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LABORATORY MANUAL FOR VERTEBRATE ANATOMY 



hypomere there arises on each side a tubular cavity, of the same nature as the blood vessels. 

 As the hypomere closes below in the median ventral line the two cavities are brought together 

 and fuse to form the heart (Fig. 51). By either method of formation the heart necessarily 

 lies in the ventral mesentery of the gut and is therefore provided with dorsal and ventral 

 mesenteries, known as the dorsal and ventral mcsocardia (see Fig. 44A). These, however, very 

 rapidly disappear, the ventral mesocardium vanishing as soon as formed. The heart then 

 swings free in the coelom, attached only at its two ends. The portion of the coelom in which 

 the heart lies is at first continuous with the general cavity of the hypomeres, but is soon cut 

 off from the rest of the coelom by the formation of the transverse septum and is thereupon 

 named the pericardial cavity. 



internal 

 carotid 



interna] 

 carotid 



gill slits 



vitelline . 

 artery caudal 

 artery 



vitelline 

 artery 



caudal 

 artery 



Fig. 52. — Diagrams of early vertebrate embryos to show the development of the main blood 

 vessels. A, earliest stage, in which the circulatory system consists of the vitelline veins, heart, ventral 

 aorta, first aortic arch, dorsal aortae, and vitelline arteries. B, later stage showing the development of 

 successive aortic arches following the first one. The method of formation of the aortic arches by buds 

 from the ventral and dorsal aortae is illustrated by the last aortic arch in B. (From Wilder's History of 

 the Human Body, courtesy of Henry Holt and Company.) 



4. The chief embryonic blood vessels. — The earliest vessels to form in the vertebrate 

 embryo are the vitelline veins. These veins develop on the surface of the yolk, sac (or in the 

 absence of a yolk sac, along the intestine) in the splanchnic mesoderm of the hypomere. They 

 pass to the embryo in the mesentery of the gut and enter the posterior end of the heart. At the 

 anterior end of the heart a blood vessel, the ventral aorta, arises and connects with the heart. 

 The ventral aorta extends forward to the anterior end of the pharynx. Here it divides in two, 

 and the two branches turn dorsally, one on either side of the pharynx. This pair of branches 

 encircling the pharynx is the first pair of aortic arches. Each vessel lies in the center of the 

 first or mandibular visceral arch. On reaching the dorsal side of the pharynx the two vessels 

 turn posteriorly and as the dorsal aortae proceed backward, situated in the median dorsal region 

 of the body wail. Each dorsal aorta on reaching the region of the yolk sac sends out a vitelline 

 artery over the surface of the yolk sac. The early embryonic circulation is thus completed 

 (Fig. 52^). 



