CELL-CLUSTERS OF THE PERICARDIUM.
In view of what was said above, regarding the close primary relation ship between mesenchyme, endothelium, and mesothelium, it seemed reasonable to expect that mesothelium also might to some extent differentiate hemoblasts, possibly leucoblasts. Careful examination of the pericardium reveals patches of proliferating and differentiating mesothelial cells (fig. 19) very similar to the cell-clusters described for the aorta (compare figs. 6 and 19). These clusters appear both on the visceral and parietal pericardium; and groups of similar cells (syncytia) occur also in the pericardial cavity (fig. 21). The latter most probably are separated portions of the pericardial cell-clusters. Also, single cells may apparently separate (fig. 20) in a manner very similar to that described for the aortic endothelium. All of these cells derived from the pericardium again have many features in common with hemoblasts. It seems quite probable that in the case of the peri cardium we are dealing with sources of extravascular hemoblasts (leucoblasts) from the coelomic epithelium. In the adult it is known that phagocytic leucocytes (macrophages) may arise from the peri toneum, and the same condition may well prevail also in the embryo. In certain experimentally produced teleost hybrids Reagan (Is) also describes the transformation of mesothelium into erythrocytes.
The mesenchyma is a fundamental hemogenic tissue. Among its proximate differentiation products are endothelium and mesothelium. These represent originally mechanical rather than functional differen tiation products. As such they might be expected to have retained the original differentiative capacity of the parent mesenchyma. The histologic facts above outlined seem to prove that such is actually the case. All the facts detailed are perfectly consistent with this inter pretation.
The possibility of origin of hemoblasts from endothelium has become associated with the monophyletic theory of blood-cell origin and con tradicts the idea of strict specificity of endothelium demanded by the angioblast theory of intra-embryonic vascularization of His. The op posed ideas of the origin and genetic relationship of the erythrocytes and the leucocytes are expressed in the monophyletic and diphyletic (polyphyletic) theories. It needs to be emphasized that the formulated processes are still largely in theory. The bulk of the best evidence,
however, seems to be in favor of a monophyletic genetic method. The above observations are in accord with, and in a degree a support to, the monophyletic theory. The evidence shows that young endothe lium and mesothelium are only slightly altered (mechanically) mesen chyma, and that the two tissues may in early stages give rise to hemoblasts. In the young embryo these hemoblasts differentiate into erythrocytes. The primitive leucocyte ("lymphocyte") is represented by this hemoblast; in later stages this hemoblast (leucoblast) may differentiate into the various types of granulocytes.
The monophyletic theory rests, then, essentially upon these three facts: (1) the origin of the early hemoblast from mesenchyme, now no longer disputed; (2) the differentiation of these hemoblasts into erythrocytes, also no longer disputed; (3) the identity, inferred from a very close structural similarity, of hemoblast and lymphocyte, which latter is regarded as the progenitor of the granulocytes. The third point is the one on which the discussion and dissension center. Grant ing the verity of the third point, it follows that the primitive leucocytes appear before the erythrocytes. Such an ontogenetic sequence is in accord with the principle of progressive differentiation and with the phylogenetic history—a very significant fact.
The sharpest attack upon the monophyletic theory of recent years has been made by Stockard (21), on the basis of his observations on Fundu/us embryos; but his more definite and positive facts, namely, the origin of the erythroblasts, "leucoblasts," and endothelium from mesenchyma as seen in the living embryo are actually in accord with the monophyletic theory and furnish strong additional support. The observation upon which he takes foothold to launch his attack, namely, the segregation of erythropoietic and leucopoietic foci, is open to the criticism of misinterpretation with respect to the latter. At any rate, his illustrations of primitive "leucocytes" do not inspire confidence; the karyorrhexis shown would alone make one suspicious of degenera tive phenomena, and their general appearance raises the question whether they are not actually (as also Reagan (16) suggests) degenera ting erythroblasts suffering nuclear changes and hemolysis.