EMBRYOLOGY AND HUMAN MORTALITY 141 



The figures show that in man, the highest product of 

 organic evolution, about 57 per cent, of all the biologically 

 classifiable deaths result from a breakdown and failure 

 further to function of organs arising from the endoderm 

 in their embryological development, while but from 8 

 per cent, to 13 per cent, can be regarded as a result of 

 breakdown of organ systems arising from the ectoderm. 

 The remaining 30 to 35 per cent, of the mortality results 

 from failure of mesodermic organs. The two values 

 stated for ectoderm and mesoderm, shown by the two 

 bars in the diagram, differ by virtue of the fact that two 

 important causes of death, cerebral hemorrhage and 

 apoplexy, and softening of the brain, are put in the 

 one case with the ectoderm and in the other case with 

 the mesoderm. The pathological arguments for the one 

 disposition as against the other of these two diseases are 

 interesting, but lack of space prevents their exposition 

 here. I have chosen rather to present the facts in 

 both ways. 



Taking a general view of comparative anatomy and 

 embryology it is evident that in the evolutionary history 

 through which man and the higher vertebrates have passed 

 it is the ectoderm which has been most widely differ- 

 entiated from its primitive condition, to the validity of 

 which statement the central nervous system furnishes the 

 most potent evidence. The endoderm has been least pro- 

 gressively changed structurally and functionally in the 

 process of evolution, while the mesoderm occupies, on the 

 whole, an intermediate position in this respect. 



Degree of differentiation of organs in evolution im- 

 plies degree of adaptation to environment. From the pre- 

 sent point of view we see that the germ layer, the endo- 

 derm, which has evolved or become differentiated least in 



