312 THE thick. 



the course of the circulation on the third day, and during the 

 later stages of incubation. 



a. The circulation at the end of the third day (Fig. 113). The 

 heart, at the end of the third day, is a single twisted tube, slightly 

 constricted at intervals which mark the boundaries of the succes- 

 sive chambers, but with no trace of a division into right and left 

 sides. The blood enters the hinder end of the heart by the two 

 vitelline veins, and passes out in front through the truncus 

 arteriosus ; this at once splits into right and left branches, each 

 of which again divides into the three anterior aortic arches, which 

 encircle the pharynx and open dorsally into the aortse. The two 

 aortge are widely separate in the head, but approach each other 

 further back, and are fused for a short distance in the body. 

 Behind this point they separate again, and each aorta gives 

 off a large vitelline artery (Fig. 113, av), which, passing out 

 beyond the embryo, opens into the capillary network of the 

 area vasculosa ; from this network the blood is collected again 

 by vessels which unite to form the vitelline veins, and so 

 returns once more to the heart. 



The great purpose of the circulation at this stage is to 

 insure the absorption of nutriment from the yolk-sac, and its 

 conveyance along definite channels to the embryo. The 

 vascular system of the embryo itself is as yet only imperfectly 

 formed, and there is no special respiratory organ. 



b. The circulation during the latter half of the period of incuba- 

 tion. The heart is now fully formed. The sinus venosus has be- 

 come absorbed into the right auricle, of which it now forms part : 

 the auricular septum is still incomplete, the large foramen ovale 

 allowing blood to pass freely from the right auricle to the left 

 auricle. The ventricular septum is complete ; and the truncus 

 arteriosus is divided into two entirely separate vessels, of 

 which one, the pulmonary trunk, arises from the right ventricle, 

 and the other or systemic trunk from the left ventricle. 



Three pairs of aortic arches are present, but these are the 

 third, fourth, and fifth of the complete series, the first and 

 second having disappeared along the greater part of their length. 

 The systemic trunk, arising from the left ventricle, leads to the 

 third and fourth pairs of aortic arches, and through these to the 



