STUDIES ON THE DEVELOPMENT OF THE HORSE. 313 



formed from the vitelline veins, receives blood from the allantois and the embryo as 

 well as from the yolk-sac. The allantoic (umbilical) veins (text-fig. 7), one at each 

 side, spring from a transverse sinus in the allantoic mesoderm at the caudal end of 

 the embryo (text-fig. 7). The blood from the embryo reaches the sinus by the short 

 Cuvierian veins formed by the junction of the anterior and posterior cardinals. Each 

 anterior cardinal (primitive jugular) vein (text-fig. 7) begins near the mandibular 

 arch ; as it runs backwards it approaches the first two visceral pouches and the otic 

 vesicle. Each posterior cardinal vein (text-fig. 7) runs forwards from the caudal end 

 of the embryo ; on the way it is intimately related to either the pronephric duct or 

 the Wolffian body. The blood flows from the sinus (text-fig. 7) by a wide opening 

 into the atrium ; the atrium, separated by a constriction from the sinus, opens by a 

 valved aperture into the ventricle — portions of the atrium and ventricle are seen in 

 figs. 38 and 40. While the sinus and atrium occupy the uppermost and left portion 

 of the space between the pharynx and the middle curved portion of the trunk, the 

 ventricle occupies the middle and right portion and lies in contact with the floor of 

 the fore-gut and the yolk-stalk (fig. 33). The truncus (bulbus) arteriosus springs 

 from that part of the ventricle lying in contact with the floor of the pharynx, and 

 runs forwards in contact with the atrium (fig. 39) to reach the middle line and give 

 oft* tw T o pairs of aortic arches (text-fig. 7) , a pair to the mandibular arches and a pair 

 to the hyoid arches. # As text-fig. 7 indicates, the first aortic arch on the left side 

 proceeds backwards in front of the first branchial pouch to meet and unite with the 

 smaller second aortic arch and thus form the left aorta. In a like manner the first aortic 

 arch on the right side unites with the second arch to form the right aorta. The two 

 dorsal aortse as they proceed backwards lie one at each side of the notochord (fig. 42), 

 and are also more or less intimately related to the pharynx and gill pouches, the 

 spinal cord and otic sacs, the pleuro-peritoneal sinus and the anterior cardinal veins 

 (figs. 38 to 41). On nearing the sinus venosus the two aortse approach each other 

 and completely fuse, but they again separate when a point about 3 mm. from the 

 caudal end is reached (text-fig. 7). 



As the two aortse proceed backwards through the posterior third of the trunk 

 they are intimately related to the notochord and the mesodermic somites. On 

 reaching the posterior border of the yolk-stalk the distance between the aortse (text- 

 fig. 7) becomes greater, and it continues to increase until they end in the allantoic 

 mesoderm. As the aortse proceed backwards the left gives off the large left vitelline 

 artery (text-fig. 7), which eventually bifurcates to form the sinus terminalis (fig. 34) ; 

 the right gives off the small right vitelline artery (text-fig. 7). Before ending 

 blindly each aorta gives off branches to the hind-gut ; some of these branches give 

 rise to a plexus with which the allantoic (umbilical) veins communicate (text-fig. 7). 



* In only having two pairs of aortic arches the 21 -days horse agrees with a human embryo of about 15 days 

 figured by His. In a figure by His of a human embryo of about 3 weeks all five aortic arches are present. If the 

 age of these human embryos is approximately accurate, the aortic arches appear later in the horse than in man. 



