398 



EMBRYOLOGY. 



embryo, they grow backwards to the terminal part of the intestine, 

 and there implant themselves in the aUantois. This is, as we have 

 seen in the first part of this text-book (fig. 132, 3 and 4 al), an 

 organ which is produced by evagination of the anterior [ventral] wall 

 of the hind gut. In most Mammals (figs. 134' al and 142 ALC) it 



attains during embryonic 

 life a quite extraordinary 

 development, for it grows 

 out of the body-cavity, 

 penetrates between the 

 other foetal membranes, 

 and is distended into a 

 large vesic;le, which re- 

 ceives the urinary fluid 

 secreted by the embryo. 

 The part of it which lies 

 in the body-cavity remains, 

 on the contrary, narrow. 



\tta\. \ The terminal part of it 



\ I // which receives the Wolffian 



and MuUerian ducts is 

 called sinus urogenitalis 

 (fig. 219 sug and 229 ug), 

 a structure which will often 

 demand our attention in 

 considering the develop- 

 ment of the external sexual 

 organs. 



The sinus urogenitalis 

 and the hind gut unite 

 to form a short, unpaired 

 region, the cloaca (fig. 229 

 d), a small depression 

 which opens out at the 

 surface of the body and 

 in very many Vertebrates — in the Amphibia, Eeptiles, Birds, and 

 the lowest Mammals, the Monotremes — persists throughout life. 

 In the remaining Mammals, however, these structures have only 

 an embryonic existence. In the first case aU the elimination- 

 products of the body are conducted to the outside through the 

 «loaca,— out of the hind intestine the faecal masses, out of the 



Tig. 229. — Diagram of the urogenital organs of a 

 Stammal at an early stage, after Allem Thomson ; 

 from Balfoue. 



The parts are seen chiefly in profile, but the Miiilerian 

 and Wolffian duets are seen from the front. 



S, Ureter ; >i, urinary hladder ; 5, nrachus ; ot, genital 

 gland (ovary or testis) ; W, left Wolffian body 

 (primitive kidney) ; z, its diaphragmatic ligament ; 

 w, WolflBan (mesonephric) duct ; m, Miiilerian 

 dnct ; gc, genital cord consisting of Wolffian and 

 Miiilerian ducts enveloped in a common sheath ; 

 i, rectum ; ug, ui-ogenital sinus ; cp, genital emin- 

 ence, -which becomes the clitoris or penis; U, genital 

 ridges from which the labia majora or the scrotum 

 are developed. 



