1 24 RE GENERA TION 



may also affect definite organs and bring about a decrease in size, 

 as when the thymus and mammae decrease from iodine poisoning, and 

 certain extensor muscles after lead poisoning. Atrophy may also be 

 brought about by pressure on a part, as when the feet or waist are 

 compressed. In old age there may be a decrease in some of the 

 organs, as in the bones, the testes and ovary, and even in the heart. 



Degenerative changes appear even in the young stages of some 

 animals, as when the tail of the tadpole is absorbed and the arms of 

 the pluteus of the sea-urchin are absorbed by the rest of the embryo. 



Especially interesting are the cases of absorption that take place 

 when organs are transplanted to unusual situations in the body. 

 Zahn transplanted a foetal femur to the kidney, where it continued to 

 grow but was later absorbed. Fischer transplanted the leg of a bird's 

 embryo to the comb of a cock, where it continued at first to grow, but 

 after some months degenerated. The spleen, the kidney, and the 

 testis have been transplanted, but they degenerate, and, in general, 

 the larger the transplanted piece the more probable its degeneration. 

 Small pieces of the skin have been transplanted from one individual 

 to another, and it has been found that small pieces maintain them- 

 selves better than large pieces. Ribbert's recent experiments in 

 transplanting small pieces of different organs have been more success- 

 ful than earlier experiments in which larger pieces were used. The first 

 difficulty seems to be in establishing a blood supply to the new part, 

 in order to nourish it. If the piece is quite small, it can absorb the 

 substances, necessary to keep it alive, from the surrounding tissues, 

 until the new blood supply has developed. 



In the lower animals grafting experiments have been more success- 

 ful, because the parts can remain alive for a longer time. It is 

 important to find, however, that even in these cases, a part grafted 

 upon an abnormal region of the body is usually absorbed. Rand 

 shows that if the tentacles of hydra become displaced, as sometimes 

 happens when a piece containing the old tentacles regenerates (Fig. 

 48, A-A 3 ), the misplaced tentacles are absorbed ; and I can confirm this 

 result. In hydra, the hollow tentacles are in direct communication 

 with the central digestive tract, and a displaced tentacle seems to be 

 in as good a position as a normal one, as far as its nourishment is 

 concerned, yet it becomes absorbed. 



Rand also found, in other experiments, that when the anterior end 

 of a hydra is grafted upon the wall of another hydra, the piece may 

 maintain itself if it is large ; but it is slowly shifted toward the base 

 of the hydra to which it is grafted, and then the two separate in this 

 region. If the graft is small, it may be entirely absorbed into the 

 wall of the animal to which it is attached. 



Marshall found that if the head of a hydra is partially split in two, 



