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Scientific Proceedings (129) 



and which are running from sinus to sinus as shown by Doan 

 in the pigeon. With endothelium, as delicate as this is, it is 

 obviously impossible in sections to always determine the exact 

 location of the red blood cells, but a large proportion of them 

 appear in rows extending from sinus to sinus, with here and 

 there parallel, investing, endothelial nuclei. The histological 

 picture obtained is strikingly similar to that described for the 

 pigeon's marrow during recovery from starvation, 3 and in view 

 of such a similarity our conclusion that most of the red cells 

 arise intravascularly seems wholly justified. 



The method that we have u?ed for this purpose has been the 

 administration of large doses of dead typhoid bacilli intra- 

 venously. Our doses have been increasingly large, usually be- 

 ginning with y 2 of a 24 hour agar slant, planted in a standard 

 manner, and increasing to 1, 2 and 3 cultures to the dose. Very 

 large outpourings of leucocytes have immediately occurred and 

 after 3 doses of this character the bone marrow has been de- 

 pleted to the maximum of its myelocytes and has not begun to 

 form new white cells to any appreciable extent. 



The conclusion that the red cells arise by proliferation of 

 endothelial cells which remain in the bone marrow in a rela- 

 tively undifferentiated state seems justified from the observa- 

 tions of Doan on the pigeon and those reported here for the 

 rabbit. That large clumps of developing red cells do occur in 

 the marrow has been observed many times, and though an en- 

 dothelial margin can not always be demonstrated, this may be 

 either the difficulty of demonstrating so delicate a structure in 

 sections, or else an overgrowth of the young cells into a blood 

 island. Either conclusion obviously explains the method of 

 entry of the adult erythrocytes into the circulation through the 

 original openings of the capillaries into the sinusoids. It has 

 been determined that endothelium gives rise to the erythro- 

 cytes of the embryo in a large number of species; and suggestive 

 evidence is presented here for the mammal, and elsewhere for the 

 pigeon, that a similar relationship exists in the adult marrow. 

 This seems to us to indicate an additional and most important 

 physiological significance for endothelium in the adult verte- 

 brate. 



3 Doan, Paoc. Soc. Exp. Biol, and Med., 1923, xx, 5. 



