302 Papers from the Department of Marine Biology. 



actual condition represented would be realized. A similar complex 

 of erythrocyte and encapsulating endothelium has been seen also in 

 a larger endothelium-lined vascular space. 



In figure 15 is shown a hemoblast separating from, but still in cyto- 

 plasmic continuity with, the endothelium of a small pericerebral blood- 

 space. 



The suggestion has been made that what is interpreted as an endothe- 

 lial cell rounding up and differentiating into a hemoblast is hi reality 

 only an endothelial cell in preparation for mitosis. This is a plausible 

 objection to the interpretation above given and must be met. I have 

 studied many dividing endothelial cells in the pericerebral mesenchyma 

 with this point in mind. It is a fact that both mesenchymal and endo- 

 thelial cells acquire a deeper-staining cytoplasm just before mitosis; 

 but a mesenchymal cell divides without rounding up and has a rela- 

 tively smaller nucleus and a less coarse and chromatic nuclear reticulum; 

 moreover, the cytoplasm invariably has a slightly less deep-staining 

 reaction. In these preparations the color of the dividing mesenchymal 

 cell is a deep pink, that of the differentiating hemoblast a brownish 

 or bluish red. The same color difference obtains between the dividing 

 and the hemogenic endothelial cell. Moreover, the intravascularly 

 dividing endothelial cell contracts only relatively slightly, thus becom- 

 ing a more or less stout spindle-shaped cell (figs. 16 and 18), but does 

 not round up in typical hemoblast fashion. In figure 18 is shown a 

 long dividing endothelial cell at the late anaphase stage; this more 

 probably represents a lateral sprout from the main vessel which is cut 

 in cross-section, but it shows the typical stout-spindle character of the 

 dividing endothelial cell in contrast to the shorter and more spheroidal 

 condition of the differentiating endothelial hemoblast. 



The endothelium of these pericerebral vascular channels has con- 

 siderable proliferative capacity, both internal and external (fig. 17). 

 Extravascularly dividing endothelial cells round up more like hemo- 

 blasts and may indeed be progenitors of extra vascular hemoblasts ; but 

 certain minor morphologic differential characters suggest that we may 

 here be possibly dealing with simply slightly modified proliferating 

 mesenchyma. 



It appears, then, that young endothelium, whether in the yolk-sac, 

 ventral area of the mesonephric portion of the aorta, or in the intra- 

 embryonic mesenchyma (pericerebral), functions in the formation of 

 hemoblasts, and in an essentially similar fashion. 1 



lr rhe following recent observations have a special bearing upon this point: Huntington (Amer. 

 Jour. Anat., vol. 16, p. 290, 1916) records for certain mammalian embryos that "other red cells 

 develop by the direct transformation of the border endothelial cells lining the early lymphatic 

 spaces." Reagan (Anat. Rec., vol. 10, p. Ill, 1915) states that in "chemically treated teleost 

 embryos" (Fundulus heteroclitus) , "both the endocardium and myocardium have in this region 

 become completely transformed into strongly eosinophilous erythroblasts." Certain investigators 

 have described peculiar mesodermal "fiber cells" in Limulus and in spiders and scorpions which 

 on the one hand transform into striped muscle, and on the other into "a special type of blood- 

 corpuscle" or perhaps true blood-cells. (Patten, The Evolution of the Vertebrates and their Kin, 

 p. 235.) 



