292 H. E. JORDAN 



Neither brush borders nor terminal bars occur on these cells. 

 Such structures have been described for the entodermal cells 

 of the human yolk-sac by Branca (2). However in my own 

 specimens of the human yolk-sac (10, 11, and 12), I could never 

 convince myself of the presence of these structures. Lewis (15) 

 likewise was unable to find them in human yolk-sacs of similar 

 ages. 



The entodermal cells in the yolk-sac of the 10 mm. pig embryo 

 are undergoing extensive mitotic proliferation. This fact, 

 viewed in conjunction with the good cytologic preservation of 

 the cells as indicated primarily by the abundance and character 

 of the presecretion filaments, should remove all doubt as to the 

 normal and healthy condition of these specimens. 



Not a single entodermal cell can be found in process of amitotic 

 division. Nor are any of these cells binucleated. This is signi- 

 ficant in view of the fact that all types of cells in the mesenchyma 

 and its derivatives show abundant examples which admit of 

 interpretation in terms of direct division. 



2) Haemopoietic. The first question under this caption con- 

 cerns the origin of the angioblast. The term 'angioblast' is 

 employed here to designate the original anlage of the vascular 

 tissue in the yolk-sac. It is obvious that no sharp line can be 

 drawn between original and secondary angioblast. Suffice it 

 to note that angioblast is still in process of formation in the 

 yolk-sac of the 10 mm. embryo. No information accrues from 

 this study touching the question of the origin of the first mass of 

 vascular anlages. Once formed, angioblast can of course spread 

 by process of growth. However, it is also still being added to 

 by previously discrete moieties. If these additions can be 

 shown to be made from the mesenchyma, it would seem to afford 

 a strong presumption against the derivation of the original 

 angioblast from entoderm [Minot (19)]. Such anlages do arise 

 by differentiation within the mesenchyma in the shape of dis- 

 crete blood-islands, as described above. I conclude for the 

 mesenchymal origin of the angioblast on the basis, then, mainly 

 of these two observations: 1) the common origin of endothe- 

 lium and haemoblasts, as described above, from niesenchyma; 



