Chapter $ 



Organizers and Tissue Differentiation, and Their 

 Relation to Organismal Differentials 



In our discussion of the factors which cause organ formation in primi- 

 tive organisms, we have referred to organizers localized in certain organs, 

 which are able to induce the production of these same organs in another 

 animal of the same species into which they have been transplanted. How- 

 ever, the "organizer" concept was not used originally in the analysis of organ 

 formation in phylogenetically primitive species, but rather in embryos of 

 less primitive organisms. The transplantation of pieces of organs may lead 

 to the development of organs or of embryonal tissues other than those which 

 function as organizers, and, in particular, the latter may induce the formation 

 of parts of an organism normally adjoining the organizer tissue. It has been 

 possible to trace this potentiality to the formation of organs and tissues and 

 the distribution of organ-forming substances from the ovum through the first 

 segments, through blastula and gastrula, to the more complex organisms. 

 Associated with these changes is a parallel development of organismal differen- 

 tials from their precursors, which also proceeds in the direction from less 

 specific to more specific substances and mechanisms. It is this parallelism in 

 these two processes and the possibility of a relationship between them which 

 we wish to analyze in this chapter. 



As stated, tissue and organ formation during embryonal life is brought 

 about partly by substances which function as organizers in association with 

 inherent, genetically determined characteristics of the tissues, which are the 

 substratum on which the organizers act. The organizers may be defined as 

 morphogenic contact substances, which serve as tissue transformers, or 

 rather, as inductors, causing the tissues with which they come in contact to 

 undergo certain changes, which, within a definite range of variability, are 

 fixed by the constitution of the tissues upon which they act. In the earlier 

 stages of embryonal development, when the plasticity and range of variability 

 of the tissues are still very great, these substances may determine which of 

 their potential differentiations the tissues will actually undergo. When in 

 later stages the structure of the organism has become more stabilized, the 

 organizers may exert quantitative rather than qualitative effects; they may 

 determine not what kind of organs are to be produced, but what their size 

 and position shall be and how many of them shall be formed; or they may 

 stimulate the tissues to develop in a certain direction rather than to stand still 

 or to undergo only relatively slight further differentiation. But this difference 

 between the effects exerted in earlier and later embryonal stages is not a radical 

 one ; it is rather a difference of degree. As might be expected, during embryonal 

 development we may have to deal not only with single inductions by or- 



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