28o SENESCENCE AND REJUVENESCENCE 



cattle might remain for a long period at a constant body-weight 

 but at the same time undergo an increase in height and a decrease 

 in the amount of fat. Evidently the skeleton undergoes growth, 

 at least in length of bones, even under these conditions, and other 

 parts must grow to some extent and in certain dimensions in 

 accordance with the growth of the skeleton, but this growth is in 

 part at the expense of the reserves. After a certain length of time 

 this growth ceases. 



Aron ('ii), working with growing dogs, succeeded in maintain- 

 ing a constant body-weight for a long time, in some cases nearly a 

 year, by limiting the quantity of food. He also found that the 

 animals increased in size, the skeleton underwent growth, and the 

 brain retained its weight or increased in weight, while the animals 

 became progessively thinner and their fat reserves and muscular 

 tissue suffered marked losses. If the food was not increased in 

 amount the animals finally died of starvation after three to five 

 months, with a slight loss of weight. But if the quantity of food 

 was somewhat increased they could still be maintained at a con- 

 stant weight and in a condition of extreme emaciation, but now no 

 further growth occurred. The results of later experiments on rats 

 (Aron, 'i2, '13) are essentially similar and these experiments on 

 animals agree well with the observations of various earlier authors 

 on children. 



Aron concludes from his experiments that the internal growth- 

 impulse exists primarily in the skeleton and that other parts merely 

 follow the growth of the skeleton as far as nutritive conditions 

 permit. This is probably true for mammals or for vertebrates as 

 regards growth in stature during later stages of development, but 

 it is certainly not true for the early stages of development of verte- 

 brates nor for many invertebrates where no skeleton is present. 

 It seems probable that in these animals growth of the more stable 

 substances of the body, in part at the expense of the less stable, has 

 occurred. The diet in these cases is merely quantitatively, not 

 qualitatively insufficient; it contains the constituents necessary 

 for the construction of the relatively stable structural substances, 

 but not in sufficient quantity for the growth of all parts. Under 

 these conditions it might be expected that growth or maintenance, 



