278 SENESCENCE AND REJUVENESCENCE 



in this field is quite beyond the present purpose, and I do not 

 regard myself as qualified to undertake it; but certain points which 

 bear more or less directly upon the problem of senescence demand 

 some consideration. 



In extensive and carefully controlled feeding experiments with 

 white rats, Osborne and MendeP have been able to show that cer- 

 tain proteins — gliadin from wheat and rye, hordein from barley — 

 are adequate for maintenance of weight and good nutritive condi- 

 tion, but not, or only to a slight degree, for growth under the usual 

 conditions. But after male and female animals had been fed during 

 some five months with gliadin as the only protein, they were mated, 

 and the female gave birth to four normal young which showed 

 normal growth as long as nourished on the milk of the mother, and 

 only later when placed on the gliadin diet showed retarded growth. 

 During gestation there must have been somewhere a synthesis 

 of the specific body-proteins in sufficient quantity to permit the 

 normal embryonic development and growth of the young. The 

 ability of the body to synthesize from a certain diet the substances 

 necessary for growth evidently differs under different physiological 

 conditions. McCollum has concluded on the basis of his experi- 

 ments that " the processes of replacing nitrogen degraded in cellular 

 metabolism are not of the same character as the processes of 

 growth," and suggests further that cellular katabolism and repair 

 do not involve the destruction and reconstruction of an entire pro- 

 tein molecule. Growth, of course, so far as it involves increase 

 in amount of proteid substances, must involve the construction 

 of new molecules. 



In an earlier chapter it was suggested that growth is funda- 

 mentally the accumulation of substances which cannot readily 

 leave the cell without change of constitution and which under the 

 usual conditions are not readily or rapidly changed so as to become 

 eliminable. If this conception is correct, the further possibility 

 suggests itself that tissue breakdown and repair, under ordinary 

 conditions, in the higher animals, may consist largely or wholly, on 



'Osborne and Mendel, 'iia, 'iih, '12a, '12b, '12c, '13, '14; Mendel, '14. See 

 also Hopkins, '12; McCollum, '11; Ruth Wheeler, '13. Osborne and Mendel give 

 numerous references to the literature of the subject. 



