1358 PHYSIOLOGY 



that many tissues do become thus differentiated at a fairly early period 

 in development, having undergone in the process an absolute modifica- 

 tion of their potentialities, which must be at any rate shared by 

 the chromosomes of their constituent cells. The extent to which this 

 limitation of powers of development occurs varies widely in different 

 animals. In the higher animals, such as man, epithelium will reproduce 

 epithelium, and liver-cells will reproduce liver-cells, while nerve-cells 

 are absolutely incapable of multiplication. On the other hand, in 

 Crustacea a whole limb may be torn off and be regenerated from the 

 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 multi- 

 plicative 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. expo- 

 sure 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 pheno- 

 menon 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 



