138 



THE HUMAN EMBRYO. 



off two vessels on each side, the first and second aortic arches, which pass 

 around the pharynx and unite again upon its dorsal side, and then, as the 

 aortae, Ao, descend along the ventral side of the nervous system, soon 

 uniting in the median line to form the single dorsal aorta which runs along 

 nearly to the tail of the embryo, where it forks; and its branches, passing one on 

 each side of the intestinal canal, enter the body-stalk and run to the chorion, 

 where they branch out. Behind the pharynx the entodermal canal merges into 



the cavity of the yolk-sac, Yk, and then beyond 

 the yolk-sac extends again into the tail of the 

 embryo, forming an expansion there which is 

 known as the bursa. From the under side of the 

 bursa runs out the allantoic diverticulum, ,4//, 

 which extends as a narrow tube of entoderm 

 through the allantoic stalk to the level of the 

 chorion, where it ends blindly. The pericar- 

 dial chamber on its caudal side is bounded by 

 the septum transversum, in which we find the 

 anlage of the liver, Li, already present, and 

 through which, on either side, the great vein 

 from the yolk-sac, the omphalo-mesaraic or 

 vitelline vein, passes to the heart. Of the 

 veins of the embryo only the umbilical, uv, is 

 shown in the figure. This vein gathers the 

 vessels from the chorion, passes through the 

 body-stalk, then runs in the somatopleure of 

 the embryo to join the omphalo-mesaraic vein 

 and enter the heart. In the figure only the 

 general course of the vein is indicated. The 

 fact that it is situated in the somatopleure could 

 not well be shown. 



Fig. 74. — Reconstruction of the 

 Anatomy of THE EMBRYO of 2.6 

 mm. in Fig. 72. 



op, Optic vesicle. A, Ventral aorta. On/, 

 Omphalo-mesaraic vein. Au, Umbili- 

 cal artery. All, Allantois. Car, 

 Cardinal vein. Vli, Right umbilical 

 vein. Ao, Dorsal aorta. Jg, Anterior 

 cardinal vein, ot, Otocyst. — {After 

 W. His.) 



Human Embryo in the Ninth Stage with Three 

 Gill Clefts Showing Externally. 



Our knowledge of this stage is quite good. The described embryos vary in 

 length from 2.6 to 4.2 mm. The chorionic vesicles are about 10 mm. in diameter, 

 varying according to the size of the embryo. Figure 73 and figure 75 represent 

 two embryos of this stage, the latter being probably somewhat more advanced. 

 The back of the embryo is normally (or at least usually) convex. The head is 

 bent to one side, usually to the right, and the tail to the other, the whole embryo 



