372 THE DEVELOPMENT OF THE CHICK 



terior cardinals passes forward to the ducts of Cuvier without 

 entering the mesonephric circulation. On the fifth and sixth 

 days the cephalic ends of the postcardinals gradually dwindle 

 and disappear (cf. Fig. 216); thus all of the blood entering the 

 postcardinals must pass through the mesonephros to the sub- 

 cardinals, which thus become efferent vessels of the mesonephros; 

 and a complete renal-portal circulation is established. 



This form of circulation continues during the period of func- 

 tional activity of the mesonephroi, and as the latter gradually 

 atrophy, the portions of the subcardinals posterior to the anas- 

 tomosis gradually disappear. A direct connection between the 

 post- and subcardinals is then established on each side, by way 

 of the great renal veins, which have in the meantime formed in 

 connection with the development of the kidney (Fig. 214). 



The crural and ischiadic veins have, in the meantime, developed 

 in connection with the formation of the hind limbs, as branches 

 of the postcardinals. Thus the hinder portion of the latter be- 

 comes transformed into the common iliac veins, and at the hinder 

 end the postcardinals form an anastomosis (Fig. 214). 



IV. THE EMBRYONIC CIRCULATION 



On the fourth day the blood is driven into the roots of the 

 dorsal aorta through three pairs of aortic arches, viz., the third 

 or carotid, the fourth or aortic, and the sixth or pulmonary. The 

 fifth pair of aortic arches is also functional for a time during this 

 day, but soon disappears. The blood passing up the third or 

 carotid arch is directed forward through the internal and external 

 carotid arteries to the head; that passing up the fourth and 

 sixth arches turns backwards to enter the dorsal aorta, so that 

 there is an intermediate area of stagnation in the roots of the 

 dorsal aorta between the carotid and aortic arches; though this 

 is more or less problematical, the arrangement of the vessels ren- 

 ders such a condition very probable. A very small proportion of 

 the blood enters the rudimentary pulmonary arteries from the 

 sixth arch. The blood in the dorsal aorta passes backwards and 

 enters (1) the segmental arteries, (2) the omphalomesenteric 

 arteries, (3) the (rudimentary) umbilical arteries, and behind 

 the latter passes into the narrow continuation of the dorsal aortse, 

 still separate in this region, known as the caudal arteries. 



The blood is returned to the heart through the sinus venosus 



