INFLUENCE OF RACE AND SEX ON GROWTH-PROCESS 485 



growth in any given growth-cycle are primarily characteristic of the 

 process itself and only in a minor degree dependent upon the dietary, 

 provided it is in all respects sufficient. We have seen that the main 

 inhibiting factor in growth arises from the accumulation of the products 

 of growth and the enhanced rapidity of tissue-disintegration which 

 ensues. The characteristic dimensions of an animal, therefore, and 

 the same, to a less striking degree, is doubtless true of a plant, are 

 determined mainly by the relative magnitude of the specific Velocity- 

 constants of the forward and the opposed reactions. These are char- 

 acteristic of the particular reactions which occur in a given race or 

 sex, and are not influenced by the mere abundance or paucity of the 

 dietary. 



That the bodily dimensions of an animal may be affected to a limited 

 extent by the abundance of the Dietary is, however, a readily ascer- 

 tainable fact. If the dietary be absolutely insufficient even to main- 

 tain bodily heat and the output of work, the tissues are called upon 

 to supply the energy-requirements, the animal loses weight and may 

 ultimately die of inanition or of acute conditions supervening upon 

 partial inanition. If the dietary insufficiency is less extreme than this, 

 growth is nevertheless slowed, and the bodily dimensions attainable 

 at maturity are smaller than is normal for the species. If, on the other 

 hand, the diet is exceedingly abundant and other environmental 

 conditions are exceptionally favorable, then the bodily dimensions 

 at maturity may come to distinctly exceed the average, although the 

 degree of supernormality which is attainable in this way is, of course, 

 strictly limited. Mice, under no matter what favorable conditions of 

 environment and abundance of food supplies, do not achieve the bodily 

 dimensions of a guinea-pig or even of a rat. 



The supply of nutrients to the tissues is, as we have seen, determined 

 primarily by the composition of the blood which, subject to short- 

 period fluctuations, remains relatively constant throughout the 

 growth and life of the animal. The "Nutrient Level" or concentration 

 of growth-substrates in the blood is maintained by a dynamic equilib- 

 rium which involves a variety of factors. On the one hand we have the 

 availability of Foodstuffs in the external environment and the ability 

 of the digestive apparatus to disintegrate them and to absorb the 

 products of their disintegration. On the other hand we have the rate 

 of utilization by the tissues and the equilibrium between the storage- 

 capacities of the tissues for the various classes of foodstuffs, for poly- 

 saccharides, fats, and amino-acids, and the concentration of these 

 substances or their products in the blood and tissue-fluids. The height 

 of the nutrient reservoir in the blood is thus governed by a balance 

 between a certain rate of inflow and a certain rate of outflow. In 

 addition to these factors, and in order to avoid an excessive accumula- 

 tion of nutrient materials in the blood, an overflow is also provided in 

 the phenomenon of Exogenous Metabolism, or the destruction of food- 



