282 H. E. JORDAN 



of the endothelial cells is like that of these cells. Moreover, 

 the endothelial cells appear to be in direct syncytial continuity 

 with the mesenchyma. Many are in mitosis, and occasional 

 nuclei appear to be dividing amitotically. It seems most prob- 

 able that they are actually mesenchymal cells modified in shape 

 by the pressure of the confined blood stream. Endothelial 

 cells which lie next the entoderm are sharply separated there- 

 from (figs. 29 and 30). The entodermal cells rest upon a deli- 

 cate but distinct basement membrane, with which the endothe- 

 lium is not in structural continuity (fig. 2). The vascular an- 

 lages (angioblast) are at certain points in direct continuity with 

 the mesenchyme, but are sharply demarked from the entoderm 

 (fig. 29). There is no evidence here that the angioblast has any 

 direct genetic relationship to the entoderm; all the available 

 morphologic data are opposed to the idea of such a relationship. 

 The endothelium includes, however, numerous cells which 

 may be arranged into a complete series connecting the above 

 described endothelial cell with a haemoblast (figs. 4, 13, 14, 15 

 and 16). The transition steps consist of a progressive rounding 

 up of the nucleus and a gathering of the cytoplasm around it. 

 At the same time the nucleus enlarges and the cytoplasm ap- 

 pears to increase in amount. Moreover the cytoplasm becomes 

 more highly basophilic and appears finely granular. The cell 

 as a whole, of fusiform shape, becomes progressively shorter 

 and finally separates from the endothelial wall either as a short 

 fusiform cell, or frequently as a spherical cell flattened at its 

 proximal pole and drawn out laterally into delicate processes 

 which gradually separate from the vessel wall (figs. 5 and 6). 

 Such cells may even become multinucleated before separation 

 (figs. 8 and 9), and undergo cytoplasmic differentiation, even 

 elaborating haemoglobin, as will be described below. The 

 multinuclear condition appears to be the result of amitotic 

 nuclear division (figs. 9 and 35). The observation of the differ- 

 entiation of endothelial cells into haemoblasts is of cardinal 

 importance, and will be more fully discussed in a later section. 



