126 MORPHOLOGY OF THE ORGANS OF VERTEBRATES. 



by rupture of the surrounding walls, the egg escapes. In the 

 higher vertebrates usually but a single egg escapes at a time, 

 but in the ichthyopsida hundreds or even thou- 

 sands may pass out from the ovary in a few hours. 

 As will be understood from the relations of the 

 ovary, the eggs upon their escape pass into the 

 coelomic cavit}', from which they pass to the ex- 

 terior, in the case of some fishes, by means of the 

 abdominal pores, but in most vertebrates through 

 the Miillerian duct. 



At first the ovaries are simple ridges project- 

 ing slightly into the ccelom, but as they increase 

 in size they become more distinct from the body 

 wall, until at last they are only supported by a 

 double fold of the peritoneum, like a mesentery, 

 the mesovarium. 



The male gonads which produce the sperma- 

 tozoa are called testes, and, like the ovaries, their 

 essential portions are derived from the germinal 

 epithelium. This in places sinks into the under- 

 lying mesenchyme as cords of cells. Later these 

 cords become hollow (canaliculi seminiferi), and 

 from certain large round cells in their walls, the 

 spermatozoa are formed by cell division. In the 

 case of the testes there is the same formation of a 

 mesenterial support (mesorchium) as was noted for 

 the ovary. 



The Urogenital Ducts. — In teleosts, ganoids, 

 and cyclostomes the pronephric duct remains with- 

 out essential alteration, functioning solely as an 

 excretory duct. In the elasmobranchs it divides 

 into two tubes. In the mesonephric region its 

 lumen becomes oval in section, the collecting tu- 

 bules emptying into the dorsal portion. It now 

 divides lengthwise back as far as the cloaca, while 

 in front the division stops just in front of the 

 mesonephros (Fig. 1 34). The dorsal of the two resulting tubes 

 is the Wolffian (Leydig's) duct, the \entral the Miillerian duct. 



Fig. 134. 

 Division of 

 pronephric 

 duct into Miil- 

 lerian (ot) and 

 Wolffian (&■) 

 ducts in elas- 

 mobranchs ; ft 

 funnels; g, 

 glomeruli of 

 mesonephros; 

 0, ostium tubae 

 formed by fu- 

 sion of prone- 

 phric funnels. 



