Hemopoiesis in the Mongoose Embryo. 297 



rated, and has not yet fully rounded up into the typical hemoblast. 

 Similar examples might be multiplied indefinitely; more are given in 

 the paper dealing with this same phenomenon in the pig embryo; but 

 enough has been shown, especially in the case of cell b, to leave no 

 further doubt, I believe, that young endothelium can indeed trans- 

 form into cellular blood elements. 



It is of interest and importance to note that the origin of hemoblasts 

 from endothelial cells also in the bone marrow, both under normal 

 and certain pathological conditions, is described by some of the leading 

 pathologists, e. g., Aschoff (see Mallory's "Principles of Pathologic His- 

 tology"). Such a "lining cell" of the marrow blood spaces is believed 

 by Aschoff and by Mallory to be capable of differentiation either into 

 an erythrocyte. a granuloblast, or by hypertrophy into a megakaryo- 

 cyte, which accords with the evidence derived from my study of the 

 yolk-sac vessels of the pig and the mongoose. Moreover, Mallory, in 

 his list of normal cellular blood elements (p. 21), substitutes for the 

 commonly described mononuclear leucocyte (transitional leucocyte) 

 his "endothelial leucocyte." This cell he derives from "the endothelial 

 cells lining blood, and to a less extent lymph, vessels by proliferation 

 and desquamation. They also multiply by mitosis after emigration 

 from vessels into the lesions." 



Since few any longer doubt that young mesenchymal cells can trans- 

 form on the one hand into certain blood-cells and on the other into 

 endothelium, it seems a priori reasonable to suppose that the only 

 slightly altered mesenchyma, the embryonic endothelium, can also 

 occasionally transform directly into hemoblasts. The same thing 

 should be true also, perhaps to a lesser extent, with regard to meso- 

 thelium, and for the same reason. According to Bremer (i) the meso- 

 thelium covering the body-stalk of a 1 mm. human embryo does in 

 fact give rise to some extent to blood-cells. 



The main purpose of this paper is to attempt to establish the thesis 

 that young endothelium has a hemogenic capacity, by showing that 

 intra-embryonically also conditions occur similar to those described 

 for the yolk-sac. These conditions relate to the endothelial origin of 

 hemoblasts, which through a close series of developmental stages can 

 be traced into erythrocytes. 



The transition to intra-embryonic conditions next to be described 

 may be made by way of figure 4, which represents a binucleated elon- 

 gated hemoblast, from a yolk-sac vessel, about to separate from the 

 endothelium with which it is still in part intimately connected and from 

 which it has undoubtedly differentiated. The binucleated cell is appar- 

 ently also about to divide into two hemoblasts, thus consummating 

 the amitotic process. 



