SENESCENCE IN HIGHER ANIMALS AND MAN 279 



the one hand, of the separation and breakdown of certain constit- 

 uent chemical groups, which are less firmly attached to the mole- 

 cule or less stable than other parts which remain as a more stable 

 nucleus, and, on the other, of the replacement of the lost parts of 

 the molecule from nutritive substances. In actual protoplasmic 

 growth, however, the whole molecule, including the more as well 

 as the less stable portions, must be built up out of the Bausteirie, or 

 in some other way. Consequently some proteins whose constituent 

 substances can supply the losses due to tissue breakdown may not 

 contain in sufhcient quantity or not at all certain components 

 necessary for the building up of new molecules, but under excep- 

 tional conditions, as in the gestation period in Osborne and Mendel's 

 rats, the organism may be able to synthesize these molecules in 

 other ways. The general relation between the rate of growth and 

 the rate of metabohsm suggests that the synthesis of the more 

 stable molecules or molecular groups occurs more readily with a 

 high than with a low rate of metabolic reaction, and this suggestion 

 is also in accord with the fact that growth, morphogenesis, and 

 differentiation occur chiefly in the earlier stages of the life history. 



The rats fed on gliadin with maintenance of weight but little or 

 no growth retain their capacity for growth for at least several 

 months and, when placed on a mixed diet, or one containing ade- 

 quate proteins, resume growth at the normal rate. But the experi- 

 ments do not as yet show whether they will retain indefinitely the 

 capacity for growth. Besides remaining young as regards growth 

 capacity, these animals also retain the general appearance of 

 growing animals of the same size. Apparently, progressive develop- 

 ment and with it senescence have been inhibited or greatly retarded. 

 Nevertheless, after long periods of such feeding the nervous system 

 shows the water-content characteristic of old animals and the pos- 

 sibility cannot be ignored that, even in the absence of growth, 

 progressive changes in the direction of greater stability of the 

 protoplasmic substratum may have occurred. 



The results of experiments on mammals with a diet which is 

 adequate qualitatively, but sufficient in quantity only for main- 

 tenance and not for growth, are quite different from those of 

 Osborne and Mendel. Waters ('08, '09) found that underfed 



