ORGANIZERS AND TISSUE DIFFERENTIATION 273 



organizers which they contain develop step by step ; they become distinct for 

 each organ, until in the end the complete set of organs and organ differentials 

 has fully developed. At the same time, also, the precursor substances of the 

 organismal differentials develop and differentiate into finer differentials, until 

 in the end the structures characteristic of the individuality have fully formed 

 in the substratum. It may be assumed that the coarser organ differentials, 

 organizers and organismal differentials develop first and that only at later 

 periods of ontogenesis the finer chemical structures differentiate in the case 

 of both the organ and organismal differentials. While these two sets of dif- 

 ferentials have thus certain important characteristics in common, they differ 

 in their chemical constitution as well as in their distribution. Whereas the 

 organ differentials and their precursors differ in every organ and tissue, the 

 organismal differentials are the same in all parts of an organism. We may 

 perhaps tentatively assume that on a common chemical basis, which is the 

 bearer of the organismal differentials, there are superimposed in various 

 places chemical structures which correspond to the various organ differentials. 

 While the general design of the latter is similar in nearly related organisms, 

 differences develop corresponding to the distance in relationship between the 

 organismal differentials. The finest, the least noticeable differences are found 

 between the organs and tissues of nearly related individuals. Yet, the wider 

 pattern of the embryonal development' of the organs and organ differential 

 substances, which takes place by means of self-differentiation and with the 

 aid of organizers, is similar throughout the whole animal series ; this applies 

 especially to the coarser, more basic organ and tissue structures, while with 

 progressing ontogenetic development a greater differentiation sets in in the 

 development of organs. These developmental similarities are maintained, not- 

 withstanding the differences which exist as to the precursor substances char- 

 acterizing the germ cells of the various classes, species and individuals. The 

 organs and their differentials undergo graded changes during embryonal life 

 and they are readily accessible to modification within a certain range, under 

 the influence of alterations in the inner or outer environment. The organismal 

 differentials, on the other hand, although they also differentiate in the course 

 of embryonal development, are, as far as is known, much more stable and 

 much less readily accessible to environmental influences ; however, during this 

 period the character of the organismal differentials limits also the variability 

 of the organs which may occur. Differences in organismal differentials which 

 the organizer tissue and the recipient tissue may possess do not preclude the 

 effective action of organizers, but the tissue and organ differentials can de- 

 velop only within the range prescribed by the nature of the organismal dif- 

 ferentials of the recipient tissue. 



We have seen that in the adult mammalian organism a tissue equilibrium 

 is established, which is strictly autogenous ; the integrity of tissue boundaries, 

 the normal interaction of tissues, depend upon the presence of the same 

 autogenous differential in all the adjoining tissues. On this autogenous char- 

 acter depends the maintenance of the normal tissue equilibrium and the normal 

 function of tissues. There is a good deal of evidence that in the adult mam- 



