492 



Vertehrata. 



and in some (Monotremes, Cetacea, Proboscidians, etc.), they retain 

 tMs position tlirougliout life. In most species, however, at the close 

 of fcetal life, or in early youth, each descends into an evagination of 

 the ventral abdominal wall ; usually the two evaginations are united 

 externally to form a pouch-like appendage {scrotum), divided by a 

 septum into two compartments, each containing one of the testes ; 



Fig. 401. Diagrammatic 

 longitudinal section of the 

 cloaca (or rectum) and the 

 copulfttory organs, A 

 of aCrocodile, B of a 

 Monotreme, C — D of 

 various other Mammalia. 

 (J anus, 6 urinary bladder, 

 cl cloaca, / corpus fibrosum, 

 h skin, i gut, r (in A) seminal 

 groove, in (B — D) seminal 

 tube, s vas deferens, u urino- 

 genital sinus (in C the de- 

 generated portion of the 

 sinus is dotted: w'), ur 

 ureter. — Orig. 



whilst its cavity communicates with the abdomen by a duct varying 

 in width, which is often closed in the adult.* In the Monotremes 

 the vasa deferentia open, together with the ureters and urinary 

 bladder, into a deep, though narrow outgrowth of the ventral wall 



* In some Mammalia (Inseotivora, Kodentia) the testes only lie in the scrotum 

 during the breeding season (when they are much enlarged) ; at other times, in the 

 abdomen. 



