154 RALPH S. LILLIE. 



of the acid consists rather in altering the character of the normal 

 proteins of the cell, and rendering these a favorable substrate for 

 the proteases present. The autolytic process is thus activated 

 by the addition of acid; the increase in the rate and extent of 

 autolysis is roughly proportional to the amount of acid added. 1 



If this interpretation is applied to the case of the activation 

 of the starfish egg by butyric acid, certain interesting possibilities 

 appear. The acid appears to act by combining chemically, as 

 already seen. If we assume that it forms a combination with 

 some structure-forming protein, which then becomes hydrolyz- 

 able by some enzyme present, the above facts appear in a clearer 

 light. The transformation of a certain definite quantity of this 

 protein into substrate for the enzyme would then form the con- 

 dition for complete activation. The rate of this transformation 

 would correspond to the rate of the activ.ation-process and would 

 be proportional to the concentration of acid. We suggest, there- 

 fore, that acids cause activation by forming a hydrolyzable com- 

 bination for some intracellular enzyme, probably one of well- 

 defined specificity. The activating influence of high tempera- 

 tures also becomes intelligible on this hypothesis, and may be 

 referred to the production of an acid (e. g., lactic) which then 

 combines to form the hydrolyzable substrate; the similarity in 

 the time-relations of heat-activation and acid-activation is thus 

 explained. The autolysis following this combination would 

 constitute the first step in development. 



What is most significant from the standpoint of the general 

 physiology of development is the evidence that a structural modi- 

 fication, in itself apparently simple, is sufficient to alter the 

 whole course of egg-metabolism in a definite and predetermined 

 manner. We must recognize that development is essentially the 

 expression or outcome of a complex chain or nexus of metabolic 

 processes, which is released by the activating reaction. Evi- 

 dently these processes include chemical interactions and syn- 

 theses of a highly diversified and specific kind ; these latter deter- 

 mine the specific character of the development in any instance. 

 Regarding their precise nature we are almost completely ignorant 

 at present. But however complex and unanalyzable the whole 



1 Bradley and Taylor, loc. cit., 1916, p. 274. 



