1116 THE REPRODUCTIVE ORGANS 



This list of regenerative possibilities, however, ^ need not remain 

 confined to the developing animal, but may also be extended to adult 

 forms, because while the growth of the latter is greatly diminished, 

 their power of reforming injured tissues is by no means lost. It is 

 true, however, that their property of regeneration is rather dormant 

 at this time, in consequence of certain inhibitory influences, but may 

 be awakened temporarily by stimulation. Thus, it is a well-known 

 fact that the adult starfish is capable of reforming a lost arm, and that 

 a worm cut into is able to develop from the posterior extremity of its 

 anterior segment a new tail, and from the anterior end of its posterior 

 segment a new head.^ In fact, even the severed arm of the starfish 

 may eventually give rise to a complete animal, while artificial mouths 

 surrounded by tentacles may be produced in sea-anemones by simply 

 incising their body- wall and keeping the wound open. Of even greater 

 interest are those experiments which show that parts of different 

 animals may be united to form a single new one.^ In this way, com- 

 pound worms have been formed which lived for many months, and 

 Harrison has even succeeded in uniting the anterior half of Rana 

 virescens with the posterior haK of Rana palustris (parabiosis). 

 Although both parts retained their special characteristics, this com- 

 pound frog gave rise to young. Most remarkable changes may also be 

 effected in plants. Thus, it is a well-known fact that a whole plant 

 may be produced from the cuttings of its branches and roots, and even 

 from its leaves. In this category also belong the transplantations 

 practised to enrich the flower and fruit bearing qualities of certain 

 plants and trees. With regard to the growth of malignant tumors, 

 it might be mentioned that connective tissues may readily be grown 

 outside the body and that this growth may be greatly accelerated by 

 extracts of tissues, particularly of embryos, spleen and malignant 

 tumors.* Tissues may also be kept at a low temperature without ap- 

 parently losing their power of regeneration. Thus, skin niay be kept 

 for 2 to 6 "weeks in cold storage and be grafted successfully at the end 

 of this period. 



Reproduction. — These examples, no doubt, suffice to show that 

 regeneration is really a form of reproduction; but a reproduction of a 

 local or restricted kind which does not pass beyond the reformation 

 of the individual tissues. Thus, while a newt may reproduce an 

 amputated toe, the newt itself is left in. its original condition. Its 

 cells are gradually used up until its existence as a living entity ceases 

 altogether. But this natural limitation of life is prevented from 



1 Joest, Transplantationsvers. an Regenwurmern, Ber. Gesellsch. der Naturw., 

 Marburg, 1895; also Morgan, the Physiology of Regeneration, Jour. Exp. Zoology, 

 iii, 1906, or ''Experimental Zoology," New York, 1907. 



2 The earliest experiments in grafting were performed upon hydra by Trembley 

 (M6m. pour servir a I'histoire d'un genre de polypes d'eau douce, Leide, 1774). 

 Later on Hunter and Durhamel grafted the spur of a cock to the comb where it 

 continued to grow. 



3 Carrel, Jour. Exp. Med., xiv, 1911, 571, and xvi, 1912, 165. 



