EMBRYOLOGY AND HUMAN MORTALITY 145 



the case of organs of ectodermic origin, and from about 

 age 22 on in cases of mesodermic origin, the death rate 

 curves rise at a practically constant rate to extreme old 

 age. The ectodermic and mesodermic curves during this 

 portion of the life span are nearly parallel, diverging 

 only slightly from each other with advancing age. The 

 curve for the death rate resulting from breakdown of 

 organs of endodermic origin has an entirely different 

 course. It rises sharply for ten years after the low point 

 in early youth, and then makes a rather sharp bend at 

 about age 22, and passes off to the end of the life span, 

 at a reduced rate of change. In consequence of this it 

 crosses the mesodermic line at age 60. From that point 

 on to the end of life deaths from breakdown of organs 

 of mesodermic origin stand first in importance. 



Figure 40 shows the same set of facts for the female, 

 and at once a number of striking differences between the 

 conditions in the two sexes appear. In the first place, 

 the breakdown of mesodermic organs is practically of 

 equal importance in determining the mortality of infants 

 with the breakdown of endodermic organs, in the case of 

 the female. This fact, of course, arises because of the 

 heavy mortality of infancy due to failure of the female 

 reproductive organs, a matter which has already been 

 discussed. The curve for breakdown of the ectodermic 

 organs follows substantially the same kind of course in 

 the female as it does in the male. The mesoderm and 

 endoderm lines cross nearly 20 years earlier in the case 

 of females than in the males. This circumstance arises 

 from the fact that throughout life the mesodermic organs 

 play a relatively more important role in the determina- 

 tion of mortality in the female than they do in the male. 



What reward in the way of useful generalization may 

 10 



