SOME CURRENT THEORIES 441 



latest paper Minot has criticized this view on the ground that if it 

 were correct we must be growing alternately old and young. While 

 I am quite ready to admit that this is to a certain extent the case, 

 it does not by any means follow, as Minot has asserted, that every 

 change in metabolic rate is either senescence or rejuvenescence. 

 Undoubtedly it is often impossible to draw a sharp line of distinc- 

 tion between the age changes and many other periodic changes in 

 the organism (see pp. 187-93), Y et m general senescence and reju- 

 venescence are relatively slow and gradual changes in metabolic 

 rate associated with certain changes in the cellular substratum, 

 which do not undergo rapid reversal or regression. Minot's criti- 

 cism is quite beside the point. There is nothing in his own theory 

 that is in conflict in any way with the idea that senescence and 

 rejuvenescence, viewed in their dynamic aspects, are changes in 

 rate of metabolism, for it is concerned with certain conditions and 

 indications of senescence in the cells rather than with the process 

 of senescence itself. 



According to Minot, dedifferentiation and rejuvenescence do 

 not occur in the body cells. At various points in the present book 

 (see especially chaps, v-vii, x, xii) I have endeavored to show that 

 dedifferentiation and rejuvenescence occur very widely in body 

 cells. No further discussion, therefore, is necessary here. Minot 

 believes, however, that the egg differs from all other cells in that it 

 undergoes rejuvenescence after fertilization. The basis for this 

 conclusion is the increase during this stage in the amount of 

 nuclear substance in relation to cytoplasm. As regards the 

 occurrence of rejuvenescence in the embryo, I am in essential 

 agreement with him, but my conclusions are based on the changes 

 in metabolic rate rather than size relations of nucleus and cyto- 

 plasm. Minot, however, has made no mention of the spermatozoon. 

 According to his view it should be one of the youngest cells in 

 existence, since it possesses in most cases practically no cytoplasm. 

 As a matter of fact, however, it shows none of the characteristics 

 of a young cell. It is if anything more highly specialized than the 

 egg, and has ceased entirely to grow; moreover, when it enters the 

 egg it loses its morphological characteristics and to all appearances 

 also undergoes dedifferentiation and rejuvenescence into an ordinary 



