184 OUTLINES OF CHORD ATE DEVELOPMENT 



pharynx. About the time the mouth opens, they extend back- 

 ward and connect with the ventral ends of the efferent bran- 

 chial arteries of the first branchial arch, in the region where 

 the carotid gland (see above) develops later. 



About the time of hatching, outgrowths of the dorsal aorta, 

 just back of the pharyngeal region, extend laterally into the 

 region of the pronephros or head kidney (see below). These 



later become very large and form 

 the vascular glomi of this kidney 

 (Figs. 66, 72); traces of these re- 

 main, long after the pronephros 

 *c itself has disappeared. 



During metamorphosis, as the gills 

 disappear, the branchial blood ves- 

 sels are considerably modified. We 

 have seen that a continuous aortic 

 arch is reestablished in each of the 

 , ao four branchial arches by the fusion 



FIG. 67. Diagram of the of the afferent and efferent arteries. 



aortic arches and their chief rrn n , v i_ i ' i/xi_'i 



branches in an adult frog. The ^^ branchial aortic arch (third 

 Ventral view, ao, Dorsal aorta; o f the whole series) remains as the 



c, carotid artery; eg, carotid . 



gland; cu, cutaneous artery; /, root of the anterior carotid artery, 



and is known as the 



subciavian artery; t, truncus (Fig. 67). The lateral dorsal aorta 



arteriosus; v, vertebral artery. , , , ~ i i ALT i 



between the first and second (third 



and fourth) aortic arches, becomes reduced to a solid strand 

 of connective tissue, and the second (fourth) pair of aortic 

 arches consequently become the roots of the dorsal aorta, and 

 are known as the systemic arches. The third (fifth) aortic arch, 

 after becoming a solid strand of tissue, disappears entirely. 

 The fourth (sixth) arch remains as the root of the pulmonary 

 and cutaneous arteries of the adult, and is known as the pulmo- 

 cutaneous arch. 



The pulmonary arteries appear just after hatching as small 

 outgrowths from the upper ends of the efferent branchial 

 arteries of the fourth branchial arch. (Figs. 66, 67). They 

 extend backward to the lung rudiments, which they reach 



