30 THE CIRCULATION OF THE BONE-MARROW. 



of attack. Until further progress toward this fundamental comprehension of 

 first principles has been made, by means of studies along different lines of approach 

 than hitherto employed, we shall still be without the basis for a rational therapy. 



Within the past two decades exceedingly valuable contributions toward 

 solving the problem of the origin and development of individual types of blood-cells 

 have been made through embryological studies. The most representative work 

 on the embryology of the blood is that carried out by Danchakoff (1908, 1909) 

 and Sabin (1920, 1921) on birds and by Maximow (1909, 1910) on the mammal. 

 Both Maximow and Danchakoff recognized the relationship between endothelium 

 and blood-cells, not only in the stage of the primitive blood-islands but also in 

 somewhat later stages; both have thought, however, that endothelium gives 

 rise only to indifferent blood-cells. Schridde (1907), on the other hand, has de- 

 scribed the direct transformation of endothelium into erythroblasts in early human 

 embryos. Maximow believed that although the early erythroblasts of mammalian 

 embryos are intravascular in origin and derived indirectly from endothelium, the 

 ultimate erythroblasts of the adult are a group of cells extra-vascular in origin. 

 This may be said to be the prevailing view to-day. The question has been reopened 

 recently, however, by the work of Sabin (1920, 1921). It was not until she had 

 actually seen, by direct observation on living chick embryos during the second day 

 of incubation, the differentiation of the red cell from early endothelium and later 

 the origin of the monocyte cell-series and clasmatocytes from the same source in 

 chicks of the third and fourth days, that the etiological importance of the endo- 

 thelium, and hence the significance of the exact pattern of the vessels of the marrow 

 in the mature organism, was fully understood. Thus the whole blood problem 

 receives a new impetus in a different direction. This work places an emphasis 

 upon the importance, not hitherto adequately appreciated, of a more compre- 

 hensive and exact knowledge of the endothelial content of adult marrow. It is not 

 a purely morphological standpoint to which the importance attaches now, nor are 

 we interested in it solely as a means by which the blood-cells gain entrance into 

 the circulation. The important question, stimulated by the work of Sabin, is 

 the very suggestive one as to the possible direct relationship between the endo- 

 thelium of the hemopoietic tissues and the blood-cells of the mature organism. 



Obviously, before attempting to determine this relationship, a thoroughly 

 comprehensive understanding of the extent and distribution of the endothelium 

 in the marrow of the long and flat bones is essential. But here again we find in 

 the literature a wide difference of recorded observation on the part of various 

 workers. The views held may be classified into three groups, together with their 

 respective supporters. (1) The earliest observations followed close upon the first 

 recognition of the bone-marrow as a hemopoietic tissue. Hoyer (1869) could 

 detect no endothelial walls in the so-called capillaries or blood-channels in obser- 

 vations on the marrow of injected rabbits. Rindfleisch (1880), using a gelatin 

 injection mass, interpreted the regularly outlined channels in his sections of bone- 

 marrow (very well illustrated in one of his plates) as indicative of tissue spaces 

 filled with blood and limited only by the medullary parenchyma, that is to say, 



