erythh()poip:si8 in yolk-sac of pk; K.MJiiivo 29;^ 



2) the sharp (leiiiurciitioii between iiie.sench^'iiia and eiitotlerin 

 in the embryos here considered. Where blood vessel and ento- 

 derm abut, the basement membrane of the entoderm and the 

 endothelial cells of the vessel are never in direct continuity 

 (fig- 2). 



The close detailed structural similarity between the meso- 

 thelial cells and the endothelial cells, and between the nuclei of 

 both and those of the mesenchyma, was noted above (figs. 13 

 to 20) . The criteria which Clark (4) applied in the chick embryo 

 for the differentiation between endothelial nuclei and mesench}-- 

 mal nuclei are inapphcable to the yolk-sac mesenchyma of pig 

 embryos of the 5 to 15 mm. stages of development. Number of 

 nucleoli, character of nucleolar contour, and depth of tingibility 

 of nucleoli are not features by which mesenchyma nuclei can be 

 differentiated from endothelial nuclei. These are marks which 

 characterize different cells (probably representing difTerent 

 functional phases) of mesenchyma, mesothelium and endothelium 

 ahke. 



The moriDhologic evidence seems to force the conclusion that 

 endothelium and mesothelium are both very similar difTerenti- 

 ation products of mesenchyma, the factor chiefly operative in 

 the differentiation being the mechanical factor of pressure, as 

 maintained by Huntington (9), Schulte (21) and others. The 

 pressure exerted upon the mesothelium operates from the rela- 

 tively more rapidly growing entoderm; that upon the endo- 

 thelium from the confined blood cells and plasma. The further 

 fact that haemoblasts arise from l^oth mesenchyma and endo- 

 thelium supports the conclusion of their essential identity. 



If the above is correct then one would expect that the meso- 

 thelium also could produce haemoblasts. My material yields 

 no data in support of this view. Indeed very careful study of 

 the mesotlielium both of the yolk-sac and the chorion, with this 

 point in view, gave only negative evidence. The mesothelial 

 cells proliferate both mitotically and apparently amitotically 

 but nothing appears closely similar to the phenomena described 

 by Bremer (3) for the chorion of the young human embryo, 

 where the mesothelium is said to invaginate the underlying 



THE AMERICAN- JOUKXAL OE A.WTOMV, VOL. 19, NO. 2 



