480 PROCESSES INFERRED FROM INDIRECT OBSERVATION 



we say that each bacterium has a like potentiality of reproduction we 

 clearly express the fact that the synthesis of bacterial cell-substances 

 which results in the production of a cell is a favoring condition for the 

 production of new cells, in other words that some substance or sub- 

 stances comprising the bacterium accelerate the production of new 

 masses of bacterial substance. In ultimate terms, therefore, the two 

 interpretations of the phenomenon are identical, the only essential 

 difference between the more familiar cases of autocatalysis, such as the 

 hydrolysis of methyl acetate, and the process of cell-multiplication, 

 being the fact that in the latter process the reaction takes place in a 

 heterogeneous chemical system, i. e., within the particulate masses 

 comprising the cells. Yet the fact that a chemical reaction takes place 

 in a heterogeneous medium does not imply that it is discontinuous. The 

 production of calcium sulphate from a mixture of calcium hydrate and 

 sulphuric acid is a continuous process despite the fact that the product 

 is divided into particulate masses, which in this instance are crystals. 

 On the other hand the instances of autocatalysis in heterogeneous 

 systems are abundant in chemical literature, the oxidation of metals 

 in contact with air being a familiar illustration of a group of autocata- 

 lyzed reactions of this type. 



The Accelerative Factor in the process of growth is, therefore, a 

 chemical substance or substances, or a chemical condition, which is 

 strictly analogous to the accelerative factor in less complex auto- 

 catalyzed phenomena. The autocatalytic character of the growth- 

 process follows of necessity, in fact, from the fundamental characteristic 

 which, more than any other, distinguishes living from non-living mate- 

 rial, namely its potentiality of unlimited reproduction. When we assert 

 that living cells all possess like potentiality of reproduction we merely 

 state in morphological terminology that the production of living matter 

 is a self-sustained or autocatalyzed phenomenon. Just as the produc- 

 tion of living from inanimate matter is essentially a chemical process, 

 so the acceleration of its production which is consequent upon the multi- 

 plication of the particulate resultants of the process is, when viewed 

 from the chemical standpoint, evidence that substances are produced 

 in the creation of living matter which have the essential property of 

 catalyzing its further manufacture. Regarding the possible nature of 

 these endogenous catalyzers, we shall have something to say in a later 

 part of this chapter. 



It remains to consider what may be the probable nature of the 

 Inhibitive Factor which ultimately brings the process of growth to a 

 standstill, which sets a limit to the normal dimensions of any given 

 species of animal, and which predominates over the accelerative factor 

 during the latter half of each growth-cycle. In the simpler instances 

 of autocatalysis, as we have seen, the inhibitive factor may be, either 

 the exhaustion of the materials undergoing transformation, or, on the 

 other hand, the accumulation and consequent " back-pressure" of the 

 products of the reaction, or both of these factors may play a part in 



