90 JOSE F. NONIDEZ 



of cells similar to the hemocytoblasts in the endothelium of the 

 blood-vessels. The splitting off of cells from the endothelial 

 lining of the latter is a well-established fact in the embryos of 

 several vertebrates. The proliferation of the endothelium in 

 certain portions of the aorta has been described in the embryos 

 of reptiles (Danchakoff, '08 c; Jordan, '16, '17), birds (Dancha- 

 koff, '08 b), and mammals (Maximow, '08; Emmel, '15. '16; 

 Jordan, '16, '17). More recently, McJunkin ('19) has shown 

 that this proliferation also takes place in blood-vessels of certain 

 organs in the adult mammal. 



This proliferation has been generally interpreted as the forma- 

 tion of primitive blood-cells or hemocytoblasts which will even- 

 tually become erythroblasts in the blood-stream. McJunkin, on 

 the other hand, has been able to prove by injecting lampblack 

 into the veins that the endothelium is endowed with an active 

 phagocytic power and that the cells produced by its proliferation 

 become the phagocytic mononuclear elements normally found in 

 the blood. 



The formation of large cells by isolation of endothelial cells 

 was taking place in all the embryos studied by the writer, irre- 

 spective of their age. However, it was more marked in early 

 embryos than in those ready to hatch. 



The endothelium of the veins is made up of flattened cells, 

 w^hile that in the arteries is thicker and contains oval or round- 

 ish nuclei, scattered in a continuous layer of cytoplasm, sur- 

 rounded by elongated mesenchyme cells (fig. 7,6), The cells in 

 both kinds of endothehum are not uniform in size. Here and 

 there cells may be seen which possess an oval, nearly round 

 nucleus of larger size than in the other endothelial elements. 

 Sometimes these large nuclei protrude either into the lumen of 

 the vessel or toward the mesenchyme. During the enlargement 

 of the nucleus the cytoplasm becomes vacuolated. The karyo- 

 some increases in size, while the coarse chromatin network gives 

 place to a delicate reticulum. An early stage in this transfor- 

 mation has been represented in figure 3, e'. Figures 4 and 5 

 show later stages. 



