44 THE CIRCULATION OF THE BONE-MARROW. 



of the red corpuscles and thus harmonize the two divergent ideas of the intra- 

 vascular versus the extra-vascular origin of erythrocytes. Under such a view the 

 red cells could be considered as coming from endothelium, but endothelium so 

 placed that the new red cells would not be in the active current of the blood as 

 actually within the sinuses. The cells would, nevertheless, be so placed with refer- 

 ence to the sinuses as to gain a ready access to the functioning lumen without calling 

 for any special destruction of the wall of the sinusoid. 



Cunningham (1922), in his study of the cellular reactions during the pro- 

 duction of exudates in the peritoneal cavity, obtains no evidence either for or 

 against the participation of the endothelium of the neighboring capillaries of mesen- 

 tery and omentum in the formation of exudative cells. He points out the difficulty 

 of differentiating reticulum and endothelium in spleen and lymph-glands of the 

 adult mammal when attempting to determine which of these cells is progenitor of 

 the circulating mononuclear. However, certain observations have led him to 

 "suggest the hypothesis that if the circulation be cut off from a group of capillaries, 

 the endothelial cells of which still obtain sufficient nourishment to prevent cell 

 death [the condition that probably exists in the bone-marrow normally], these 

 cells may undergo a cataplastic reversion to the syncytial angioblastic or embryonic 

 endothelial type, with subsequent differentiation into clasmatocytes." Sabin 

 (1921) has proved conclusively the endothelial origin of the clasmatocyte in the 

 embryo. Furthermore, the work of Macklin and Macklin (1920), who found that 

 areas of endothelium in new-formed capillaries appear to become transformed into 

 clasmatocytes, and the work of others along similar lines, make it practically 

 certain that the protean possibilities of endothelial differentiation in various parts 

 of the functioning mature organism are only beginning to be appreciated. 



The mere knowledge and recognition of the presence of this extensive distri- 

 bution of endothelium in bone-marrow not regularly functioning as a blood-channel 

 is a step in the direction of the determination of its relation to the blood-cell pro- 

 duction of the marrow and at least a presumptive indication for further studies, 

 with this possible specific relationship as an objective hypothesis. 



This investigation is the direct outcome of Dr. F. R. Sabin's work on the origin 

 of blood-cells in the chick embryo and was undertaken at her instigation. Through- 

 out the development and interpretation of the reported findings, her constant help 

 and criticism have been indispensable. It is a pleasure to express also my gratitude 

 to Dr. R. S. Cunningham for his advice and many helpful suggestions, and to 

 Mr. James F. Didusch for the excellent illustrations accompanying the text. 



SUMMARY. 



(1) The arterial supply of the bone-marrow is secured via the medullary 

 artery, the periosteal vessels along the shaft, and some vessels near the articular 

 extremities which supply the epiphyses as well. The arterioles are relatively few 

 in number. 



(2) Normally there are a few "transition capillaries" functioning as the 

 intermediary communication between arterioles and venous sinusoids. 



