84 CHARLES R. STCCKARD 



Contrasted with the almost universal distribution of the 

 white cells within the animal kingdom the erythrocyte is confined 

 to the vertebrates phylum and to certain particular cases among 

 the invertebrates. The respiratory function of the invertebrate 

 blood is often claimed to be confined to the fluid or plasma 

 mass, and only among certain members of the higher groups is 

 a cell developed with the function of carrying oxygen to the 

 body tissues and even this cell can not be said to possess the 

 regular typical characters of the vertebrate erythrocyte. 



The vertebrate erythrocyte along with the typical verte- 

 brate mouth, the pharyngeal gills, the dorsal nerve cord, the 

 notochord, and bony skeleton and the many other possessions 

 characterizing the vertebrate group, separates it in gulf-like 

 fashion from the invertebrates. The white blood cells bridge 

 this gulf but the red blood corpuscle differs from that of the 

 invertebrate in a way comparable to the difference between the 

 vertebrate mouth and that of the invertebrate, both serve the 

 same function but are structurally unlike. Just as the mouth 

 and pharyngeal gills and vertebral column have no invertebrate 

 forerunner, so no cell within the invertebrate animals can at the 

 present moment be sought out or designated as the certain an- 

 cestor of the red blood cell. 



The cells of the vascular walls are closely similar in both 

 vertebrates and invertebrates, as pointed out above. In both 

 animal divisions they probably arise and develop in the same 

 fashion. The white blood corpuscles probably do also. Yet 

 the red cells, although they too originate from the mesenchyme 

 in the vertebrates, are not in any way certainly descended from 

 the invertebrate oxygen carrying cell or the wandering leuco- 

 blast or amoebocyte. 



There is certainly no phylogenetic or comparative morphologi- 

 cal evidence to warrant one in deriving vascular endothelium, 

 leucocytes, and erythrocytes from a common cell ancestry ex- 

 cept, of course, they are all derived from the mesenchyme or 

 same germ-layer. 



The fundamental histological study of the early developmental 

 stages of the blood elements in vertebrates was contributed by 



