EARLIEST BLOOD-VESSELS IN MAN 453 



the body-stalk, which he considered the anlages of the yolk-sac 

 vessels. This valuable very young embryo has been kindly given 

 to the Harvard Embryological Collection, so that I have had the 

 opportunity to study it carefully; and in spite of fact that the 

 unfortunate breakage of many sections makes complete recon- 

 structions impossible, enough can be seen to give a sure basis to 

 the following account of its vascular anlages. After the study of 

 the Minot and Grosser embryos it is not difficult to recognize that 

 the groups and rings of cells in question are in fact tangential 

 sections of the mounds of the mesothelial sheet covering the body- 

 stalk ; the lumen of the rings is the very loose mesenchymal core of 

 the stalk. As is seen in the drawing (fig. 4) the mesothelial cover- 

 ing is not complete in this embryo, but leaves large areas of the 

 surface uncovered, where the processes of the mesenchymal cells 

 of the core, with numerous intervening fibrils, form the only border 

 between the body-stalk and the coelom, as is the case along the 

 inner border of the chorion proper. Where the mesothelial cells 

 are present they occasionally project into the core of the body- 

 stalk, lining a funnel-shaped diverticulum of the coelom, as is seen 

 also in the older embiyos. Mitotic figures are frequent at such 

 points. The inner end of such projections are continued, usually 

 in a curved direction, as irregular hollow spaces (fig. 4, a), as cords 

 or as small groups of cells, without lumen. One of these cords, 

 three or four cells long, runs in the chorion parallel to its inner 

 border. It seems very probable that from the cell groups other 

 cords also run in the chorion, but the destruction of certain sections 

 makes positive proof of this impossible. One especially large and 

 well defined group of cells lies at the base of the body-stalk, in the 

 mesenchyma near the coelomic border, and directly over it is 

 found a mesothelial sheet and inpocketing; but here again the 

 condition of the sections makes it impossible to affinn that the two 

 structures are actually continuous. In the chorion there are none 

 of the large irregular spaces seen in older specimens ; but, extending 

 from the base of the body-stalk for a considerable distance, per- 

 haps a quarter of the way around the chorion, are cords of cells 

 which, on reconstruction, are found to form an irregular net, in one 

 • plane, parallel to the surface of the coelom. This net shows no 



