448 >. JOHN LEWI« BREMER 



It was with the extension theory in mind, therefore, as opposed 

 to the ideas of many authors that endotheUal cells arise in situ 

 from mesenchymal cells in various parts of the body, that this 

 investigation was undertaken to trace the origin of the vascular 

 endothelium in man, and to locate the anlages of the earliest 

 blood-vessels. Heretofore it has been generally supposed that in 

 man, as in other vertebrates, the first endothelial anlages appear 

 as the angioblast in the yolk-sac, between the entoderm and the 

 splanchnic mesoderm. Opposing views have been expressed as to 

 the part the two layers play in the formation of the blood-islands, 

 which since the time of His have been recognized as, in part at 

 least, the fore-runners of both blood corpuscles and endothelium; 

 some authors maintain that the vascular cells are derived from the 

 mesoderm, others that they are metamorphosed entodermal cells. 

 The early vascularization of the chorion and body-stalk in man, 

 before the presence of intra-embryonic vessels, and before the 

 formation of somites, has long been noted, but usually considered 

 as evidence of a very rapid growth from the yolk-sac anlages. 



In young human embryos, with the medullary plate of about 

 1 mm. in length, and with recognizable yolk-sac vessels, several 

 authors have described, in the chorion, chorionic villi, and body- 

 stalk, irregular spaces in the mesoderm, some lined with endothe- 

 lium, some without definite lining; and recently Grosser^ and 

 Debeyre'' have separately mentioned, beside the irregular spaces, 

 true blood-islands in the body-stalk, near the allantois. 



In still younger embryos, with no vessels or blood-islands in 

 the yolk-sac, Jung^ and later Herzog^ have called attention to 

 accumulations of cells, sometimes arranged around a lumen, 

 situated at the periphery of the mesoderm of the yolk-sac and 

 body-stalk, bordering the extra-embryonic coelom. Jung's de- 

 scription of these is as follows, p. 104: 



Auch finden sich (in the embryonic mesoderm) keine sicher zu 

 erkennenden Gefasse. Allerdings sieht man an der in die Exocoelom- 

 hohle hineinragenden Peripherie des Mesoblastes mehrfache Zellan- 



2 Grosser, O. Anat. Heftc. Bd. 47, p. 649, 1913. 



' Debeyre, A. Journal de rAiiat. et de la Physiologic. Vol. 48, p. 44S, 1912. 



* Jung. Miinchen. mod. Wochenschr. Jahrg. 54, p. 1343, 1907. 



* Herzog, M. Am. Jour. Anat. Vol. 9, p. 3G1, 1909. 



