REPRODUCTION. : 83 
The allantois is compressed and devoid of a cavity, but abun- 
dantly supplied with blood-vessels by the allantoic arteries and 
Fie. 91.—Vascular system of the human foetus, represented diagrammatically (Huxley). 
H, heart ; 7'A, aortic trunk ; c, common carotid artery ; c’, external carotid artery ; c’’, 
internal carotid artery; s, subclavian artery ; v, vertebral artery; 1, 2, 3, 4, 5, i 
arches ; A’, dorsal aorta; 0, omphalo-mesenteric artery ; dv, vitelline duct ; 0’, omphalo- 
mesenteric vein ; v’, umbilical vesicle ; vp, portal vein ; Bs liver ; uv, wu, umbilical arteries ; 
w",w’, their endings in the placenta ; wu’, umbilical vein ; Dv, ductus venosus ; vh, hepatic 
vein ; cv, inferior vena cava; vil, iliac veins; az, vena azygos; vc’, posterior cardinal 
vein ; DC, duct of Cuvier ; P, lung. 
veins, which of course terminate in capillaries in the villi. 
.Compare the whole series of figures. 
Fie. 92.—Human ova during early stages of development. A and B, front and side view of an 
ovum Saupe oes to be about thirteen days old ; e, embryonic area (Quain, after Reichert) ; 
fo) 
C, ovum of four to five weeks, showing the general structure of the ovum before formation 
of the placenta. Part of the wall of the ovum is removed to show the embryo in position 
(after Allen Thomson). 
At this stage the condition of the chorion suggests the type 
of the diffuse placenta which is normal for certain groups of 
animals, as will presently be learned. 
The subsequent changes are much better understood, for 
