44 COMPARATIVE ANATOMY 



ducts, called the vasa efferentia. The sperm, which is 

 formed in the testes, passes through the vasa efferentia into 

 the collecting tubes of the kidney, and thence by the ureter 

 to the vesicula seminalis and to the exterior. Thus in the 

 male frog there is a common duct for both generative and 

 urinary products, a fact of which we shall see the significance 

 hereafter. The ovaries vary very much in size according to 

 the season of the year. In the breeding season in early 

 spring they are very large, and fill the body cavity to 

 distention ; in summer and autumn they are much 

 smaller. They correspond in position with the testes — i.e. 

 they are attached to the dorsal wall of the body cavity by 

 a peritoneal fold, the mesovarium, and lie on the ventral sides 

 of the kidneys. Each ovary is a large irregularly-lobed sac, 

 which, when viewed externally, seems to be filled by a mass of 

 spherical black and white bodies about the size of No. 5 

 shot. These are the ova or eggs. There are no ducts 

 continuous with the ovaries ; the ova, when ripe, escape by 

 rupture of the ovarian walls, and fall into the body cavity, 

 whence they are carried to the exterior by a pair of much 

 coiled ducts, with funnel-like mouths opening into the body 

 cavity. These are the oviducts. They lie external to the 

 kidneys and ovaries, their mouths opening on either side into 

 the anterior part of the body cavity not far from the middle 

 line. Behind the mouth each oviduct has the form of a 

 rather slender white tube, which gradually expands, maintains 

 an even size throughout a much coiled course towards the 

 posterior end of the body, and then widens out suddenly to 

 form a pear-shaped dilation — the so-called uterus. The two 

 uteri open close together into the cloaca just in front of the 

 openings of the ureters. The walls of the coiled part of the 

 oviducts are abundantly furnished with glands, which swell 

 up during the breeding season and cause the oviducts to 

 increase greatly in size. The peritoneal fold by which the 

 ovaries are suspended is known as the mesometrium. (See 

 fig. 6.) 



The cloaca, into which the urinary and generative ducts 

 open, is a short tube continuous with the rectum. From 

 what precedes, it follows that it has, besides the opening of the 

 bladder on the ventral side, one pair of orifices on the dorsal 

 side in the male — viz. those of the common uro-genital duct ; 



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