38 VASCULOGENESIS IN THE CAT 



Delafield's haematoxylin and eosin, or Heidenliain's haematoxy- 

 lin. Reconstructions were made by the Born method in four 

 cases; for others, in small easily defined areas, where conditions 

 were simple, the quicker but less satisfactory graphic method 

 was employed. 



For convenience of analysis, the data are presented under two 

 headings : 



1. The early formation of vessels beneath the ectoderm, with 

 special reference to the umbilical vein. 



2. The early formation of vessels above the entoderm, es- 

 pecially the omphalo-mesenteric plexus and the aorta. 



THE UMBILICAL VEIN 



The early stages in the development of this vessel afford an 

 opportunity to study the formation of endothelium in a cir- 

 cumscribed region and under comparatively simple conditions. 

 It appears first near the lateral margin of the incipient intra- 

 embryonic coelom and is separated from the entoderm by the 

 whole thickness of the as yet imperfectly cleft mesoderm. The 

 inner germ layer patently is excluded from direct participation 

 in the formation of this endothelium, and its appearance, ante- 

 dating the establishment of continuity with any derivative of 

 the splanchno-pleuric plexus, must entail the acceptance of its 

 origin in situ. I am not aware that the independence of the um- 

 bilical vein and the associated hypectodermal vessels has ever 

 been impugned. It was recognized by His and more recently 

 has been considered by Mollier. In the cat it retains its iso- 

 lation to the stage of 14 somites. It would seem, however, that 

 its miportance from the theoretical standpoint has not been uni- 

 versally appreciated. The position of the umbilical vein at the 

 amnio-embryonic angle suggests that the vascular potentiali- 

 ties of intra- and extra-embryonic portions of the germ laj^ers 

 are not, of necessity, totally disparate. The extent of the vessel, 

 its comprehensive drainage area, its high physiological import- 

 ance, seem to requii'e that its mode of formation be respected 

 in general interpretations of vascular development. A doctrine 

 such as that of the origin of endothelium from entoderm, or by 

 the growthJ|of^anlages|formed in the splanchnopleure alone, neg- 



