358 THE DEVELOPMENT OF THE CHICK 



auricular aperture (Fig. 204). At the stage of ninety-six hours the 

 general form of the sinus is that of a horseshoe situated between 

 the atrium and the septum trans versum; the ends of the horse- 

 shoe, or horns of the sinus venosus, are continued into the ducts 

 of Cuvier. The sinu-auricular aperture lies on the right, and 

 here the cavity of the sinus is largest; the right horn of the sinus 

 is relatively short and the left horn forms a transverse piece on 

 the anterior face of the septum transversum, which gradually 

 curves dorsally and enters the left duct of Cuvier. 



The right and left boundaries of the sinu-auricular aperture 

 project into the cavity of the right auricle as folds that meet 

 below the aperture and diverge dorsally (Fig. 204), thus forming 

 sinu-auricular valves; a special development of the muscular 

 trabeculaB running along the roof of the right auricle from the 

 angle of these valves corresponds to the septum spurium of mam- 

 malia. The sinus septum arises as a fold of the roof of the sinus 

 between the entrance of the left horn and the vena cava inferior; 

 it grows across the sinus into the sinu-auricular aperture and 

 thus divides the latter (cf. Fig. 231). Subsequently, the sinus 

 becomes incorporated in the right auricle, and the systemic 

 veins thus obtain independent openings into the latter (see account 

 of development of the venous system). The sinu-auricular 

 valves disappear during this process. 



II. THE ARTERIAL SYSTEM 



The Aortic Arches. In the Amniota six aortic arches are 

 formed connecting the truncus arteriosus with the roots of the 

 dorsal aorta. The first four lie in the corresponding visceral 

 arches; the fifth and sixth are situated behind the fourth visceral 

 pouch; the fifth is a very small and transitory vessel, the exist- 

 ence of which was not suspected until comparatively recently 

 (v. Bemmelen, Boas), and the sixth or pulmonary arch was pre- 

 viously interpreted as the fifth. The discovery of the fifth arch 

 has brought the Amniota into agreement with the Amphibia 

 as regards the number and significance of the various aortic arches. 



The fate of the aortic arches in the chick is as follows (see 

 Figs. 205, 206): the first and second arches disappear as already 

 described (Chap. VI), and the anterior prolongation of the dorsal 

 aortse in front of the third arch constitutes the internal carotid; 

 the ventral ends of the first and second arches form the external 



