Hemopoiesis in the Mongoose Embryo. 303 



From the above there seemed to be indicated a hemogenic capacity 

 also on the part of the endothelium of the mesonephric glomerulus. 

 This region was carefully studied. But there is apparently no intra- 

 vascular differentiation of endothelium into hemoblasts. An occa- 

 sional cell has the appearance of being at an early phase of separation 

 from the vascular endothelium ; but in consequence of the irregularity 

 of the plane of section, due to the delicate nature and the contorted 

 condition of the glomerular capillaries, a confident interpretation seems 

 impossible. The normal hemogenic capacity of the glomerular endo- 

 thelium is certainly very meager, if not actually nil. 



In an earlier paper (8) I stated the conclusion that in the pig embryo 

 the glomerular endothelium liberated cellular elements (hemoblasts) 

 extravascularly. Study of the sections of the mongoose embryo (of 

 relatively younger stages of development) compels a revision of this 

 conclusion. In the mongoose embryo the visceral layer of the Bow- 

 man's capsule of the mesonephric tubule, while closely applied to the 

 glomerulus. is nevertheless easily distinguishable from the endothelium 

 of the glomerular capillaries. The endothelial cells are flat plates; the 

 cells of the capsule are cuboidal or pyriform hi shape. An occasional 

 cell appears between the two layers. This may be an endothelial cell 

 separating extravascularly, but such interpretation must remain 

 uncertain. 



In the pig embryo the capsular epithelium is still more closely 

 applied to the glomerulus, and the cells of both layers are very similarly 

 flattened. Generally the endothelial nucleus is more vesicular than 

 that of the capsule cell. The cells of the capsule are frequently 

 rounded up and appear to be in early stages of separation. These are 

 the cells which I interpreted as endothelial elements separating extra- 

 vascularly; but in the light of conditions in the mongoose embryos, 

 this conclusion does not seem warranted. In the mongoose embryo 

 the vascular endothelium of the mesonephric glomerulus is apparently 

 hemogenically inactive; but the presence of the pyriform cells in the 

 capsular membrane (fig. 22), certain of which remain attached by the 

 merest thread of protoplasm coupled with the fact of an occasional 

 free cell within the lumen of the capsular portion of the mesonephric 

 tubule suggests that certain of these cells may separate in the embryo 

 to become macrophages. A similar process is described by Mallory 

 in the case of certain infections of the human kidney (e. g., acute 

 capsular glomerulo-nephritis) . 



It is probably incorrect to regard the endothelium of the mesonephric 

 glomerular capillaries as "embryonic" and relatively undifferentiated. 

 This endothelium has to perform a specialized secretion process in 

 connection with the nephric function of the mesonephros. This may 

 explain the inability normally to produce hemoblasts. On the other 

 hand, the capsular portion of the tubule plays a lesser role in this secre- 



