1214 PHYSIOLOGY 



tissues of the stump. Destruction of the optic lens in the salamander is 

 followed by its regeneration from the anterior part of the optic cup, a tissue 

 which had no part in its primary formation. Worms will form a new head 

 after decapitation. In these animals therefore the cells in many parts of the 

 body possess the power of directed growth, if need arise, in case of injury, 

 and are able to form tissues of many different kinds. 



I have mentioned the small size of the larva formed from isolated cells 

 of the segmenting egg as a proof that the number of cell. divisions of the 

 somatic part of the developing animal is predetermined and limited. This 

 conclusion must not be taken too absolutely. Many of the tissues, even of 

 the highest animals, possess the power of almost unlimited regeneration by 

 cell- multiplication as a response to injury. Under normal conditions the 

 growth of such tissues is limited, not by absence of power to divide, but as 

 a result of a mutual interaction between them and the surrounding cells. 

 We might almost speak of a struggle for existence between the various tissues 

 of the body, which in the healthy organism results in an equilibrium, or 

 balance of multiplicative powers. If this balance is upset by any means, 

 such as stimulation of certain cells by the presence of intra-cellular parasites, 

 or their destruction by irritants or other abnormal conditions (e.g. exposure 

 to X-rays), one tissue may enter into active growth at the expense of the 

 surrounding tissues, and the result is a morbid growth such as cancer. It is 

 possible that in the latter case a new factor comes into play. All tissues 

 of the body, as we. have seen, begin to die from the time that they are 

 born. They have a certain span of life, a certain limitation to their genera- 

 tions, which results in the phenomenon of senescence, such as occurs in a 

 culture of protozoa. In protozoa this phenomenon is the signal for rejuvena- 

 tion by conjugation. It is possible that in cancer something of the same 

 nature occurs. It is at any rate significant that in a rapidly growing 

 cancer many of the dividing cells present the phenomenon of heterotype 

 mitosis, a phenomenon which is otherwise found only in the sexual cells 

 preparing for conjugation and for the production of a new individual. Given 

 adequate conditions of nutrition, there seems to be no limit to the growth of 

 cancer-cells. In mice a cancer may be transferred from one individual to 

 another by inoculation, and this process may apparently go on indefinitely, 

 so that finally a mass of cancer-cells may have been produced equal in volume 

 to many thousands of mice, and persisting long after the mouse from which 

 it was first taken would have died under natural conditions. 



In sexual reproduction the new individual partakes of characteristics 

 of both its parents. It therefore resembles neither of its parents in all 

 details. The conjugation of the two parent cells from which it is derived has 

 been preceded by a throwing out of half the chromosomes from each parent 

 cell. It is therefore natural to ascribe the variations which occur among the 

 members of one family to a qualitative difference in the chromosomes which 

 have been eliminated in the formation of their respective egg- cells. Can 

 we regard the chromosomes as representing separate qualities of the individual 

 or must we assume that all qualities are represented to a greater or less 



