XIII 



PHYLUM CHORDATA 



153 



on each side, open into the urino-genital sinus. The latter has 

 a median aperture into the general cavity of the cloaca situated 

 on the summit of a prominent urino-genital papilla. The oviducts 

 (Mulleriaii ducts) of the female are repressnted in the male by 

 rudiments of their anterior portions. The entire kidney is some- 

 times regarded as a mesonephros. but the posterior portion, de- 

 veloped entirely behind the part which is converted in the male 

 into the epididymis, and having its own ducts, is sometimes 



neph 



med.,ur.sin, 



FIG. 779. Chiloscyllium. Right kidney 

 and urinary sinus of female, ma!. *'/. sii>. i'.--, 

 median urinary sinus; neph, kidney; 



v.i-. */'/><'.$, right urinary sinus. 



FIG. 780. Dog-fish, egg-case. (After 

 Dean.) 



looked -upon as corresponding to the metanephros of the higher 

 Vertebrates. 



The ripe ovum, rupturing the wall of its Graafian follicle, 

 escapes into the abdominal cavity, whence it reaches the interior 

 of one of the oviducts ; there it becomes fertilised by sperms 

 received from the male in the act of copulation, and then becomes 

 enclosed in a chitinoid case or shell (Fig. 780) secreted by the 

 shell-gland. 



