirectly concerned in organization 

 in that it alters the gradient pattern in the ectoderm, that pattern being 

 the real organizer. The chorda-mesoderm itself develops longitudinal and 

 transverse organization, but in its earher stages there is no definite evi- 

 dence of anything more than a gradient pattern in it, and with differen- 

 tial inhibition the higher levels of this pattern, that is, anterior and median 

 regions, are most inhibited.^^ Implanted into other than the normal posi- 

 tion, it may sometimes induce a new gradient, as in the case in which the 

 induced embryo is more or less opposed in orientation to the host. Here 

 the inductor gradient determines the gradient pattern of the induced em- 

 bryo, and so its organization, and comes nearer being a true organizer than 

 in normal development; but even here, according to this view, the gradi- 

 ent pattern induced in the ectoderm is the real organizing factor. 



The foreign inductors, if living, may or may not possess gradient differ- 

 ences; whether living or dead, their action is usually more or less local 

 and does not induce an orderly whole but merely certain tissues or organs 

 in more or less unordered complex. If these inductors are primarily activa- 

 tors, the character of the induced part depends chiefly on two factors- 

 level of host gradient in which it arises and degree or intensity of activa- 

 tion. Either of these may be predominant in determining the result. These 

 inductors, however, give us no information as to the manner in which or- 

 ganization occurs. Weiss (1935) admits this as regards foreign inductors 

 but regards the natural inductor not only as activator but as organizer. 

 Actually, however, there is often much organization in the complexes in- 

 duced by foreign inductors, but it is not normal. If these inductors are not 

 organizers, the organization must result from pattern of some kind already 

 present in the part subjected to inductive action, and the only sort of pat- 

 tern that seems adequate to account for the varied results is a gradient 

 pattern, wholly or predominantly nonspecific regionally. But, as already 

 noted, foreign inductors can probably determine new gradient patterns 

 in the host tissue; if so, they are indirectly organizers. 



Needham, Waddington, and Needham (1934) hold that two factors, 

 "evocation" and "individuation," are concerned in induction. Evocation 

 consists in bringing about development of an embryonic axis in the ecto- 

 derm; individuation, in determining the regional character of that axis. 

 The foreign inductors— living, dead, or extracts— are, in general, only 

 evocators. This conception of induction is open to certain criticisms. 



^3 See pp. 257-65; also Bellamy, 1919; Lehmann, 19370; Cohen, i( 



