The Biology of Senescence 



lived (Ferris, 1932). In both these species, however, the form 

 having the higher reproductive rate in early life dies younger, 

 an observation which supports Miller's suggestion that death 

 results from 'exhaustion'. Egg laying in Apsilus vorax continues 

 until death (Cori, 1925) and in Proales, appears itself to be 

 adversely affected by somatic senescence, the egg-substance 

 failing to enter the eggshell, and eggs of bizarre size and shape 

 and of low hatchability being produced (Jennings and Lynch, 

 1928). Old populations of P. sordida consist of two types of 



». — Survival curves for males, mictic (M) and amictic (A) females of 

 Lecane inermis (from Miller, 1931). 



senile individuals, some thick and opaque, and others abnorm- 

 ally transparent, with pigmentation of the gastric glands. There 

 is considerable individual variation in the length of survival 

 once senescence is established. 



Impairment of function in all the species which have been 

 studied is so general during senescence that it is not possible to 

 identify a pace-maker organ in the process, though in some 

 cases it appears to be the digestive system which first deterior- 

 ates. The pattern is fully consistent with some or all of the 

 somatic cells having a fixed survival-time under normal meta- 

 bolic conditions — a highly important precedent for the study 

 of other types of metazoan senescence. The senescent change 



86 



