INDIVIDUALITY OF EMBRYONAL TISSUES 235 



transplantation. Here, too, all kinds of combinations succeed, including the 

 union of smaller parts, which as such would not be capable of independent 

 life, with larger parts of larvae; in these experiments abnormalities, such as 

 organisms possessing two heads, may develop under certain conditions. Also, 

 in homoiotransplantation analogous organs tend to find and join each other 

 and usually it is impossible to recognize later the original line of demarcation. 

 No incompatibilities due to differences in individuality differentials develop, 

 and such combinations of organisms may even pass through metamorphosis. 

 If difficulties do arise, they are of a non-specific nature. 



Similar were the results in heterotransplantation if the partners belonged 

 to nearly related species. In this case also the analogous organs of the two 

 partners had the tendency to unite and thus the two partners developed into 

 one homogeneous organism, in which no scar could be recognized at the point 

 of union; but when non-analogous organs of embryonal partners happened 

 to join, a scar did form, or else the organs separated after some time. As we 

 have noted previously, in the joining-together of pieces of adult lumbricidae 

 there developed at first a scar, which only secondarily was replaced by the 

 specific tissues. While the rates of growth in the two partners could be 

 independent of each other, the rates of differentiation were about the same, 

 substances circulating in both partners determining presumably the latter 

 effect. As we shall see later, Uhlenhuth, in transplanting eyes in salamander 

 larvae, found a similar correspondence in the rate of differentiation and in 

 the time of metamorphosis of host and graft. There resulted, thus, not only 

 a harmonious union of the two embryos belonging to different species, but in 

 certain cases even the blood vessels of one partner could grow into the other 

 partner apparently without causing any incompatibility. 



We find, then, that heterodifferentials do not need to prevent the direct 

 union of the specific tissues in analogous organs without interference by 

 connective tissue; this was true also of parts of the nervous system, even in 

 cases in which the diameters of the components differed in the partners. On 

 the other hand, if non-analogous organs happened to meet, as stated, the 

 union took place by means of connective tissue, except in the case of ecto- 

 dermal and entodermal epithelia. These observations suggest that under these 

 conditions tissue differentials functioning as contact substances regulate the 

 interaction of tissues from analogous organs at the point of junction, although 

 the species differentials of the corresponding tissues differ in the two partners. 



While after union of embryos from different species of Rana, the results 

 were similar to those obtained in homoiotransplantation, such combinations 

 were not able to maintain themselves for longer than two to three weeks when 

 species as distant as Rana esculenta and Bombinator igneus were united. 

 Although for some time in the beginning the partners could develop normally 

 and the double organisms begin to feed themselves, after awhile they became 

 sickly and progress ceased. Therefore, in the case of transplantation of more 

 distant species, heterotoxins apparently led to various abnormal conditions 

 in the animals. In Born's experiments circulatory disturbances became mani- 

 fest after about fourteen to sixteen days; there was either edema or no 



