Hemopoiesis in the Mongoose Embryo. 299 



lium and hemoblasts, coupled with proliferative activity on the part 

 of the constituent cells, should render unnecessary any discussion as 

 to whether these clusters may not be groups of hemoblasts deposited 

 from the circulating blood and caused by pressure to adhere to the 

 vessel wall. The latter interpretation was given by Minot (is) to these 

 clusters in human and rabbit embryos, in opposition to Maximow (ll), 

 who described them in the rabbit as arising by the proliferation 

 of the endothelium. Minot's objection to Maximow's original inter- 

 pretation was based on his failure to observe either in the human or 

 the rabbit embryos any continuity between the protoplasm of the 

 endothelial cells and that of the "mesamoeboids" of the cell-clusters, 

 or any considerable number of mitotic figures in the endothelial cells 

 in the neighborhood of the clusters; "and, finally, because the endothe- 

 Ual nuclei are differentiated, while the nuclei of the cells of the clusters 

 are not differentiated." The mongoose material, however, shows a 

 definite cytoplasmic continuity between endothelial cells and the 

 proximal cells of the clusters. Moreover, while endothelial mitoses 

 are rare in the neighborhood of the clusters, various stages in the 

 amitotic division of endothehal nuclei are abundant. As in the case 

 of certain other tissues of various forms undergoing rapid growth 

 under certain conditions (e. g., blastoderm of pigeon, Patterson; tendon 

 cells of new-born mouse, Nowikoff, etc.), the endothelial cells may here 

 proliferate by the amitotic mode. Finally, the nuclear differentiation 

 of the subjacent endothelial cells is of a lesser degree, judged especially 

 by the small size of the nuclei and their spheroidal shape than that of 

 the more peripheral hemoblast transition elements, many of which 

 contain oval and kidney-shaped nuclei. Further countervailing evi- 

 dence to the position that the clusters are accretion products from the 

 circulation is detailed in another paper (9) and need not be repeated 

 here. 



The aortic endothelium produces not only cell-clusters which con- 

 tribute hemoblasts and erythroblasts, but single cells also separate 

 from the endothelium in the same manner as described for the yolk-sac. 

 Such cells may appear at any point in the wall of the mesonephrie 

 portion of the aorta, though they are more likely to be in the ventral 

 region. A group of two cells appears in the mid-dorsal line of the 

 aorta, subjacent to the notochord. Figure 7 illustrates an endothelial 

 cell along the lateral wall which has rounded up centrally and has 

 assumed hemoblast features; proximally it spreads out in delicate 

 processes for a considerable distance, and is still continuous with the 

 endothelium. Figure 8 represents an endothelial hemoblast separating 

 from the dorsal wall of the aorta. Figure 9 illustrates still another 

 method of separation; this hemoblast is directly continuous on one 

 side with the endothelium. In figure 10 is shown a 2-cell ''cluster" 

 of hemoblasts from the lateral wall of the aorta. Figure 1 1 illustrates 



