304 COMPARATIVE ANATOMY. 



Wolffian duct ; while the latter, which persists as the permanent 

 kidney (comp. note on p. 298), empties its secretion by means of 

 separate ducts into the Wolffian duct, which thus serves to conduct 

 both urinary and genital products. In the female (Fig. 238A) 

 the genital gland has no connection with the Wolffian duct, and 

 the ova pass to the exterior by means of the Miillerian duct. 



A narrower anterior, and a broader middle and posterior portion can 

 usually be distinguished in the kidneys of Elasmobranchs. The outer border 

 is usually notched, and this, together with the arrangement of the nephro- 

 stomes in the embryo, points to the original segmental arrangement of the 

 organ. The segmental character, however, disappears later on ; in the adult 

 the nephrostomes are without exception much less numerous than the vertebra 

 of this region, but their number and size varies much in different genera and 

 even in individuals. 



The kidneys of Sturgeons appear to show many points of 

 similarity to those of Elasmobranchs ; further investigations are, 

 however, necessary before their relations can be fully explained, 

 and the same may be said with regard to the kidneys of Dipnoans 

 and bony Ganoids. In the Dipnoi, the existence of nephrostomes, 

 although not proved, is very probable ; the urinary organ of these 

 animals corresponds to the mesonephros. In Ceratodus, the duct 

 lies freely in a peritoneal fold, while in Protopterus it is embedded 

 in the substance of the kidney. In both forms the kidney 

 is lobulated; it is relatively much smaller in Ceratodus than in 

 Protopterus. The lobes do not correspond to the segmentation 

 of the vertebral column. 



A close examination of the organ which has usually been spoken of as the 

 kidney in Teleostei and Ganoids shows that a larger or smaller portion of it 

 more particularly the anterior part consists of an adenoid or lymphoicl 

 substance. 



Amphibia. In these, the most primitive condition is met 

 with in the Gymnophiona, in which the kidneys (Fig. 239, 

 Ni) consist of long narrow varicose bands, usually extending from 

 the heart to the anterior part of the cloaca, which latter is often 

 much elongated. In the embryo they consist of definite masses, 

 which are arranged segmentally (that is, correspond with the 

 segmentation of the vertebral column), and in each of them a 

 glomerulus, a nephrostome, and an excretory duct can be 

 distinguished (compare Fig. 234A). 



This condition sometimes persists in the anterior portion of the 

 kidney, while, owing to secondary processes of growth, as many as 

 twenty nephrostomes are later on met with in a single body- 

 segment. The number of nephrostomes in the entire kidney may 

 amount to a thousand or more. 



As regards the urinary duct and the relations of the entire 

 renal apparatus to the generative organs, the Gymnophiona in all 

 essential points resemble other Amphibians. 



