EARLIEST BLOOD-VESSELS IN MAN 451 



The net in the chorion can be traced into many of the chorionic 

 viUi, and into the body-stalk as shown in figure 1 ; but in the body- 

 stalk there is a great difference in the character of the elements 

 composing it. The shreds or cords lying in relatively large meso- 

 dermal clefts open out into large endothelial angiocysts, from 

 which cords again lead directly into large spaces in the mesodenn, 

 without separate endothelial lining, or else end abruptly. I shall 

 speak later of the histological appearances of these spaces, and for 

 the present call them the "unlined spaces." Other cords, not 

 connected with the net, can be traced further toward the embryonic 

 shield, along the allantois, but there is no direct connection with 

 the yolk-sac net. The unlined spaces and the terminal cords are 

 always near the surface of the body-stalk. 



Grosser first called special attention to the epithelial layer of 

 mesodermal cells, the mesothelium, which forms the coelomic 

 surface of the yolk-sac and of the body-stalk in his 1 mm. embryo, 

 ending abruptly at the junction of body-stalk and chorion. More- 

 over, he pointed out that this mesothelium, instead of forming a 

 smooth surface, dipped in irregularly, giving in sections the 

 appearance of festoons. This disposition of the mesothelium is 

 found in all the young human embryos which I have studied, 

 except the Bryce-Teacher ovum, in which there is as yet no 

 coelomic cavity. A model of the body-stalk of the Minot embryo, 

 in which the mesenchymal core is left out, leaving the mesothe- 

 lium as a sheet of wax, enclosing the vascular net, shows the outer 

 or coelomic surface formed of rounded ridges and irregular 

 mounds, between which are deep clefts or depressions; on the 

 inner surface of the mesothelium these clefts are seen as sharp 

 ridges or pointed funnel-shaped ingi*owths. The model shows 

 clearly that these ridges and ingrowths touch here and there, and 

 are apparently continuous with, either the angioblast cords or 

 the unlined spaces of the vascular net. 



Through the great generosity of Professor Grosser I was enabled 

 to repeat my reconstructions from his embryo, which is faultlessly 

 preserved, stained, and sectioned. The results correspond with 

 the findings in the Minot embryo, except that in Grosser's embryo 

 it is possible to trace the net from the chorion through the body- 



THE AMERICAN JOURNAL OF ANATOMY, VOL. 16, NO. 4 



