THE DEVELOPMEN1 OF THE VASCULAR SYSTEM. 



203 



foramen ovale which persists throughout foetal life, but closes soon after 

 birth. The atrio-ventricular canal also becomes divided into two passages 



Sinus venosus 



Left valvula venosa 



Right valvula venosa 



Right ventricle 



Right atrio- 5 



ventricular canal 



Right ventricle 



Atrial septum 



Pulmonary vein 



Left atrium 



Left atrio- 

 ventricular canal 



Left ventricle 



I \ 



Interventricular furrow Ventricular septum 



FIG. 173. Dorsal half of heart (ventral view) of rabbit embryo of 5.8 mm. Born. 



by a ridge from the dorsal wall and one from the ventral wall uniting with 

 each other and finally with the septum atriorum (Fig. 172). Thus the two 

 atria would be completely separated if it were not for the foramen ovale. 



Aortic septum 



Interventricular opening /_ 



Right atrio-ventricu- (. 

 lar orifice " ~~i 



Right ventricle - ~c 

 .*?;/-. \ 



'] 



Ventricular septum *-- 



Pulmonary artery 



- Aorta 



_. Left atrio-ventricular orifice 



*>- - Left ventricle 



FIG. 174. Ventricles and proximal ends of aorta and pulmonary artery of a 7.5 mm. human 

 embryo. Lower walls of ventricles have been removed. Kollmann's Atlas. 



During the separation of the atria, a division of the ventricular portion 

 of the heart also occurs. On the caudal side of the ventricular portion a 



