ORGANIZERS AND TISSUE DIFFERENTIATION 261 



the same organizer may induce only quantitative changes in number, velocity 

 and intensity of developmental processes. Furthermore, quantitative relations 

 between the tissue acting as organizer and the tissue representing the sub- 

 stratum may play a certain role. Hence, if the substratum tissue is very 

 extensive, it may offer an effective resistance to the activity of the organizer 

 and appear inert; on the other hand, if the organizer tissue is very large, 

 it may induce in the substratum changes which are greater from a quantitative 

 point of view, although they may be of the same character as those effected 

 by smaller pieces of organizer tissue. There seems to be active here, a quan- 

 titative relationship not unlike that characteristic of certain chemical inter- 

 actions which determine the ultimate kind of equilibrium to be attained. We 

 noted similar effects of the relative size of host and graft in transplantations 

 in phylogenetically primitive organisms. 



This struggle between the inductive activity of the organizer and the re- 

 sistance of the substratum is also exemplified in the interaction between the 

 organizer belonging to one species or order of animals and the substratum 

 belonging to another species or order. In this case, the direction in which 

 the differentiation of the affected tissue shall take place may be determined 

 by the organismal differentials or their precursors in the substratum, rather 

 than by the precursors of the organismal ^differentials in the organizer tissue. 

 The organizer may transmit merely the impulse to further differentiation of 

 the tissue in the direction of certain organ formations ; but the character of 

 these organs is modified by the characteristics of the species or order to which 

 the substratum tissue belongs. The organismal differentials or their pre- 

 cursors do not exhibit a modifiability under the influence of organizers com- 

 parable to that which the specific substances of the various organs and tissues, 

 the organ and tissue differentials, display. 



Thus in the analysis of the organizer action use was made, especially by 

 Spemann, Zeinitz and Schotte, of transplantations into different species and 

 orders, either the organizer tissue being transplanted into a distant host, or 

 the tissue serving as substratum being grafted into a different species in such 

 a way that it came in contact with the organizer of the host. In both these 

 instances the inductions expected took place. These experiments furnished at 

 the same time further data as to the transplantability of tissues representing 

 early embryonal stages. It was in this way possible to graft successfully tissues 

 belonging not only to different species, but even to different orders, and the 

 latter type was called xenotransplantation. But in xenotransplantations there 

 was sometimes noticeable on the part of the grafts a tendency not to enter 

 into perfect union with the adjoining host tissue; however, the time during 

 which the strange tissues were kept under observation in these experiments 

 was short, because the main aim was the analysis of the organizers rather 

 than of the organismal differentials or their precursors. Nevertheless, as far 

 as such investigations make conclusions possible, they seem to confirm the 

 view expressed in the preceding chapters, that in early embryonal tissues the 

 organismal differentials, or rather the mechanisms through which their 

 existence becomes manifest, are not yet fully developed and the range of 



