HUMAN EMBRYO IN THE NINTH STAGE. 



141 



the arterial process, Ht, is seen on the right. The heart is continued forward by 

 the large aorta (Fig. 74, A ), which gives off five branches on each side of the neck. 

 These branches again unite on the dorsal side and run backward to unite with 

 the fellow-stem, and so form the single median dorsal aorta, Ao, which runs way 

 back and terminates in two branches, An, which , 

 curving round, pass out through the body-stalk 

 and supply the circulation of the chorion. The 

 five branches in the neck are known as the 

 aortic arches. The column around each branch 

 constitutes the so-called branchial arch. Each 

 branchial arch is further marked out by the gill 

 cleft in front of it and behind it, as shown in 

 figure 74. The reconstruction of the third em- 

 bryo in the side view (Fig. 78) affords further 

 information concerning the disposition of the 

 heart and the large blood-vessels. The veins, 

 as is there shown, are (i) the anterior cardinals, 

 /, which are often referred to as the jugular veins, 

 although they are not identical with the j ugulars 

 of the adult; (2) the cardinals (compare Fig. 74, 

 Car), or posterior cardinals as they are already 

 called; the posterior and anterior cardinals, 

 coming from the caudal and cephalic regions re- 

 spectively, unite to form a single transverse 

 stem, the duct of Cuvier, D. C. (the posterior 

 cardinals receive their blood chiefly from the 

 Wolffian bodies, and later undergo complicated 

 metamorphoses) ; (3) the large umbilical or 

 allantoic veins, Al.v, which pass up from the 

 chorion through the body-stalk into the soma- 

 topleure until at the level of the septum trans- 

 versum, above the liver, Li, they empty into 

 the duct of Cuvier; (4) the omphalo-mesa- 

 raic or vitelline veins, om, which come up from 



the yolk-sac on either side and meet the ducts of Cuvier at the venous end of 

 the heart. This figure also shows the disposition of the aortic arches and an 

 early stage of the primitive internal carotid artery, car. The muscular, but 

 not the endothelial, heart is represented in the reconstruction. 



Ot. 



Fig. 78.— Reconstruction of the An- 

 atomy OF THE Human Embryo of 

 4.2 MM. Shown in Fig. 74. 

 Otocyst. y, Jugular vein. car. 

 Carotid artery. /, First aortic arch. 

 Au, Auricle. Ven, Ventricle. Li, 

 Liver. om, Omphalo-mesaraic vein. 

 j4/, Allantoic diverticulum. ^ri. 

 Allantoic artery. ^4/. v. Allantoic vein. 

 Am, Origin of the amnion. D.C, 

 Duct of Cwwi&r.— {After JV. His.) 



