COMPARATIVE ANATOMY OF THE CIRCULATORY SYSTEM 203 



There subsequently develop around the pharynx additional aortic arches connecting the 

 ventral and dorsal aortae. In typical vertebrates six such pairs of aortic arches appear, one 

 pair to each pair of visceral arches (Fig. 52^). The aortic arch runs in the center of the vis- 

 ceral arch. The aortic arches develop in order, the second after the first, then the third, and 

 so on. Posterior to the pharyngeal region, the two dorsal aortae soon fuse to form a single 

 dorsal aorta which continues into the tail as the caudal artery. From the aorta, as development 

 proceeds, branches arise for the various parts of the body. In fishes the middle part of each 

 aortic arch later breaks up into a system of capillaries which occupies the gill filaments, but 

 in non-aquatic forms the arches remain unbroken. 



It will now be perceived that provision must be made for the return of the venous blood 

 of the embryo to the heart. For this purpose two pairs of veins arise in the somatic mesoderm 

 of the body wall. These are a pair of anterior cardinal veins returning blood from the anterior 

 part of the body and a pair of posterior cardinal veins returning blood from the posterior part 



FIG. 53. Later stage of the development of the circulatory system, showing the chief veins. 

 Veins blank, arteries black, a, internal carotid; b, dorsal aorta; c, anterior cardinal vein; d, the six 

 aortic arches; e, the six gill slits; /, the conus arteriosus of the heart continuing into the ventral aorta; 

 g, main part of the heart; h, sinus venosus of the heart; *, duct of Cuvier or common cardinal vein; 

 jy posterior cardinal vein; k, vitelline vein; /, yolk sac; m, subintestinal vein; , vitelline artery; 

 o, lateral abdominal or umbilical vein; p, allantois (urinary bladder); q, allantoic or umbilical artery; 

 r, caudal artery; s, caudal vein. (Slightly modified from Vialleton's Intents de Morphologic des 

 Vertebres.) 



of the body. In order for these veins, which are situated in the dorsal body wall, to reach the 

 heart, a bridge of mesoderm is formed on each side extending between the somatic and 

 splanchnic mesoderm. We have already learned that this pair of bridges is the beginning of 

 the transverse septum; and by the union and extension of the bridges the septum is completed. 

 At the level of the posterior end of the heart the anterior and posterior cardinal veins on each 

 side unite to a common vessel, the duct of Cuvier or common cardinal vein. The common 

 cardinal vein then passes to the heart from each side by way of the transverse septum between 

 the two walls of which this vein is inclosed. In addition to the cardinal veins there appears 

 in vertebrate embryos a pair of veins in the lateral or ventral abdominal walls, known as the 

 lateral or ventral abdominal veins, which enter the heart along with the common cardinal veins. 

 They are also called the umbilical or allantoic veins in the embryos of amniotes. The main 

 veins of the embryo at this time are illustrated in Figure 53. 



The two vitelline veins are soon extended posteriorly in the embryo by means of a tribu- 

 tary, the subintestinal vein, which courses in the mesentery of the gut and constitutes the chief 

 vein of the digestive tract. It is shown in Figure 53. It continues into the tail as the caudal 

 vein, making a loop around the anus. In embryos without a yolk sac the subintestinal and 

 vitelline veins appear as one continuous vein. 



