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 mesonephric 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 11 illustrates a 3-cell group, located just to the side of the mid-ventral line. This group is still partially attached dorsally to the endothelium and may be regarded as a later step in the process of endothelial hemoblast formation from the yolk-sac shown in figure 4.
In figure 12 is illustrated a 4-cell group from the ventro-lateral wall. Three cells are arranged in horizontal series and jut out directly into the lumen of the aorta; the three cells, moreover, are at successively later stages of differentiation. The proximal cell has intermediate features between an endothelial cell and a hemoblast (a); the distal cell has erythroblast characteristics and is very similar to some of the adjacent aortic erythroblasts (c). Close to the proximal cell of this group the endothelium is thickened and contains an increased number of nuclei, similar to the attached pole of the multicellular clusters. For purposes of comparison, both dimensional and structural, a few of the adjacent cells of the lumen are shown. A series of successive stages of differentiation is indicated by the letters a to d; a is a typical hemo blast ; b, a typical megaloblast; c, typical normoblasts, and d, a late erythroblast (normoblast) stage.
From the foregoing it seems clear that the endothelium of the aorta in the mesonephric portion may proliferate locally in the ventral or latero-ventral or even dorsal wall, giving rise ventrally to cell-clusters which grow in size, the constituent cells of which undergo a coincident differentiation into hemoblasts and young erythroblasts. Similar cell
clusters of larger size are nowhere to be found in the yolk-sac vessels; such are apparently not formed during the yolk-sac homopoiesis; this fact, if countervailing evidence were needed, would sufficiently dis credit any interpretation of the aortic cell-clusters in terms of masses of hemoblasts carried to the aorta by the blood and caused to adhere to the ventral wall by reason of pressure and the adhesive properties of their cytoplasm. Coincident with the production of hemoblasts in the aorta through a cell-cluster phase, the endothelium of the same portion of the aorta at any point, though more generally ventrally, may pro duce also, through the transformation of individual endothelial cells, hemoblasts in an exactly identical manner as above described for the vessels of the yolk-sac. These two processes—hemoblast production by endothelial cell-clusters and by transformation of individual endo thelial cells—are essentially similar; the difference involved is one of the degree of endothelial proliferative and differentiative capacity, not one of kind. A few of the cells of the larger cluster appear to have also phagocytic properties.
The question arises as to why the hemogenic activity of the aortic endothelium is generally limited to the ventral area of the mesonephric portion. Why do the larger clusters form only ventrally? This portion differs from other portions in that this is the region along which the larger ventral arterial blood-vessels to the abdominal viscera migrate caudally to their definitive location, the process involving a shifting of the larger stems (celiac artery, superior and inferior mesen teric arteries) from higher to lower ventral connecting segmental twigs. This region would seem to contain a less highly differentiated type of endothelium—that is, one of greater proliferative capacity, providing thus for an unequal growth between the dorsal and ventral walls. Herein may possibly reside the capacity for some of these endothelial cells to differentiate into hemoblasts. If this interpretation is correct, then the younger endothelium of the intra-embryonic vessels should also be able to transform into hemoblasts. Such youngest type of endothelium should be present in the numerous small blood-channels in the anterior head region next the brain.