240 THE MESODERMAL DERIVATIVES 



each side gives rise to the laryngeal artery ( to the larynx and muscles 

 of the hyoid apparatus) and the oesophageal arteries (to the shoulder 

 and brachial arteries of the limb). The paired systemic arteries join 

 within the body to form the single dorsal aorta, sometimes called the 

 descending aorta, which in turn gives rise to many branches. These 

 include the renals to the glomi of the pronephros and glomeruli of 

 the mesonephros; the large coeliac artery to the stomach, liver, pan- 

 creas, and intestine; the gonadals; the lumbars; and finally the iliacs 

 to the hind legs. 



The aortic arch which develops into the third branchial arch (fifth 

 visceral) disappears. Those in the fourth branchial arches, however 

 (sixth visceral), develop connections with blood islands in the skin 

 and the lungs to become the pulmo-cutaneous arteries. In the frog, 

 the skin represents about 60 per cent of the necessary respiratory sur- 

 face. Some time after metamorphosis the connection of this fourth 

 pair of branchial aortic arches severs connections with the dorsal 

 aorta (now known as the systemic trunk), leaving only a vestigial 

 strand of tissue known as the ductus Botalli or ductus arteriosus. In 

 this manner the systemic arteries to the viscera and hmbs are sepa- 

 rated from the respiratory arteries to the skin and lungs. 



The arterial circulation of the frog is not entirely efficient for the 

 simple reason that each truncus arteriosus is divided only partially 

 by two septa into three channels, and some of the recently aerated 

 blood from the lungs will be sent again to the lungs. One of the chan- 

 nels leads to the carotid arches; a second joins the systemic arches; 

 a third leads from the right side of the heart, carrying venous blood to 

 the pulmo-cutaneous arches. The largest exit is to the systemic trunk, 

 and, until the time of metamorphosis, the two main anterior arteries 

 must pass through the gill circulation where the blood can adequately 

 exchange its carbon dioxide for oxygen. The systemic trunks give rise 

 to the pharyngeal arteries, which supply the mandibular arches, and 

 to the subclavians, which supply the forelimbs, and then they join 

 to form the single dorsal aorta. This large vessel then gives off several 

 thoracic and lumbar arteries to the back, a large coeliac artery and 

 smaller mesenteric arteries to the viscera, iliac arteries to the limbs, 

 and a caudal artery to the tail. 



The Venous System. The vitelline veins are the first blood vessels 

 to develop and they appear as blood islands ventro-lateral to the yolk 

 mass. The fused anterior vitelline veins comprise the meatus venosus, 



