628 



CHORD AT A. 



life the arterial trunk, which at first is simple, is divided into a 

 pulmonary artery, arising from the right half of the heart and 

 carrying venous blood, and an aorta ascendens, with arterial blood, 

 connected with the left half. In contrast with the reptiles, the 

 right aortic arch is entirely lost, the left persisting. 



The urogenital system is of great importance in the separation 

 of the group into smaller divisions (fig. 651). In both sexes this 

 consists of practically the same parts in early embryonic life. 

 These are the early formed Wolffian body ( W ) ; the permanent 

 kidneys, which appear later and are not shown in the diagram ;. 



FIG. 651. 



FIG. 65i. 



FIG. 651. Diagram of embryonic mammalian urogenital system. (From Balfour,. 

 after Thompson.) cl, cloaca; cj>, genital process: go, genital cord; i, rectum; 

 te, ridge for formation of labia or scrotum ; rn, Mullerian duct ; ot, gonad ; mi, 

 urogenital sinus ; W, Wolfflan body ; w, Wolffian duct; 3, ureter; A, urinary blad- 

 der ; 5, continuation of latter to allantois (urachus). 



FIG. 652. Urogenital system of male beaver. (From Blanchard.) a, castor eum 

 sacs ; b, openings of their ducts into preputial canal; c, tip of penis ; d, preputial 

 opening ; e, anal glands; f, their ducts; 0, anus ; 7i, base of tail; i, corpora caver- 

 nosa ; 7c, Cowper's glands; /, seminal vesicles; 7n, vasa deferentia; n, testes; o, 

 urinary bladder with ureters. 



the urinary bladder (4), a part of the allantois which extends (5) 

 into the foetal appendages; the three ducts, the Mullerian (m), 

 the Wolffian (w), and the ureter (3). These ducts no longer 

 empty into the intestine, but into the allantoic structures, the 

 ureters into the base of the urinary bladder, the Wolffian and 



