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TEE AMEBIC AX MUSEUM JOURNAL 



But there is in addition a disease 

 whic-li represents altogether an abnor- 

 mality of tissue growth. This is can- 

 cer. Cancer is an abnormal, excessive, 

 and continuous tissue growth or pro- 

 liferation. In comparing the conditions 

 which determine the growth of normal 

 tissues with those of cancerous tissues, 

 which latter we can graft in a way 

 very similar to normal tissues, we ob- 

 tain a deeper insight into the character 

 of cancerous growth. 



In the following we shall limit our 

 discussion to a few of the factors which 

 come into play if we graft a normal tis- 

 sue from one animal to another or from 

 one part of the body to another part of 

 the same animal. The greater number 

 of the experiments in grafting tissues 

 are carried out in smaller animals, like 

 guinea pigs, rats, mice, and rabbits. 



In transplantation of animal tissue 

 we are concerned in a general way witli 

 three conditions: (1) the character of 

 the host, that is, the animal or tissue in 

 which the graft is made; (2) the char- 

 acter of the graft, that is, the tissue to 

 be transplanted; and (3) the relation- 

 ship between the host and the graft. 

 We know that a wounded tissue will 

 heal much more rapidly and perfectly 

 in a young than in an old person. 

 There is something present in the 

 young that is deficient or wanting in 

 the old. Likewise a graft grows better 

 in a young host than in an old one. 

 This characteristic is particularly no- 

 ticeable in experimentation on trans- 

 plantable tumors in mice; it also ap- 

 plies, at least in certain cases, to the 

 transplantation of tissues that are not 

 abnormal. Experiments of this sort 

 indicate that the young animals differ 

 from the older ones in the character 

 of the substances circulating in the 

 body fluids. What then may these 

 substances be? Are they the ordinary 

 food constituents — the proteins, fats, 

 carbohydrates, and mineral substances, 

 or the two more elusive yet specific 

 substances known as "vitamines"' — 



both of which are necessary for main- 

 tenance as well as growth? Or are 

 they special growth substances such 

 as are known to exist in the corpus 

 luteum of the ovary, in the thyroid, and 

 probably in the pituitary gland ? 



Although these questions cannot be 

 definitely answered at the present time, 

 the results of many and varied experi- 

 ments indicate that the advantage 

 which the young animal possesses is not 

 due to the quantity or quality of the 

 food stuffs but more likely to some sort 

 of specific growth substance. It has 

 been shown ^ that substances circulating 

 in the tadpole cause those remarkable 

 metamorphic growth processes which 

 occur in the life of the developing sala- 

 mander. Tissue grafted on a tadpole 

 from another tadpole undergoes meta- 

 morphosis at the same time as the host. 

 Experiment has determined that the 

 corpus hiteum of the ovary gives off at 

 certain times a substance which, circu- 

 lating in the body of the host, will 

 cause a placental structure to be pro- 

 duced. On the other hand, it has been 

 found difficult to graft tissues in ani- 

 mals bearing young, but here there are 

 indications that abnormal substances 

 are circulating in the body fluid which 

 exert an injurious influence on the 

 transplant, or perhaps neutralize, in 

 some way, the specific growth sul> 

 stances which are normally present. 



The fate of the graft is dependent 

 also upon the character of the grafted 

 tissue itself. A good evidence of this 

 fact is that growing embryonic tissue 

 which is not yet fully differentiated is 

 more susceptible to successful trans- 

 plantation than adult tissue which is 

 fully differentiated. 



In each organ there are formed dif- 

 ferent constituents which vary greatly 

 from one another in the ease with 

 which they lend themselves to trans- 

 plantation. On the whole, the simpler 

 structures, the ones developing first on- 



^ Uhlenhuth. Proc. Soc. Exper. Biol, and Medi- 

 cine, Vol. XIV, 88, 1917. 



