4 i4 SENESCENCE AND REJUVENESCENCE 



up to these stages and then stops or becomes abnormal. If these 

 stages are passed successfully, further development is likely to 

 follow its usual course. It is easy to see why, if anything is wrong, 

 it should become evident during these stages, for they represent 

 the period when the intrinsic metabolic activity of the cells is greater 

 than at any other period of the life history, and the physical condi- 

 tion of the protoplasm which is of course correlated with the high 

 rate of metabolism must likewise be most susceptible to change at 

 this time. Internal or external factors, which produce little or 

 no effect when the metabolic and protoplasmic susceptibility is 

 lower, may at this time bring about changes which either lead to 

 death or profoundly modify the further course of development. 



The different behavior of the two eggs in relation to fertilization 

 which was mentioned in an earlier section (pp. 405-6) appears in 

 the tables. The starfish egg shows scarcely any increase in sus- 

 ceptibility just after fertilization, while in the sea-urchin egg the 



increase is marked. 



TABLE X 



Nereis limbata 

 KCN 0.005 m l- 



Stage of Average Survival Time 



Development in Hours and Minutes 



2-4 cells 13-45 



Early gastrulae 11.30 



Early larvae hatching 7 . 30 



Larvae 8 hours after hatching 3 . 30 



Larvae with two pairs of setae 45 



Full-grown larvae i . 40 



Advanced larvae 2 . 30 



In Nereis, an annelid worm, the susceptibility increases up to the 

 larval period and during this period begins to decrease. Undoubt- 

 edly the great increase in susceptibility in the early larval stages is 

 due in part to the appearance and increase of motor activity and 

 functional stimulation. The larva is a highly organized animal 

 with sense-organs and muscles, and its rate of metabolism is higher 

 than that determined by conditions existing within its cells because 

 it reacts to external stimuli. But even during the larval period 

 very considerable changes in susceptibility occur which must belong 



