THE ANATOMY OF A SIX MM. PIG EMBRYO 99 



The Arteries. — These begin with the ventral aorta, which takes origin from the 

 bulbus cordis. From the ventral aorta are given off five pairs of aortic arches. 

 These run dorsald in the five branchial arches (Figs. 104 and 105) and join the 

 paired dorsaLoi_desMnding_ aorta. The first and second pairs of aortic arches are 

 very small and take origin from the small common trunks formed by the bifurca- 

 tion of the ventral aorta just caudal to the median thyreoid gland. The fourth 

 aortic arch is the largest. From the fifth arch small pulmonary arteries are devel- 

 oping. There is evidence that this pulmonary arch is really the sixth in the 

 series, the fifth having been suppressed in development (cf. Fig. 272 B). Cranial 

 to the first pair of aortic arches, the descending aortae are continued forward 

 into the maxillary processes as the internal carotids. Caudal to the aortic arches 

 the descending aorta; converge, unite opposite the cardiac end of the stomach, 

 and form the median dorsal aorta. From this vessel and from the descending 

 aortae paired dorsal intersegmental arteries arise. From the seventh pair of these 

 arteries (the first pair to arise from the medial dorsal aorta) there are developed 

 a pair of lateral branches to the upper limb buds. These vessels are the sub- 

 clavian arteries. From the dorsal aorta there are also given off ventro-lateral 

 arteries to t he glo meruli of the inesonephros, and median ventral arteries. Of the 

 latter the celiac artery arises opposite the origin of the hepatic diverticulum. The 

 vitelline artery takes„originj3y_two^ or three trunks caudal to the dorsal pancreas. 

 Of these tru nks t he posterior isthe lajrger and persists as the superior mesenteric 

 arter}^^ Thyng (Anat. Record, vol. 5, 1911) has figured three trunks of origin in 

 the 7.8 mm. pig. These unite and the single vitelline artery branches in the wall 

 of the yolk sac. 



Opposite the lower limb buds the dorsal aorta is divided for a short distance. 

 From each division there arises laterad three short trunks which unite to form the 

 single umbilical artery on each side. The middle vessel is the largest and appa- 

 rently becomes the common iliac artery. A pair of short caudal arteries, much 

 smaller in size, continue the descending aortse into the tail region. 



The Veins. — The vitelline veins, originally paired throughout, are now repre- 

 sented distally by a single vessel, which, arising in the wall of the yolk sac, enters 

 the embryo and courses cephalad of the intestinal loop (Fig. 102). Crossing to 

 the left side of the intestine and ventral to it, it is joined by the superior mesenteric 

 vein which has developed in the mesentery of the intestinal loop. The trunk 

 formed by the union of these two vessels becomes the portal vein. It passes along 

 the left side of the gut in the mesentery. Opposite the origin of the dorsal pan- 

 creas it gives off a small branch, a rudimentary continuation of the left vitelline 



