1266 PHYSIOLOGY 



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 intracellular 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 generations, 

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

 of protozoa. In protozoa this phenomenon is the signal for rejuvenation 

 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 ef 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 

 extent in every chromosome? In the case of many qualities, especially 

 those which distinguish the species as apart from the individual variation 

 or family characteristic, we must probably accept the latter idea as correct. 



