THE URO-GENITAL SYSTEM 383 



transmitted, a single nephridium with its nephrostome became 

 differentiated for this purpose, and that later on there came a 

 longitudinal splitting of the primary mesonephrotic duct, be- 

 ginning above and progressing gradually, for the better ac- 

 commodation of the sexual products. The employment of a 

 nephrostome instead of vasa efferentia is quite a fundamental 

 difference, but rudiments of these latter vessels are to be de- 

 tected in association with the ovaries, and thus the use of the 

 former may have been a later adaptation. 



The uro-genital relations of the selachians seem to have been 

 inherited directly by the amphibians (Fig. 109, a and c), for 

 the two correspond closely; in the male there is the same re- 

 lationship between testes and sexual kidney, and the meso- 

 nephrotic duct is a common ureter and ductus deferens. A 

 rudimentary oviduct tapering anteriorly to a blind end, is usu- 

 ally found attached to the side of this latter tube. In the fe- 

 male the oviduct is often very long and convoluted, and its 

 walls are often glandular and furnish membranous and gelat- 

 inous encasements for the eggs. In a few instances the lower 

 part of the tube is expanded into a uterus for the retention 

 of the larva. 



Corresponding to the lack of internal copulation there are 

 no external organs, but there are various instincts developed 

 which have for their purpose the mingling of the sexual 

 products. Thus the males of some aquatic salamanders pro- 

 duce conical spermatophores, which rest upon the sand at the 

 bottom of the pond and are taken up by the cloaca of the fe- 

 male ; a similar purpose is seen in the amplexation of frogs and 

 toads, in which the males embrace the females during ovi- 

 position and void the seminal fluid over the egg masses as soon 

 as laid. 



A fundamental change of relationship is seen in the Am- 

 niota, caused by the appearance of the third kidney, the meta- 

 nephros. This organ, which possesses a separate ureter and 

 is thus a complete urinary system in itself,' assumes the entire 

 control of this function, and leaves to its predecessor, the 

 mesonephrotic system, nothing but reproductive functions. 



