72 THE BIOLOGICAL PROBLEM OF TO-DA Y 



upon A, as stock, a shoot of C is grafted, while 

 upon this shoot of C, as stock, a shoot of B in 

 turn is grafted. 



In the matter of these different grades of dis- 

 harmony, a comparison may be made between 

 sexual and vegetative affinities. In many cases 

 the spermatozoa of one species will not impregnate 

 the eggs of another species. In other cases, the 

 alien spermatozoon may penetrate the egg and 

 unite with its nucleus, making, however, an un- 

 satisfactory combination in various degrees of 

 infertility. Sometimes the fertilised egg divides 

 only a few times and then dies ; sometimes develop- 

 ment proceeds to the stage of the blastula, the gas- 

 trula, or even further ; but it then comes to an 

 end, through intrinsic causes beyond our ken, and, 

 finally, complete destruction follows. 



Our acquaintance with what happens in trans- 

 plantation of animal tissues is smaller than in the 

 sphere of botany. 



Long ago, Trembley attempted to cause, by graft- 

 ing, the union of two pieces of hydroid polyps into 

 a single individual. He divided, across their 

 middles, two specimens of Hydra fusca, and then, 

 in a watch-glass, applied the upper end of one to 

 the lower end of the other. In one case he was 

 rewarded by the occurrence of complete union ; for, 

 after a few days, on feeding the upper end with a 

 worm, it was passed on into the lower end. Later 

 on buds arose, both above and below the point of 

 union. Trembley, however, was unable to graft on 

 each other parts of different species, parts of the 



