308 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 transversum; the ends of the horse- 
shoe, or horns of the sinus venosus, are continued into the ducts 
of Cuvier. The sinu-auricular aperture les 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 (ig. 204), thus forming 
sinu-auricular valves; a special development of the muscular 
trabecule 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 (ef. 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 
ralves disappear during this process. 
Il. Tuer 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 le 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 
aorte in front of the third arch constitutes the internal carotid; 
the ventral ends of the first and second arches form the external 
