596 



THE EMBRYOLOGY OF THE FROG 

 THE ARTERIAL SYSTEM (Fig. 345) 



The Lateral Dorsal Aortae, which are the first to appear, are paired 

 and lie dorsal to the pharynx. They are formed from a series of sepa- 

 rate spaces in the mesenchyme of the head region which then connect 

 and form the vessels of the cranial region. 



The Dorsal Aorta (Fig. 345, ao) is the name given the lateral dorsal 

 aortae as soon as they fuse medially on the dorsal side of the embryo tc 

 form a single vessel. This vessel extends to the caudal extremity of the 

 embryo. 



The Aortic Arch. An aortic arch forms in each branchial arch, firsl 

 as a little vascular space when the embryo is about 4.5 millimeters long 



Diagrams of the heart and chief arteries of a tadpole. A, The vessels 

 of a tadpole at the stage when three external gills are present ; B, the 

 arrangement when secondary gills are in use ; C, the adult arrangement. 

 o.c., Anterior commissural vessel ; a.cb., anterior cerebral artery ; a/., 

 afferent branchial arteries ; ao., dorsal aorta ; car., carotid artery ; e.g., 

 carotid gland ; cu., cutaneous artery ; d.b., ductus Botalli ; ef., efferent 

 branchial arteries ; ht., heart ; hy., efferent hyoidean artery ; {., connecting 

 vessel ; I., lingual artery ; md., efferent mandibular artery ; p.c., posterior 

 commissural vessel ; pl.c., pulmocutaneous arch ; pul., pulmonary artery ; 



sys., systemic arch; tr., truncus arteriosus ; v., ventricle; I-IV., branchial 



aortic arches. (After Bourne.) 



This space connects ventrally with the truncus arteriosus and dorsall} 

 with the lateral dorsal aorta (Fig. 341, ab). It is now called an aortic 

 arch. Thus there are four pairs of these aortic arches, being in reality 

 the third to sixth pair, and the first and second are formed in the mandib 

 ular and hyoid arches. 



The Afferent Branchial Arteries (Fig. 345, af). As the externa 



