262 



Scientific Proceedings (129) 



hours and a veritable honey-comb design is outlined by the ves- 

 sels, a small number only of which seem to be completely open 

 to the circulation. There are very many developing red blood- 

 cells, all within strands of endothelium. In some places it ap- 

 pears almost as though the endothelium were being actually re- 

 placed by a strand of developing red blood-cells, though the 

 regularity and continuity of outline is nowhere broken. There 

 are numerous small groups of from five to eight young granu- 

 locytes in various stages of development, all located extravas- 

 cularly in the parenchyma. Succeeding stages increase the com- 

 plexity of the picture. However, insofar as we have been able 

 to observe, the red blood-cells have appeared only intravas- 

 cularly, and the white blood-cells extravascularly. No analyses 

 of hyperplastic marrows have as yet been attempted. 



124 (2084) 



On the intravascular development of erythrocytes in the bone 

 * marrow of the adult rabbit. 



By R. S. CUNNINGHAM and C. A. DOAN. 



[From the Department of Anatomy, Johns Hopkins University, 

 Baltimore, Maryland.] 



Bone marrow has been one of the most difficult tissues to 

 understand because it has proved so hard to reduce it to a suf- 

 ficiently simple state for analysis. The older methods, con- 

 sisting chiefly in classification of cell types, have led to the al- 

 most universal acceptance of the monophyletic theory, and in 

 general to the conclusion that the developing red blood cells are 

 formed in parenchymal spaces outside the vascular system, hence 

 differing from the manner of development found in the embryo. 

 Since it has been accepted that the red cells develop extravas- 

 cularly, it has been obviously necessary to determine their mode 

 of entry into the circulation. The two principal explanations 

 offered have been : (1) That the endothelial lining of the vas- 

 cular bed was incomplete, as in the spleen, and consequently 

 the young cells could be forced through these openings; and 



