418 A Three Weeks' Human Embryo 



pharynx nearly to the hypophysis. This portion of the vascular system 

 resembles closely others described of the same age, but reminds one, in 

 the extreme difference in size of the aortic arches, of the condition de- 

 scribed by Miss Lehmann," where in lower mammals the six arches are 

 not all complete at any one time. 



The two dorsal aortas unite (Fig. 3) near the cephalic border of the 

 liver and the caudal margin of the arms, giving off just before their union 

 the brachial arteries and soon after, the small vitelline or omphalo- 

 mesenteric artery (Figs. 10, 17). Branches could occasionally be traced 

 to the tubules of the mesonephros (Fig. 6). In contrast to the other 

 arteries the umbilicals are large and in the body stalk anastomose across 

 the middle (Fig. 5), but continue as a pair for some distance in the body 

 stalk. 



Veins. — The veins are remarkable for the great variation in caliber. 

 The much branched jugular (precardinal) can be traced from a point 

 lying between the eyes and cerebrum (Fig. 7), keeping near the surface 

 (Fig. 8), sending branches through, and then passing mesad of the 

 Gasserian ganglion (Fig. 9), laterad of the ear vesicle and the ganglia 

 of the 7th and 8th nerves (Fig. 11), and by a breaking-up in the 9th and 

 10th ganglia, comes to lie mesad of the 10th ganglion as it passes over 

 into the vagus nerve, then unites with the cardinal (Fig. 11, at left) and 

 the umbilicals (Fig. 2, at left) to form the ducts of Cuvier. The course 

 of the jugular in the head seems to illustrate one phase of the change of 

 position of the bloodvessels with relation to the nerves as demonstrated 

 by Dr. Mall " in a recent article. 



The ducts of Cuvier unite across the middle to form the sinus venosus; 

 and this connects by a small opening with the great sinusoid " of the liver 

 formed in the course of and by the union of the vitelline veins (omphalo- 

 mesenteric) (Fig. 10). 



The umbilical veins coming from the body stalk are joined by veins 

 from the legs, become enlarged and break up in the body wall into a great 

 sinus (Figs, 5, 6, 10, 11), and finally as they enter the duct of Cuvier 

 become so small as to be scarcely traceable (Fig. 2, at left, and Fig. 10, 

 at right). 



The Nepheic System. 



General. — The nephric system (Fig, 17), as is usual at this age, con- 

 sists of a pair of Wolffian ridges extending along the back from the arm- 



" Lehmann, Harriet, Anat. Anz., XXVI, 1905. 



"Mall, F. P., Amer. Jour. Anat., IV, 1904. 



"Minot, C, Boston Soc. Nat. Hist., Proc. XXIX, 1900. 



