284 



ESSENTIALS OF ZOOLOGY 



the two vitelline veins, as has been described in the case of the 

 chick. It lies medianly below the fore-gut, and in front it 

 diverges to form the first arch, which on reaching the dorsal 

 side runs backwards as the primitive dorsal aorta supplying 

 the embryo and sending the blood back to the placenta 

 or to the yolk sac. The heart expands and becomes con- 

 stricted into a series of four vesicles which can be identified 

 as sinus venosus, auricle, ventricle, and conus arteriosus. It 

 is also bent into an S-shape. The sinus venosus receives the 

 blood from the yolk sac and the placenta and also from the 

 body of the embryo, and is ultimately absorbed into the wall 



Cardinals 



Duct. Pul.art.. , 



Int. carotid Ao. arch. art. \ Ao. Ant. Post. 

 I / / 



Ext. carotid Conus Vent. Aur. Vitelline vessels Allantoic vessels 



FIG. 136. Blood circulation of embryo diagrammatic. 



of the right auricle. The originally single auricle becomes 

 divided by a septum which descends across the cavity from the 

 dorsal wall, and fuses with the wall in its descent to the margin 

 of the ventricle. The auricle is thus separated into right and 

 left auricles. The ventricle similarly develops a septum from 

 the apex of the bend which is prominently marked, and this 

 septum grows towards and finally fuses with the auricular 

 septum, thus dividing the ventricular cavity into two separate 

 ventricles, right arid left. For a period the auricles remain in 

 communication through a median foramen, the foramen ovale ; 

 and by means of a valve derived from the opening of the 

 sinus venosus, the Eustachian valve, the blood laden with 

 food and oxygen from the placenta and entering the right 



