ENDOTHELIAL HEMOGENESIS IN INTRAEMBRYONIC MESENCHYMA.
Search for evidence in support of this hypothesis is abundantly rewarded. Figure 13 represents a blood-channel which has just formed out of the pericerebral mesenchyma. The lumen of the channel still contains strands of dissolving mesenchyma. The four cells present represent four different stages in the transformation of a mesenchyme cell into a hemoblast, leading through an endothelial cell. Cell a is still typically mesenchymal, but is clearly in process of transformation into an endothelial cell, for two of its processes already form portions of the wall continuous with that portion formed by the typical endothelial cells b and c. The original endothelial cell now represented by c and d apparently divided its nucleus, thus forming a binucleated cell; the distal portion of this cell underwent differentiation into a typical hemoblast, while the proximal end represents a transition stage between a typical endothelial cell and a true hemoblast. In this single section of a small area, including four nuclei, the whole process of mesenchymal and endothelial hemogenesis may be seen in abbre viated form; and the process is essentially identical with the manner in which mesenchyma and endothelium function hemogenically in the yolk-sac wall.
Figure 14 is of a cross-section of a capillary vessel including a single endothelial cell, and with a diameter just sufficient to accommodate a single erythrocyte. Since this section is from the same vascularizing area as that of figure 13, the most probable interpretation that sug gests itself is in terms of cells c and d of figure 13. If cell c, still con tinuous with the general mesenchyma, had differentiated into an endothelial cell, and d into an erythrocyte, we would have exactly the condition shown in figure 14. The endothelial wall here is still continuous with the mesenchyma, as shown by the strands of proto plasm at the upper pole. If the cytoplasm immediately enveloping the second nucleus of an originally binucleated cell differentiated into hemoglobin-containing protoplasm and thus into an erythrocyte, while the more peripheral cytoplasm remaining in functional association with the second nucleus differentiated into endothelial protoplasm, the 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 in 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 nuclearreticulum; moreover, the cytoplasm invariably has a slightly lees 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.