412 SENESCENCE AND REJUVENESCENCE 



and finally became nearly uniform again in early larval stages. 

 Meyerhof has shown that the heat production of the sea-urchin 

 egg increases steadily up to the larval stage; at the sixty-four-cell 

 stage it is about twice as great as during the first hour after fertili- 

 zation; when the larva begins to swim it is three times as great, 

 and at a stage four hours later, four times as great. Heat produc- 

 tion in the Aplysia embryo decreases during the first few cleavages, 

 then increases rapidly to the larval stage, when it becomes nearly 

 uniform, i.e., the changes in heat production in Aplysia are essen- 

 tially parallel to the changes in oxygen consumption and carbon- 

 dioxide production as determined by Buglia. 



All of these data indicate that at least the oxidation processes 

 increase in rate during the earlier stages of development, and the 

 general behavior of the developing embryo, the increase in the 

 amount of metabolically active cytoplasm and nuclear substance, 

 and the decrease in amount of yolk where yolk is present suggest 

 that not merely oxidation but metabolic activity in general 

 undergoes a marked increase during this period. In short, this is a 

 period of physiological rejuvenescence. 



CHANGES IN SUSCEPTIBILITY DURING EARLY STAGES 



Lyon ('02) found that the susceptibility to cyanide of the sea- 

 urchin egg underwent a gradual increase during the course of 

 cleavage, and I have determined the susceptibility to cyanide dur- 

 ing early development in a number of animal species. In these 

 experiments the susceptibility was measured in most cases by the 

 limits of recovery, that is, the length of time in the cyanide solu- 

 tion at which recovery ceased to occur on return to water. It was 

 also possible in most cases to determine the survival time by 

 observing the death changes in the cyanide. A part of the results 

 of these experiments appear in the following tables. For the sake 

 of simplicity only the average survival times are given, viz., the 

 average length of time in cyanide necessary to prevent any visible 

 degree of recovery after return to sea- water. These tables serve 

 merely to give a general idea of the changes in susceptibility and 

 do not show the differences or the differe nt rates of change in the 

 susceptibility of different regions of the embryos. 



