FERTILIZATION IN THE LIFE-HISTORY 33 



crement was probably at least 1,000 per cent daily. 

 From the fifteenth to the twentieth day of gestation 

 the rabbit adds only 212 per cent daily to its weight- 

 Four days after birth the increment is about 17 per 

 cent daily; twenty- three days after birth only about 

 6 per cent; two months after birth less than 2 per 

 cent; at two and one-half months less than i per 

 cent; and after full growth is attained, about two 

 hundred and twenty days after birth, the weight no 

 longer increases regularly. The processes of breaking 

 down and repair in the organism tend to balance. He 

 showed that similar principles hold in the development 

 of man, guinea-pig, and hen. 



We have here, indeed, a general principle of develop- 

 ment, which presumably holds for all metazoa, which 

 may be expressed by saying that in embryonic stages 

 the period of most rapid loss of growth power occurs, 

 and, if this is identical with the process of growing old, 

 that senescence is most rapid in exceedingly early stages 

 of development. The underlying conception has long 

 been appreciated by naturalists; it is an old idea, which 

 it would be difficult to trace to its ultimate source, that 

 organisms start out with a high charge of growth energy, 

 which is gradually dissipated in the process of growing 

 old. 



The point of maximum efficiency of the organism in 

 the life-history does not, however, correspond with the 

 starting-point, the fertilized egg, but to an early follow- 

 ing stage of development. The germ cells themselves 

 have, indeed, in the process of their differentiation 

 completely lost growth capacity, and exhibit also in 

 terms of differentiation all the signs of senescent cells. 



