304 Papers from the Department of Marine Biology. 



tory process, and in the mongoose embryo is relatively much less highly 

 differentiated, as indicated by the still cuboidal character of its epithe- 

 lium. This fact, in view of the mesenchymal origin of this portion of 

 the tubule, may underlie the possibility of a capsular cell to separate 

 from its epithelium and become a free intratubular element, perhaps 

 a macrophage. 



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. 



DISCUSSION. 



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 



