260 THE BIOLOGICAL BASIS OF INDIVIDUALITY 



ganizers, but with chains of such transformations. An organizer induces a 

 certain tissue and organ formation ; there is associated with this transforma- 

 tion the production of a new organizer, which, on its part, induces a specific 

 differentiation in the surrounding tissue and this again may lead to the forma- 

 tion of an additional organizer exerting a specific function. 



There are, then, two sets of factors which fix the structure, chemical con- 

 stitution, metabolism and function of tissues and organs: (1) The inherent 

 characteristics of tissues and their range of modifiability, which may lead 

 to development by "self-differentiation," and (2) the character of the or- 

 ganizers which are parts of the inner environment of the tissues. There is 

 reason for assuming that, in general, without the action of the organizers the 

 development of the early embryonal tissues would be very imperfect and 

 rudimentary. This is indicated by the behavior of isolated embryonal tissues 

 made to grow in vitro; even here, differentiation of tissues seems to depend 

 largely upon the interaction between adjoining tissues and their organizers, 

 and if this interaction is lacking, further differentiation does not take place. 

 However, the artificial growth stimuli as such, acting in vitro, tend to prevent 

 further differentiation. The less evident the action of the organizers is in the 

 differentiation of tissues, the more the tissues appear to develop as the result 

 of inherent conditions by way of self -differentiation. Development by self- 

 differentiation usually leads to a more restricted formation of organs than 

 that which takes place under the influence of organizers. In general, with 

 advancing differentiation and increasing fixity of tissues in the course of 

 embryonal life, self-differentiation comes to play a greater role and the 

 tissue will depend less upon specific environmental organizer effects. 



Furthermore, with the increasing development and differentiation of the 

 organizer tissue, the organizer may change or the organizer effect may be lost, 

 although even tissues in an advanced stage of differentiation, such as retina 

 or brain, may still exert some of the organizer action which the more primi- 

 tive precursors of these tissues exerted. Likewise, with increasing differen- 

 tiation, different parts of the organizer tissue may begin to undergo modifica- 

 tions in respect to the organizer functions they exert. 



Both organizer and conditions inherent in the recipient tissue or substratum 

 are then of importance in embryonal development; therefore a tissue may 

 develop by self-differentiation in the absence of an organizer and it may be 

 modified in its development by the presence of an organizer. Of course, there 

 may be always hidden in the apparent process of self-differentiation some 

 previously exerted effects of organizers. Now, this interaction between these 

 two sets of factors may assume the character of a competitive struggle between 

 the inertia of the substratum, with its varying potentialities, and the inductive 

 activity of the organizers. Thus the same organizer may be able to accomplish 

 a certain transformation with one tissue with which it comes in contact, 

 but not with another. Or the difference may be of a quantitative character 

 rather than an absolute one; the organizer may be able to induce a certain 

 change more readily in one organ than in another; or in one tissue the or- 

 ganizer may readily induce newformation of a certain kind, while in another 



