xii PHYLUM CHORDATA 359 



of tubules by whose agency the process of secretion is car- 

 ried on, the whole being richly supplied with blood-vessels. 

 Eventually the tubules open into a duct, the ureter. In the 

 lizard and the rabbit there is present a median thin-walled 

 sac, the urinary bladder, in which the urine is stored, to be 

 discharged at intervals. In the rabbit the ureters open into 

 the bladder, and the latter opens on the exterior by a median 

 canal, the urethra. In the lizard the ureters and the bladder 

 have independent openings into the cloaca, and the bladder 

 is filled only by regurgitation from the latter chamber. 



The sexes are distinct in all three. There are two testes, 

 each with its duct or vas deferens. In the female there are 

 two ovaries, which are solid bodies in which the ova lie im- 

 bedded. In the dogfish, when mature, the ova are of large 

 size, containing a great quantity of food-yolk. The ova of 

 the rabbit are extremely small, while those of the lizard are 

 of a size intermediate between those of the other two. 

 Each ovum is enclosed in a follicle — the Graafian follicle — 

 with a wall composed of small cells. When the ovum 

 approaches maturity the follicle projects on the surface of 

 the ovary, and eventually the wall becomes ruptured and 

 the ovum escapes into the body-cavity. 



The oviducts, of which there are two, are not connected 

 with the ovaries, each opening anteriorly into the body- 

 cavity by a wide opening. In the dogfish and the lizard 

 the oviducts remain practically distinct from one another 

 throughout; in the rabbit the posterior parts are united to 

 form a median chamber, the body of the uterus, and a 

 median passage, the vagina, leading to the exterior. The 

 ova in all three, when discharged from the ovaries, enter the 

 wide openings of the oviducts and are impregnated during 

 their passage backwards. In both the dogfish and the lizard 

 each fertilised ovum becomes enclosed while in the oviduct 



