The Biology of Senescence 



the growth of individual D. magna. In the majority of specimens 

 (Fig. 25#) the two cycles of growth followed one another, the 

 growth-potential in the second cycle falling almost to zero with 

 increasing age; this fall is accompanied by a decrease in tgg 

 size and number, and the animal finally dies after a short period 

 in which growth has almost ceased. In individuals of the second 



40 



30 



tu 

 if 20 



LU 



z I- 



i 



o 



z 



10 



20 



DAYS 



30 



Fig. 25 (b). — Growth of D. magna — second type: growth-phases 

 superimposed (Edlen, 1938). 



type (Fig. 25b) the two growth cycles were superimposed — in 

 these growth was very rapid, there was no prepubertal 'shelf in 

 the curve of body size, and fertility was lost early, although the 

 life-span appeared to be normal. The third type (Fig. 25^) 

 showed only the first cycle of growth, but no gonadal function 

 developed, and adult size was not attained. These forms died 

 early. Edlen considered that the developing ovary exerts a 



94 



