ERYTHROPOIESIS IN YOLK-SAC OF PIG EMBRYO 281 



the endothelium (figs. 13 to 18). In the Giemsa stain the nuclei 

 of these three tissues are similarly colored bluish orange, while 

 the cytoplasm stains a lighter blue. Occasional cells may be 

 seen in mitosis, but there is no clear evidence to indicate that 

 their prohferation products may differentiate into haemoblasts. 

 The proliferation is most probably related only to the extension 

 of the mesothelial covering. Certain cells, however, are more or 

 less rounded, simulating early stages in the formation of haemo- 

 blasts from endothelium (fig. 20) . 



c. The mesenchyma 



The mesenchyma is of very variable amount in different 

 portions of the wall (figs. 2, 4, 29 and 30) ; in certain regions it is 

 so sparse as to be barely discernible between the entoderm and 

 the mesothelium ; in other regions it may greatly exceed in width 

 that of the tallest portions of the entoderm. It is a loose-meshed 

 syncytium containing numerous spaces and occasional small 

 blood islands, and larger and smaller blood vessels or sinusoids 

 (figs. 29 and 30). Around certain spaces the mesenchymal 

 cells may become arranged so as to very closely simulate endo- 

 thelial cells. Indeed it seems impossible to differentiate between 

 such a cell and certain endothelial cells from blood-cell-contain- 

 ing channels. It seems difficult to avoid the conclusion that 

 endothelium is thus differentiated from the mesenchyma, the 

 differentiation depending here as in the case of the structurally 

 apparently identical mesothelium, upon the mechanical factor 

 of pressure (fig. 29). Many of the mesenchymal nuclei are in 

 some phase of mitosis, and occasional nuclei appear to be divid- 

 ing amitotically. 



d. The endothelium 



The cells lining blood-cell-containing channels are flattened 

 elements, of fusiform shape in sections. The commonest type 

 of cell contains a vesicular oval nucleus, practically identical 

 with that of the mesenchyma and the mesothelium (figs. 4, 13 

 and 29) ; and also the delicate reticular cytoplasmic structure 



