ONTOGENY 



259 



dorsally through aortic arches to a pair of dorsal aortae. The 

 latter convey the blood to all parts of the body and to the yolk 

 sac, from which it is returned to the heart by cardinal veins 

 and vitelline veins (from the yolk sac). The yolk-sac circulation 



Fig. 161. — Circulatory system of chick embryo of 72 hours incubation; 

 embryonic membranes not shown. Starting at the ventricle (v) the blood 

 is pumped forward through the ventral aorta to the aortic arches of which the 

 four of the right side are shown (in solid black). The gill clefts lie between the 

 arches. Above, the aortic arches unite to form a dorsal aorta each of which 

 sends a carotid artery (right one shown) to the head. Posteriorly, the aortae 

 unite to form the dorsal aorta (da) which in the middle of the trunk region gives 

 off a right and left vitelline artery (va) carrying blood to the yolk capillary 

 circulation to be aerated and charged with food from the yolk. The blood 

 returns to the embryo by right and left vitelline veins (w) which unite and join 

 the sinus venosus of the heart. Blood is collected from the embryo by right and 

 left anterior and posterior cardinal veins (ac and pc) which form on each side a 

 common cardinal (c), also joining the sinus venosus. The mingled venous 

 blood from the cardinal veins and the aerated blood from the vitelline veins 

 passes from the sinus venosus to the atrium (a) and then to the ventricle, d, 

 diencephalon; lb, leg bud; m, mesencephalon; my, myelencephalon; o, otic 

 vesicle; ol, olfactory pit; s, somite (muscle); t, telencephalon; wb, wing bud. 



serves to aerate the blood and to supply the embryo with nutri- 

 tion. The blood in passing through the gill arches is not aerated, 

 since that is done in the yolk sac at this stage of development. 

 The resemblance of the aortic arches to the afferent and efferent 

 branchial arteries of the fish or of the tadpole is merely ana- 



