RECONSTRUCTION MODEL OF A HORSE EMBRYO TWENTY-ONE DAYS OLD. 343 



terminates in a short truncus aorticus (figs. 58, 62, PL XIX). The truncus aorticus 

 lies dorsal to the ventricle, cranial to the atrium, and, at its termination, it is on the 

 right side of the atrio-ventricular canal (fig. 62, PL XIX). 



Truncus Aorticus. — The short truncus aorticus extends from the bulbus cordis 

 to the ventral wall of the fore-gut, where, immediately ventral to the ventral ends of 

 the first branchial pouches, it terminates by dividing into four branches, two on each 

 side. The more cranially situated branches are the ventral roots of the first aortic 

 arches ; the more caudally situated branches are the second aortic arches. 



The Aortic Arches. — Two aortic arches are present on each side. 



The ventral root of the first arch runs cranialwards from the truncus aorticus, 

 below the aperture of the first branchial pouch, to the ventral end of the mandibular 

 arch ; there it turns dorsally and becomes the first aortic arch. 



The first aortic arch runs dorsally, along the cranial border of the aperture by 

 which the first branchial pouch communicates with the pharynx (figs. 58, 63, 

 PL XIX), and at the level of the dorsal margin of the aperture it turns caudally 

 and becomes the dorsal root of the first arch. The dorsal root of the first arch 

 terminates by joining the dorsal end of the second arch at the level of the otic 

 vesicle and ventral to the anterior cardinal vein (fig. 58, PL XIX). 



The second aortic arch runs directly dorsally, through the second branchial 

 arch, from the truncus aorticus to the caudal end of the dorsal root of the first 

 aortic arch, with which it unites to form the commencement of the dorsal aorta 

 (fig. 58, PL XIX). 



The Dorsal Aortse. — The dorsal aortse run from the dorsal ends of the second 

 aortic arches to the caudal end of the embryo, where they terminate by breaking up 

 into branches which join the capillary plexus of the allantois (figs. 58, 59, 60, 61, 

 PL XIX). In the head region each dorsal aorta lies medial to the dorsal extension 

 of the second branchial pouch of the same side, and then along the dorsal surface 

 of the lateral margin of the more caudal part of the pharynx. In the thoracic region 

 it is run along the dorso-lateral border of the gut, dorsal to the pleuro-pericardial 

 canal of the same side (text-figs. 25 and 26, pp. 341, 342). Near the caudal end of 

 the fore-gut, and in the transverse plane in which the vitelline veins join the sinus 

 venosus, the two dorsal aortse fuse together to form a single trunk. This trunk 

 lies in the median plane, dorsal to the yolk-sac and ventral to the notochord, until 

 within a short distance from a point near the cranial end of the cleft which indicates 

 the position of the hind-gut. There it separates again into a right and a left dorsal 

 aorta which run caudally, in the angles between the spinal medulla and the dorsal 

 wall of the hind-gut, to their terminations in the allantoic region (figs. 58, 59, 60, 61, 

 PL XIX). A short distance from its termination the caudal portion of each dorsal 

 aorta gives off large branches to the wall of the yolk-sac, and a number of branches 

 which pass ventrally to the allantoic capillary plexus already described (p. 340). 



The Anterior Cardinal Veins. — Each anterior cardinal vein commences near the 



