382 CARNEGIE INSTITUTION OF WASHINGTON. 



have lived for two years or more upon such rations which have included 

 a single added protein. Furthermore, animals have been bred and 

 reared into the fourth generation on the same type of diet, having in 

 general the following composition: 



Per cent. 



Protein, e. g. edeatin 18 



"Protein-free milk" 28 



Starch 24 



Butter fat 18 



Lard 12 



Although our pubhshed experiments have afiforded results which 

 disprove the widespread view that the capacity to grow, or the growth 

 impulse, is lost with age independently of whether it has or has not 

 functioned during the period usually associated with increase in size, 

 a number of questions relating to the persistence of the growth impulse 

 remain unsolved. Thus, although it might be admitted that brief 

 periods of inhibition of growth were without detriment to the animal, 

 it has been stated that animals in which growth has been suppressed for 

 a very long period can never reach the full size and physical equip- 

 ment characteristic of unretarded individuals, although they may show 

 some resumption of growth when suitable diet is furnished. 



Again the question has been raised as to whether the age at which 

 suppression of growth begins determines in any way the future out- 

 come of the capacity to grow. Our records now show numerous 

 prompt responses to the opportunity to complete growth at all ages, 

 even when the retardation has been brought about long before the 

 period of sexual maturity is reached. We now have records of resump- 

 tion of growth in a number of animals which were stunted for more 

 than 500 days — twice the age at which full size is ordinarily reached by 

 the normally developing individual in the same environment. We can 

 further say that the procreative functions are not necessarily impaired 

 by stunting before the age at which breeding is ordinarily possible. 



The results of our experiments up to date in the retardation of 

 growth in animals may be summarized as follows : the growth impulse, 

 or capacity to grow, can be retained and exercised at periods far beyond 

 the age at which growth ordinarily ceases. In the case of our experi- 

 mental animals, albino rats, in which increment of body-weight 

 ordinarily ceases before the age of 300 days, resumption and comple- 

 tion of growth was readily obtained at an age of more than 550 days. 

 It is now reasonable to ask whether the capacity to grow can ever be 

 lost under these conditions unless it is exercised. Even after very pro- 

 longed periods of suppression of growth, the animals can subsequently 

 reach the full size characteristic of their species. In this respect there 

 is no impairment of the individual. The satisfactory resumption of 

 growth can be attained not only after stunting by underfeeding, but 

 also after the cessation of growth which results when the diet contains 



