CHAPTER V 

 EMBRYOLOGY AND HUMAN MORTALITY 



the preceding chapter attention was confined 

 strictly to the organological incidence of death. It is 

 possible to, push the matter of human mortality still 

 farther back. In the embryological development of the 

 vertebrate body, there are laid down at an early stage, 

 in fact immediately following the process of gastrulation, 

 three morphologically definite primitive tissue elements, 

 called respectively the ectoderm, the mesoderm and the 

 endoderm. These are termed the germ-layers, and em- 

 bryological science has, for a great many forms, succeeded 

 in a broad way in tracing back to the primitive germ 

 layer from which it originally started its development, 

 substantially every one of the adult organs and organ 

 systems of the body. Itj makes no difference to the validity 

 or significance of the discussion which we are about to enter 

 upon, in what degree of esteem or contempt in biological 

 philosophy the germ layer theory or doctrine, which oc- 

 cupied so large a place in morphological speculation 50 

 years ago, may be held. We are here concenied only with 

 the well-established broad descriptive fact, that in general 

 all adult organ systems may be traced back over the path 

 of their embryological development to the germ layer, or 

 combination of germ layers, from which they origin- 

 ally started. 



Having arranged, so far as possible, all causes of death 

 on an organological basis, it occurred to me to go one 



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