482 PROCESSES INFERRED FROM INDIRECT OBSERVATION 



transformation, but the initial mass multiplied by the constant ratio 

 of the velocity-constants of the forward and reverse reactions. 



In the case of the growth of Bacteria in a limited quantity of culture 

 medium, McKendrick assumes that the inhibitive factor is simply the 

 exhaustion of available foodstuffs, i. e., that it corresponds to the first 

 of the alternative possibilities outlined above. In the growth of 

 animals, however, it is difficult to see how the limited availability of 

 Foodstuffs could be a deciding factor in the inhibition of normal growth, 

 for the medium in which our cells actually live and grow is the lymph 

 (or "tissue-fluid"), which is constantly supplied and renewed from the 

 blood. Now the mechanisms of the body are, as we have seen, so 

 devised that the composition of the blood is maintained in a condition 

 of extraordinary uniformity. It is true that its content of the more 

 particularly nutritional constituents fluctuates with the fluctuating 

 absorption of nutrients from the alimentary canal, but these short- 

 period fluctuations result in the long run in the mairtenance of a 

 remarkably steady flow of nutrient materials to the tissues. The blood 

 derives its nutrient constituents from the external environment and in 

 fact contains them not merely in sufficient proportion to maintain an 

 equilibrium of body-weight, but, even in adult animals, in considerable 

 excess of the necessary minimum, the destruction of this excess consti- 

 tuting the "Exogenous Metabolism" as contrasted with "Endogenous 

 Metabolism," or irreducible minimum of nutrient-consumption incident 

 to the maintenance of life. The medium in which our cells live, there- 

 fore, is under normal dietetic conditions a medium of almost constant 

 composition and, for the purposes of tissue-synthesis, it is inexhaustible 

 since it is continually renewed. The Substrates of growth must there- 

 fore be regarded as being of constant concentration and the inhibiting 

 factor of growth must be sought elsewhere than in the exhaustion of 

 available nutrients. 



On the other hand, if a portion of the tissues of an adult animal be 

 injured or destroyed, the process of growth immediately recommences 

 and is expressed in the phenomenon of Regeneration which, if mechanical 

 factors do not impose an insuperable obstacle, continues until the 

 complete restoration of the lost tissues has been accomplished. In 

 other words, removal of the products of growth immediately reinaugu- 

 rates the growth-process, just as the removal of the products of a 

 "balanced" chemical reaction at equilibrium immediately reinitiates 

 the forward reaction. We must infer, therefore, that in the growth of 

 mammals, at least, it is the accumulation of the Products of Growth 

 which normally inhibits the process and not the exhaustion of nutritive 

 materials. In Plants the supply of nutritive materials to the cells is 

 more fluctuating and dependent upon the environment, and here we 

 may expect to find, and do actually find, a much more conspicuous part 

 played by the supply of nutrients in determining the final attainable 

 dimensions of the organism. Nevertheless plants of a given species, 

 even under the most favorable nutritional conditions, do not exceed 

 certain definable limits in their dimensions at maturity, and they 



