422 C. JUDSON HERRICK 



These are independent variables. The first factor can be meas- 

 ured directly in terms of the carbon dioxid output or oxygen 

 consumption of the tissue, and also indirectly by a very ingenious 

 method elaborated by Child which depends upon a differential 

 susceptibility of the tissue to various poisons. Tissues with 

 higher metabolic rate are more susceptible to certain concentra- 

 tions of potassium cyanide, alcohol, ether, and other drugs than 

 are tissues of lower metabolic rate. In many lower organisms 

 this difference in susceptibility can be directly observed through 

 structural changes caused by the drugs. The second factor is 

 expressed functionally by the range of diversity of behavior and 

 structurally by complexity of tissue and organ differentiation. 



3. In general, young tissues have a higher rate of metabolism 

 and less complexity of organization than older tissues, other 

 things being equal. During the life cycle, from cleavage stages 

 of the egg to advanced senescence, there is a progressive reduc- 

 tion in the amount of metabolism per unit of weight and a pro- 

 gressive increase in complexity of permanent stable organiza- 

 tion; and throughout the cycle some parts of the body retain 

 their juvenile characteristics longer than others. Growth, 

 senescence, tissue differentiation, and regeneration all have 

 certain features in common, one of which is the progressive 

 stabilizing of the more labile embryonic protoplasm by laying 

 down a framework of relatively immobile structurally differ- 

 entiated substances. "According to this view, senescence is 

 primarily a decrease in rate of dynamic processes conditioned 

 by the accumulation, differentiation, and other associated 

 changes of the material of the colloid substratum. Rejuvenes- 

 cence is an increase in rate of dynamic processes conditioned by 

 the changes in the colloid substratum in reduction and dediffer- 

 entiation " (Senescence, p. 58). 



4. The ripe germ cells are really " old " in the sense defined 

 above, i. e., their metabolic rate is low and their protoplasmic 

 structure is stable. By the act of fertilization the quiescent 

 germ cells are stimulated to a high rate of metabolism, with 

 corresponding transformation of their highly differentiated and 

 stable protoplasm into a less differentiated and more labile form. 

 Both fertilization and asexual reproduction are processes of 

 rejuvenescence. ' If these conclusions are correct, agamic and 

 gametic reproduction are fundamentally similar processes, 



