406 SENESCENCE AND REJUVENESCENCE 



ovary and does not begin development until the sperm enters, may 

 live for a week or more. The death of the unfertilized starfish egg 

 is not comparable to death from old age in organisms in general, 

 but is the result of the peculiar physiological condition of the egg 

 almost on the boundary line between parthenogenesis and zygo- 

 genesis. The conclusions concerning natural death which Loeb 

 has drawn from the behavior of this egg are certainly not applicable 

 to death from old age (see pp. 307-9). A few other eggs show 

 somewhat similar behavior, but in all of them a more or less similar 

 physiological condition exists and their behavior cannot be made the 

 basis for conclusions as to the nature of death in general. 



In experiments of my own the susceptibility of various animal 

 eggs to cyanide before and after fertilization has been tested, both 

 by observing the occurrence of the death changes and by determin- 

 ing the limits of recovery in a given concentration. The sea-urchin 

 egg and the eggs of Nereis, Chaetopterus, and Hydroides, among the 

 annelids, are all somewhat more susceptible to cyanide after ferti- 

 lization than before, although the difference is not very great. In 

 the starfish egg, however, the susceptibility increases markedly in 

 unfertilized eggs when maturation begins, and there is little or 

 no further change on fertilization. Since increased susceptibility 

 means increase in rate of metabolism, these results agree in general 

 with those obtained by other methods, although the increase in 

 susceptibility to cyanide is not as great as might be expected if the 

 rate of oxidation increases from three to six times with fertilization. 

 The results with the starfish egg indicate, as Loeb suggested, that 

 here the chief increase in rate of oxidation occurs with maturation. 



PARTHENOGENESIS 



The naturally parthenogenic egg is evidently a cell which pos- 

 sesses the capacity to react to its physiological or physical isolation 

 from the parent body or from the former source of nutrition or 

 to the change of conditions associated with its extrusion from the 

 body by the initiation of a normal development. Although oxygen 

 consumption and susceptibility of parthenogenic eggs before and 

 after isolation have not been determined, the observations on the 

 starfish egg which is on the verge of parthenogenesis and the very 



