ORGANIZERS AND TISSUE DIFFERENTIATION 263 



However, the injured or killed material does not behave exactly like living 

 tissues from the same or related species or orders of animals ; the former 

 seems to be less specific, as shown by the fact that it induces a smaller number 

 of transformations in the tissues on which it acts, and the fine differences 

 between different parts of a certain organ used as organizer are lost under 

 these conditions. Thus, while in the normal medullary plate of Triton different 

 portions are differentiated — the anterior portion inducing formation of brain, 

 eyes, nose, ear vesicles and balancer, the posterior portion inducing formation 

 of spinal cord and tail — in the medullary plate produced by killed material 

 these specific differences between anterior and posterior portions are no 

 longer present, the different parts acting alike. It is especially the development 

 of neural tube from ectoderm of the gastrula which can be induced by dead 

 organizer material. The production of certain mesodermal structures, such 

 as kidney, musculature, bone and extremities, can only barely be initiated 

 by killed organizer tissue, and at best these organs and tissues are formed only 

 in small quantities. But coagulated embryo extract of the chick may call forth 

 not only the development of nerve tissue in gastrula ectoderm, but even 

 of chorda and musculature. It seems, after all, that there is no absolute, but 

 only a graded difference in the ability of dead tissue to function as organizer 

 and in the specificity of the transformations brought about by it, as compared 

 with the effects of living tissues of the same kind. Needham and Waddington 

 distinguish two types of actions of organizers: (1) The organizer reproduces 

 or tends to reproduce the axis of the embryo and, ultimately, a more or less 

 whole, early embryonal stage of the organism, which furnishes the sub- 

 stratum for its operation. This process is designated as "evocation" and the 

 organizer involved is called an "evocator." It is a relatively non-specific action, 

 which may be shown also by dead material, and it represents a much more 

 simple chemical effect than that exerted by (2) the individuator which pro- 

 duces certain subdivisions of the axis. The latter type of action is exhibited 

 only by living tissue. Somewhat related views have also been expressed by 

 Weiss. 



The ability of xenoplastic tissues to act as organizers suggests that the 

 organizers are either entirely devoid of organismal differentials, or bear 

 organismal differentials with a very slight degree of differentiation. This en- 

 ables the transplanted tissues which contain the organizer to exert their func- 

 tion in the host, notwithstanding the great difference in organismal differen- 

 tials ; or their precursors, in host and transplant; likewise, the experiments 

 with dead organizer material suggest that the organizers, at least those possess- 

 ing the more restricted, the evocator functions of the killed tissues, are devoid 

 of organismal differentials. In contrast to the lack of organismal differentials 

 in the inducting and transforming substances, the living substratum on which 

 they act does possess organismal differentials. 



We may enlarge somewhat on these more general statements by citing 

 some specific experiments. If we transplant prospective medullary plate into 

 regions where the ectoderm normally develops into epidermis, the transplant 

 may, in its new location, merely form skin. Conversely, prospective epidermis 



