HUMAN EMBRYO OF THE NINTH STAGE. 



145 



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, supplies 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 



FIG. 91. OUTLINE OF THE ENTODERMAL CANAL OF 

 A HUMAN EMBRYO OF 4 . 2 MM. 



Hy, Hypophysis, i, 2, 3, Lines marking the 

 position of the pharyngeal gill-pouches. Lu, 

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

 tois. W, Wolffian duct. Ch, notochord. 

 (After W. His.) 



Op 



-V 



FIG. 92. RECONSTRUCTION OF THE ANATOMY OF A 

 HUMAN EMBRYO, 3 . 2 MM. LONG, SHOWING THE 

 ANTERIOR END VIEWED FROM THE VENTRAL SIDE. 



Op, Optic vesicle. Ht, Heart. Li, Liver. V, Allan- 

 toic vein. Au, Auricle of the heart, i, 2, 3, 4, 

 Aortic arches. 



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

 88 and 92). 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 con- 

 tinuous with the arteries and veins at either end of the heart, and an outer meso- 

 dermic tube which is confined to the heart and is unconnected with the blood-vessels. 

 The venous end of the heart lies near the yolk-sac. It is convex toward the head. 

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

 ventral side, the venous process .of the heart (Fig. 92, Au} is seen on the left and 

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

 the large aorta (Fig. 90, 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 



