438 SENESCENCE AND REJUVENESCENCE 



of the cell is the death of the individual cell. The changes in the 

 cells during their development, the appearance of metaplasmic 

 structural substances, which is usually regarded as differentiation, 

 Miihlmann interprets as a dedifferentiation and regression from 

 the embryonic condition and as a secondary result of the gradual 

 starvation of the cells. 



As regards the biological importance of the relation between 

 surface and volume, I am not aware that it has been proved in 

 any case to be a fundamental factor in limiting growth. Growth 

 is not simply a matter of nutrition: in the higher animals a very 

 definite limit of size exists, no matter hov/ great the supply of 

 nutrition, and in many lower animals extensive reconstitutional 

 growth may occur, even in a stage of extreme reduction from star- 

 vation. On the other hand, the growth of embryonic cells may 

 be inhibited by correlative influences from other parts, even 

 though an abundant supply of nutrition is present. In many cases 

 animal eggs receive their nutrition chiefly or wholly through a 

 minute fraction of their surface (see Figs. 184, 185, p. 345) yet 

 are able to attain an enormous size as compared with other cells 

 of the body. Similarly, in many cells of the multicellular body, 

 the nutritive surface is evidently only a small fraction of the total 

 surface of the cell, e.g., in many glandular tissues, yet life and 

 function continue. And in the unicellular infusoria food enters 

 through a definite mouth and passes into the entoplasm, where a 

 nutritive surface is formed about each food particle. In such cases 

 the externa] surface of the cell has no relation to its capacity for 

 ingesting food. Oxygen doubtless enters through the ceU surface, 

 but it undoubtedly enters more or less rapidly according to con- 

 ditions in the cell. In fact, the whole theory of the biological 

 importance of the relation between surface and volume rests rather 

 upon a process of logic than upon the data of observation and 

 experiment, and when we examine the behavior of cells and organ- 

 isms it is difficult to find adequate support for it. 



As regards Miihlmann's hypothesis, the conclusion that old age 

 is an advanced stage of cell starvation rests chiefly upon assertion 

 rather than proof. As a matter of fact, in starvation the nervous 

 system loses less than other tissues, while in old age, according to 



