416 



SENESCENCE AND REJUVENESCENCE 



egg which has used up all its yolk. It is a point of some interest 

 to determine at what stage the embryonic period of rejuvenescence 

 ends in such cases. The survival times for these two forms are 

 given in Tables XI and XII. 



TABLE XI 



Fundulus heteroclitus 

 Saturated Phenyl Urethane in Sea-Water 



TABLE XII 



Tautogolabrus adspersus 

 KCN 0.005 m l- 



Stage of Development 



15 minutes after fertilization 



1-2 cells 



4-8 cells 



Many cells 



Periblast 



Embryo just appearing 



Heart beating 



Xewly hatched 



Length of Time after 



Fertilization in 

 Hours and Minutes 



o-i5 

 0.50 



2 



5 

 7 



20 

 42 



Average Survival 



Time in Hours and 



Minutes 



10.45 

 IO. IO 



7-3 



o. 20 



Phenyl urethane was used instead of cyanide in determining the 

 susceptibility of the Fundulus egg, because the membrane of this 

 egg is impermeable to cyanide, as it is to many other substances, so 

 that even in high concentrations development is not retarded, while 

 for phenyl urethane the permeability is practically complete. 



Rejuvenescence occurs in Fundulus during the early stages of 

 development, as indicated by the increase in susceptibility, but 

 as soon as the embryo begins to form, it gives place to senescence. 

 The great increase in susceptibility between establishment of the 



