INDIVIDUALITY OF EMBRYONAL TISSUES 243 



which helps to determine the compatibility of the partners in experiments in 

 which different species are joined together. 



There is reason for assuming that regulative substances of a similar 

 character to those present in the morphogenic interaction between parts of 

 a natural individual, regulate also the interaction between coalesced and 

 secondarily unified organisms. This tendency to form more or less normal 

 individuals out of abnormal combinations may lead to the production of a 

 single organism from two partners, or to the later separation of the two 

 joined-together parts, each of which then gives origin to a single individual, 

 or to the domination of one partner over the other, which latter undergoes 

 various degrees of degeneration. These observations apply both to trans- 

 plantations in phylogenetically primitive classes of animals and to fusions 

 of early ontogenetic stages. However, notwithstanding these similarities, one 

 has the impression that the regulative and integrative mechanisms, which 

 are so pronounced in the case of the phylogenetically most primitive adult 

 organisms, are perhaps not effective to quite the same degree in embryonal 

 forms of phylogenetically more advanced organisms, although here, also, 

 various fargoing regulations may take place. The typical organizer actions 

 which may be observed in transplantation of very primitive adult organisms, 

 and which are so important during embryonal development, are only very 

 rarely evident under the conditions prevailing in these combinations between 

 ontogenetically primitive organisms. 



In experiments with eggs and young embryos we have not to deal with the 

 same organismal differentials which are active in adult organisms. There 

 are indications that neither the specific organismal differentials which charac- 

 terize the adult individual, nor the mechanisms which react against strange 

 differentials are as yet fully developed ; still, protoplasmic specificities which 

 distinguish different species evidently exist even in such ontogenetically early 

 forms. Some of these specificities presumably represent stages in the develop- 

 ment of the organismal differentials and their means of manifestation, and 

 all intermediate gradations between the precursors present in the fertilized 

 egg and the fully formed substances and mechanisms in the adult form may 

 be found. Furthermore, as in phylogeny, so also in ontogeny there is notice- 

 able an inverse parallelism between the degree of plasticity of organs and of 

 the integrative potentialities active in an organism, on the one hand, and the 

 the degree of development of the organismal differentials, on the other hand. 



