228 STRUCTURE OF THE VERTEBRATES 



to become the heart; (3) the first aortic loop connecting the ven- 

 tral vessels with the dorsal; (4) the paired dorsal aortae; and 

 (5) branches from the aortae completing the circuit as they pass 

 outward from embryo to yolk. 



Modifications rapidly appear in the embryo. The vitelline 

 veins begin a fusion to form the heart and a single ventral 

 aorta; and the aortae fuse from the posterior end of the pharynx 

 to the tail, forming a single dorsal aorta. Synchronously with 

 this the carotid arteries grow anteriorly from the first aortic loop 

 into the head region, and small arteries grow into the tissues from 

 the dorsal aorta. The blood carried out to the body is returned 

 to the developing heart by the cardinal veins. There are two 

 anterior cardinals draining the head and brain, and two post- 

 cardinals which carry blood from the posterior tissues and the 

 developing kidneys. 



In the meantime the aortic loops have increased in number. 

 The primitive gill outpocketings have appeared along the pharyn- 

 geal wall ; and the aortic, or branchial, loops pass between them. 

 At this stage the chick embryo is essentially like the dogfish 

 adult. In the latter the branchial arches break into capillaries 

 in the gills and form afferent and efferent branchial arches. In 

 the amniotes the arches remain as continuous vessels which 

 undergo various modifications. 



Amniote embryos of this stage develop an allantois (page 69) 

 from the posterior end of the gut. Veins and arteries are carried 

 with it, and these eventually lie in contact with the chorion. 

 Thus a vascular, moist membrane is next to the porous shell, and 

 acts as a respiratory structure for the growing embryo. In 

 placentals the allantois induces the development of a placenta 

 (page 93) and the allantoic vessels have the same function as 

 they do in the chick. In time the allantoic veins fuse with the 

 vitelline veins as the omphalomesenteric veins which pass 

 through the liver to the heart. The right omphalomesenteric dis- 

 appears and the left remains as the hepatic portal vein which 

 enters the liver and breaks into capillaries. The anterior ends of 

 the original pair remain as the two hepatic veins. 



At this stage the system of the amniote embryo does not differ 

 essentially from that of the adult dogfish, with the exception of 

 the allantoic vessels. Future changes in the embryo can best be 



