SEXUAL REPRODUCTION AND DEVELOPMENT OF INDIVIDUAL 221 



extending through the gill arches are called aortic arches (Fig. 204). 

 The typical number of these arches in the vertebrate is six, but in the 

 embryos of most higher vertebrates the fifth one is quite transitory. 

 The first and second pairs degenerate, except for remnants which 

 unite with the third in forming the carotid arteries. The fourth pair 

 of arches becomes the systemic arteries in amphibia and reptiles, but 

 in birds only the right side of the arch persists while in mammals 

 only the left. The sixth pair of arches give rise to the pulmonary 

 arteries. 



Myc. 



A 



Bui. 



Vent. 



S.K 



Tr.Ari. 



V.ao. 



Tr. Art. 



At.J.. 



Fig. 66. — Ventral view of the development of the heart of the pig-. The stages 

 shown bring out the changes involved in converting the primitive tubular vessel 

 into a four-chambered heart. 



A, from 7 somite embryo ; ^, 13 somite embryo ; G, 17 somite embryo ; D, 25 

 somite embryo ; E, from embryo 3.7 mm. in length ; F, from 6 mm. embryo. At, 

 atrium (r, right; I, left) ; Bid, bulbus arteriosus; Endc, endocardial tubes; Myc, 

 cut edge of epimyocardium ; S. V., sinus venosus ; Tr. A7-t., truncus arteriosus ; V. 

 ao. r., ventral aortic roots; Vent., ventricle (r, right; Z. left) ; V. O. M., omphalo- 

 mesenteric veins. (From Patten, Embryology of the Pig, P. Biakiston's Son and 

 Company. ) 



. Urogenital System. — It is convenient to present the excretory and 

 genital systems under one head because certain parts are developed 

 in common and others in close relation to each other. The principal 



