410 MINOT. [Vot. II. 
villi, Vz, Vz, of the frondosum. In the angle, Sz, where the two 
membranes first separate there are very few villi, so that there 
is a comparatively clear space left, which is known as the cir- 
cular sinus. It is not, as some of the older writers have believed, 
a distinct vessel, nor does it extend as a clear space completely 
Cut 26.— Placenta at full term, doubly injected by Dr. H. P. Quincy, to show 
the distribution of blood-vessels upon the surface; the arteries are drawn light; the 
veins dark. XX 0.7 diams. 
around the placenta ; but, on the contrary, it is interrupted here 
and there by an ingrowth of villi. In the cut, the spaces oc- 
cupied of maternal blood are left white; the foetal blood-vessels 
are drawn black. 
The chorionic circulation is complete in itself. The single 
vein and the two arteries of the umbilical cord spread out over 
