434 BIOLOGY: GENERAL AND MEDICAL 



found that it grew fast and appeared like a successful 

 graft, but changed after a few months, degenerated, and 

 was cast off. 



Thus it appears that with the exception of those cases 

 in which the organism as a whole experiences the 

 " physiological necessity" for the engrafted tissue, the 

 general tendency is for the graft to slowly change and 

 disappear. 



4. The amputated part is transplanted to another animal 

 or plant of a different kind. 



Here we are confronted by the theoretical and practi- 

 cal difficulties arising from the physiologico-chemical 

 divergences existing among different species, genera, 

 families, orders, phyla, etc. In most cases these can 

 be prejudged by the anatomical differences, but there 

 are exceptions. 



From the facts at our disposal we are now able to 

 state that the closer the blood-relationship of the organ- 

 ism furnishing the graft and the organism receiving it, 

 the more probable the success of the experiment. 



The experiments thus far reviewed have shown that 

 very slight differences, even such as arise among indi- 

 viduals of the same species, may interrupt the successful 

 progress of tissue implantations and suggest that the 

 greater differences between individuals of different 

 species may entirely prevent them. Let us see how 

 these theoretical suggestions are borne out by the facts 

 obtained by experiment. 



We have already seen that hydras are susceptible 

 of experimental manipulations of many kinds and can be 

 successfully grafted together in many different ways. 

 When the conjoined hydras are of different species, 

 however, the results are 'different; thus Wetzel conjoined 

 Hydra fusca and Hydra grisea and observed complete 

 union in five hours. But later a constriction appeared 

 where the fragments had been united, the head-piece 

 produced a foot near the line of union, and the lower 

 end produced a circle of tentacles. After eight days 



