146 



THE HUMAN EMBRYO. 



Ot 



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

 back and terminates in two branches, Au, 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 con- 

 stitutes 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 90. 

 The reconstruction of the third embryo in the side 

 view (Fig. 93) affords further information concern- 

 ing the disposition of the heart and the large 

 blood-vessels. The veins, as is there shown, are 

 (i) the anterior cardinals, J, which are often re- 

 ferred to as the jugular veins, although they are 

 not identical with the jugulars of the adult; (2) the 

 posterior cardinals (compare Fig. 90, Car}; the 

 posterior and anterior cardinals, coming from the 

 caudal and cephalic regions, respectively, unite to 

 form a single transverse stem, the common cardinal, 

 D.C (the posterior cardinals receive their blood 

 chiefly from the Wolfiian bodies, and later undergo 

 complicated metamorphoses); (3) the large umbili- 

 cal or allantoic veins, Al.v, which pass up from 

 the chorion through the body-stalk into the somato- 

 pleure until at the level of the septum trans- 

 versum, above the liver, Li, they empty into the 

 common cardinal; (4) the omphalo-mesaraic or 

 vitelline veins, om, which corne up from the yolk- 

 sac on either side and meet the common cardi- 

 nals at the venous end of the heart. This figure 

 also shows the disposition of the aortic arches and 

 Art, Allantoic artery. Al.v, Allan- an early stage of the primitive internal carotid 



FIG. 93. RECONSTRUCTION OF THE AN- 

 ATOMY OF THE HUMAN EMBRYO OF 

 4 . 2 MM. SHOWN IN FIGURE 89. 



Ot, Otocyst. J, Anterior cardinal vein. 

 car, Carotid artery. 7, First aortic 

 arch. Au, Auricle. Ven, Ventricle. 

 Li, Liver, om, Omphalo-mesaraic 

 vein. Al, Allantoic diverticulum. 



toic vein. Am, Origin of the amnion. 

 D.C, Common cardinal. (After W. 

 His.) 



artery, car. The muscular, but not the endothelial, 

 heart is represented in the reconstruction. 



Human Embryo in the Tenth 'Stage with Four Gill-clefts Showing Externally. 

 Few embryos belonging to this stage have been obtained. The one shown in 

 figure 94 was carefully studied and described by W. His. Its probable age is 

 twenty-three days. The embryo forms almost a complete circle, the tail being 

 close to the head. The limb-buds have appeared. . The heart is large and causes 

 a marked swelling of the body beneath the branchial arches, i, 2, 3, 4, all four of 

 which show clearly on the surface. The entodermal canal has attained nearly 

 the condition shown in figure 27, B. 



