14 CHARLES F. W. McCLURE 



latter was never observed to be actively phagocytic. The pos- 

 terior lymph-hearts are not developed on the ninth day after 

 fertilization in Bufo. 



5. Endothelium of certain veins and aortic arches. Dye granules 

 were found in the cytoplasm of the endothelial cells of the renal- 

 portal veins, which include Jacobson's vein (postcardinal) and the 

 postcava (fused subcardinals) . They were also found in the endo- 

 thelium of the postcardinals anterior to the mesonephros and in 

 the endothelium of the veins of the pronephros. The endothelium 

 of the portal vein and its tributaries in the mesentery was found 

 to be phagocytic; also that of a tributary of the portal vein 

 which came from the pancreas. Dye granules were occasionally 

 housed in an endothelial cell of the external and inferior jugular 

 veins and occasionally in an endothelial cell of certain aortic 

 arches. The endothelium of none of these three vessels can be 

 regarded, however, as being consistently phagocytic. The aortic 

 arches near the heart are frequently stained a diffuse deep blue in 

 larvae in which the body is over-loaded with dye. This stain is 

 usually removed in the living animal after the latter has remained 

 in tap water for twenty-four hours before killing, and is not to be 

 confused with the presence of dye granules in the endothelium of 

 'the aortic arches above-mentioned. 



With the exception of the instances mentioned above, no dye 

 granules were observed in the endothelium of any other veins 

 or arteries in the body. The endothelium of the peripheral arteries 

 and veins and of the dorsal aorta and its branches did not present 

 the slightest indication of possessing a phagocytic function. 



The tendency shown by the endothelium of certain lymphatics, 

 veins and even arteries to ingest and store the dye, certainly sug- 

 gests the possibility that vascular endothelium in general may 

 have been primitively phagocytic. It is especially interesting to 

 note in this connection that trypan blue is stored by the endothe- 

 lium of the renal-portal veins in the adult carp (Wislocki, '17); 

 also by the endothelium of the larger blood-vessels within the 

 gills and by the renal and digitiform gland endothelium of an 

 adult selachian (Hoskins, '17). 



