HIGHER INVERTEBRATES AND AMPHIBIA 219 



A second problem prominent in the earlier experiments concerned the 

 possibility of changing species characters of parts of organisms by means of 

 heterotransplantation. Is the host able to impress his own organismal differ- 

 ential on the transplant? We now know that species, as well as individuality 

 differentials are gene derivatives and are therefore essentially fixed, although 

 their manifestations may be modifiable within certain limits. 



If we now consider the investigations in lumbricidae which have a bearing 

 on the problems with which we are especially concerned, namely, the evolu- 

 tion of the organismal differentials and their relation to the degree of plasticity 

 of the organism in its response to environmental changes, it has been shown 

 that, as a general rule, homoiotransplantation of pieces which are viable 

 succeeds readily, host and transplant remaining permanently united. Follow- 

 ing transplantation, a union of the corresponding organs, such as integument, 

 vessels, intestines and nerve strands, takes place, and movements as well as 

 mitotic divisions in the tissues play a role in this process ; thus one harmonious 

 individual is produced in which the organ systems derived from different 

 individuals function well, and it is only by means of differences in pigmen- 

 tation that the homoiogenous constituents of such individuals can, in some 

 cases, be distinguished. It seems that especially the union of the nerves of the 

 two partners is important in homoio- as well as in autotransplantation ; if 

 the nerves do not properly unite, then regeneration may occur at the point 

 of junction of the pieces and a new head may grow out, or the partners 

 separate, even if outwardly the union between the partners has been perfect. 

 Apparently the nerves play, here, an important part in determining regenera- 

 tion, and we may recall the fact that also in Planaria Santos found indica- 

 tions that the cephalic ganglia may determine head formation. It is apparently 

 the contact with corresponding living nerve tissue which keeps the nerves in 

 a quiescent state, preventing their regenerative outgrowth and thus their 

 stimulating effect on the growth of other surrounding tissues. However, the 

 evidence as to the significance of nerve tissue in regenerative and integrative 

 growth processes, especially in cells in planarians, is still contradictory, and 

 in lumbricidae even a defect in the union of the body walls of the two pieces 

 may lead to a newformation of a head, irrespective of the presence of nerve 

 fibers. 



While, then, in this class of animals there are apparently no differences 

 between auto- and homoiotransplantation, yet by means of certain experi- 

 mental procedures it is possible to bring out such differences ; thus, if three 

 pieces are joined together, the middle piece being inserted in an inverse 

 direction, this combination remains alive permanently only in autotransplan- 

 tations, while in homoiotransplantations some difficulties appear. But, there 

 remains the possibility that the superiority of autotransplantation may be due 

 to the fact that in this instance the pieces fit together better mechanically and 

 that individuality differentials are not concerned in this result. Similar obser- 

 vations were made in Hydra by H. D. King in joining together more than 

 two homoiogenous pieces. 



In lumbricidae, the differentiation of the organism is farther advanced 



