THE CIRCULATION OF THE BONE-MARROW. 43 



channels which become functionally patent under the stimulus of the new conditions. 

 In view of what we know of the capillaries elsewhere, may it not be that, under ex- 

 cessive demand for blood-cells, when we recognize grossly an increased activity and 

 vascularity of the marrow (red marrow versus yellow marrow), these otherwise 

 collapsed capillary channels become patent and function to help meet the crisis? 

 They may thus be a very important unit of the defensive mechanism of the body. 

 Drinker and his associates were unable, however, to demonstrate satisfactorily 

 these accessory channels in the mammal following the return of the blood-volume 

 to normal after a large hemorrhage, when it might be expected that all possible 

 avenues of delivery for cellular elements would be functioning. 



On the other hand, in view of Sabin's (1920, 1921) derivation of red blood- 

 cells, clasmatocytes, and monocytes in the chick embryo from endothelium, there 

 remains still another possible function for the marrow-capillaries, or rather the 

 endothelium of the marrow-capillary. It will be remembered that the endothelium 

 of these capillaries is embryonic in appearance. In hyperplastic marrow injec- 

 tions, Drinker has described the disappearance of a detectable endothelial lining 

 to the vessels and ascribes the lack of extravasation of injection granules, even with 

 these apparently open vessel walls, to the close packing together of the developing 

 cells, which he believes grow into and through the yielding endothelium. If the 

 red cells were formed intravascularly in an extra-circulatory capillary bed with 

 embryonic endothelium as then source, the apparent cellular border described 

 by Drinker might be these developing cells inside a greatly distended capillary, 

 with wall so stretched and endothelial cells so altered by rapid proliferation as to 

 be unrecognizable as such. The fact that there is no parencl\ymal diffusion with 

 injections, even though the wall appears to be patent, would seem to suggest this. 

 After saponin injection, Drinker and his collaborators noted the appearance of 

 nucleated red cells in the peripheral circulation prior to an increase in the leucocyte 

 count. They ascribe this to the fact that the developing red cells are in "closer 

 proximity to the circulating blood." This would be literally true were their devel- 

 opment assuredly intravascular. Even though the extravascular origin of the 

 erythrocytes in the adult mammal is practically universally accepted to-day, 

 Drinker, it would seem, has more nearly sensed the only justifiable attitude tenable 

 at the present time when he states: "Red cells are apparently formed outside the 

 blood-stream and enter the moving current as a result of growth pressure. It 

 will be noticed that we have not declared for the extravascular origin of the erythro- 

 cytes, but have simply said that they arise outside the bloodstream." In our 

 present state of knowledge this is all that can be said. 



Finally, there is the possibility that these strands of endothelium are never 

 opened up to the circulation as such in the bone-marrow, but represent filaments 

 of cells, like the angioblastic chains described by Sabin (1920) in the embryo, which, 

 in repeated cycles, multiply and make new generations of red corpuscles, the pre- 

 existing cone-shaped openings into the sinusoids marking the avenue of entrance 

 for the cells into the blood-stream. Such an interpretation would explain the 

 discrepancies in connection with the relation of endothelium to the formation 



