74 CARNEGIE INSTITUTION OF WASHINGTON. 



tinguished from bordering endothelium. Passing downward into the 

 primitive capillary net, one finds that most of the hemoglobin-containing cells 

 have disappeared, apparently having liquefied into plasma, and there are left 

 only the fluid-containing endothelial tubes. 



Growth of Veins by Sprouting. 

 In studying the vessels in the area vasculosa of the living chick, Dr. Sabin 

 has been able to observe the process of sprouting in veins that had already 

 acquired a considerable thickness of adventitia, a phenomenon pertaining to 

 mature vessels and quite different from the process which Miss Finley studied. 

 Solid endothelial sprouts appear to push their way between the adventitial 

 cells as easily as through the looser tissue around capillaries. Furthermore, 

 the sprouts were seen to grow out as veins, having a covering of adventitial 

 cells. The estabhshment of this fact is of importance in understanding the 

 repair of vessels after injury. That veins can regenerate as veins, without 

 the preliminary development of a capillary bed, greatly increases the rapidity 

 with which vessels can be renewed in wound-healing. This explains previous 

 experiments in which Dr. Sabin showed that in intestinal anastomosis the 

 vessels from one of the apposed surfaces of the intestine can be injected from 

 the other surface on the fourth day after the operation. In the modifications 

 that constantly occur in the vascular pattern, in addition to sprouting there 

 is also destruction of branches. This could be observed in the living tissue. 

 This process is characterized by a collapse of the endothelium, obhterating the 

 lumen of the vessel, and this is followed by degeneration of both the endo- 

 thelial and adventitial cells. 



Vascttlarization op Bone-Marrow. 



It is a comparatively simple matter to demonstrate by the usual injection 

 methods the main blood-channels of the bone-marrow, and consequently the 

 distribution and character of the nutrient arteries and the venous sinusoidal 

 plexuses are quite well understood. This is not true, however, for the detailed 

 ramifications of the smaller vessels and for the extent and continuity of the 

 bone-marrow capillaries. These are so concealed by the bone-marrow cells 

 that it has heretofore been impracticable to analyze them satisfactorily. 

 Dr. C. A. Doan, using pigeons, has overcome this difficulty by depleting the 

 marrow of its free cells through starvation. By combining starvation with 

 carefully executed injection procedures, he has been able to demonstrate the 

 existence of an extensive intersinusoidal capillary plexus which appears 

 normally to be in a state of collapse, but which becomes distended under the 

 conditions of the injection. The significance of this capillary bed as a possible 

 source of new blood-cells is pointed out by Dr. Doan, and his specimens 

 provide the anatomical basis for a reconsideration of the general question of 

 hemopoesis in the adult organism. Differing from the spleen, the endothelium 

 of the marrow constitutes a closed system, forming a continuous lining 

 throughout all the vascular ramifications. 



Although Dr. Doan, by his starvation experiments, was able to demonstrate 

 the extensive closed capillary bed in the bone-marrow of the pigeon, one could 

 not say that the same capillaries occur in mammals or deny the validity of the 

 prevalent conception of an extra-vascular origin of red cells in the marrow of 

 adult mammals. It proved impossible satisfactorily to reduce mammalian 

 marrow by starvation or by several toxic agents. Dr. Cunningham and Dr. 



