INDIVIDUALITY OF EMBRYONAL TISSUES 237 



were presumably of a subtle nature and proceeded more slowly. Even indi- 

 viduals belonging to different genera and families could be temporarily joined 

 together. 



On the whole, these experiments bear certain similarities to mammalian 

 parabiosis; apparently heterotoxins are active in both. Inasmuch as in these 

 transplantations of parts of embryos we have to deal, not with the peculiarities 

 of some of their constituent tissues or organs, but with conditions common 

 to all the tissues, which are affected the more unfavorably the greater the 

 distance in relationship between the two partners, we are justified in attribut- 

 ing the incompatibilities which may develop between them to differences in 

 their organismal differentials. These embryonal organisms show a sensitive- 

 ness to heterogenous differentials similar to that noted in certain invertebrates, 

 as for instance, the lumbricidae; in both cases incompatibilities arise if the 

 species of the partners are far removed from each other phylogenetically. The 

 mutual tolerance of heterogenous constituents seems to be greater in the 

 embryonal than in the adult anuran amphibia, which latter, as we have seen, 

 are on the whole very sensitive to the effects of heterotransplantation. We 

 have seen, in a preceding chapter, that in adult amphibia restitution processes 

 are restricted to the appendages of urodeles. On the other hand, in amphibian 

 larvae of Rana, Harrison has shown that it is possible to obtain furthergoing 

 integrations. When pieces of tail were grafted so that their aboral poles were 

 in contact with the oral poles of the host and the oral surfaces of the grafts 

 were cut off, the influence of the larger piece induced processes of adaptation 

 in the grafts, which made them part of the host. In this respect a larva of an 

 anuran amphibian resembles, therefore, a hydrozoon or a pianarian; but in 

 other respects the integrative ability of these larvae is much less pronounced 

 than that of the more primitive adult invertebrate organisms. As a rule, in 

 amphibian larvae abnormal combinations of several pieces do not undergo 

 those various regulative processes leading to the reestablishment of normal 

 individuals, which take place so readily in primitive adult animals ; the larvae 

 of amphibia correspond in this respect rather to adult lumbricidae. 



II. Transplantation of Embryos and Eggs in Invertebrates. The experi- 

 ments of Born in amphibia were soon afterwards extended to invertebrates. 

 It is especially the eggs and embryos of echinoderms, of Ascaris and Chae- 

 topterus, which were used in these investigations, in which Driesch, Morgan, 

 zur Strassen, Jacques Loeb, de Haan, Goldfarb, and others participated. 

 Although these experiments were not undertaken primarily for the sake of 

 the study of organismal differentials, still some valuable data in this regard 

 were obtained. 



Two cells or cell complexes may be joined together in two ways : (a) 

 Through agglutination, a process which will be more fully discussed in a 

 later chapter, dealing with tissue formation and organismal differentials ; (b) 

 through coalescence of agglutinated cells, due to solution processes which 

 take place in the ectoplasmic cell-layer, especially of eggs or their very early 

 cleavage stages. If the union consists merely in an agglutination process, sev- 

 eral further possibilities exist. Either the two organisms remain distinct and 



