140 



THE HUMAN EMBRYO. 



on each side. The anterior branch forks and runs through the first and 

 second arches. The posterior branch forks, one fork going to' the third, 

 and the other, after again forking, suppHes the fourth and fifth branchial 

 arches. This arrangement of the aortic branches is typical. Between the 

 bases of the first and second arches is a small protuberance which is the anlage 

 of the tongue and is named by His the tuberculum impar. Studies of the sections 

 demonstrate that the cavity of the abdominal region (splanchnocele) has on each 

 side of its dorsal surface a longitudinal ridge, the commencement of the Wolffian 

 body. The ridge already contains traces of the canals of the Wolffian body. 



Fig. 76. — Outline of the Entodermal Canal 

 OF A Human Embryo of 4.2 mm. 



Hy, Hypophysis. /, 2, j, Lines marking the 

 position of the pharyngeal gill pouches. Lu, 

 Lungs. Li, Liver. Yks, Yolk-sac. Al, 

 Allantois. W, Wolffian duct. Ch, notochord. — 

 {After W. His.) 



Fig. 77. — Reconstruction of the Anatomy of 

 A Human Embryo, 3.2 mm. Long, showing 

 the Anterior End Viewed from the Ven- 

 tral Side. 



Op, Optic vesicle. Hi, Heart. Li, Liver. V, 

 Allantoic vein. Au, Auricle of the heart. I, 

 2, 3, 4, Aortic arches. 



Of especial interest is the arrangement of the circulatory apparatus (Figs. 74 and 

 77). In the first figure the arteries are shaded dark; the heart is an S-shaped 

 tube which is really double, consisting of an inner endothelial tube continuous 

 with the arteries and veins at either end of the heart, and which is confined to the 

 heart itself and has nothing to do with the blood-vessels proper. The venous 

 end of the heart lies near the yolk-sac. It is con vexed toward the head. The 

 arterial end of the heart is convexed toward the tail. When viewed from the 

 ventral side, the venous process of the heart (Fig. 'j'j, Au)\s seen on the left and 



