352 COMPARATIVE ANATOMY 



duct is rudimentary, and the ova pass to the exterior by means of 

 the Mlillerian duct. 



This special differentiation of tlie hinder part of the ineso- 

 nephros, and the formation of special ducts in connection with it, 

 seems to foreshadow the condition which occurs in the Amniota 

 (pp. 346 and 356). 



The anterior (sexual) part of the kidney is usually broader than the 

 posterior (renal) part. The outer border is usually notched, and this, 

 together with the arrangement of the nephrostomes in the embryo, points 

 to the original segmental arrangement of the organ. The segmental char- 

 acter, however, disappears later on ; in the adult the nephrostomes are 

 much less numerous than the vertebrae of this region, but their number 

 and size vary much in difl'erent genera and even in individuals, and they 

 apparently do not persist in all. 



The morphology of the kidneys and renal ducts in G-anoids (Figs. 

 286 and 287) requires further investigation. They seem on the 

 whole to resemble those of Teleosts, though in the Sturgeon they 

 apparently show points of similarity to those of Elasmobranchs. 

 As in the Teleostei, a well-developed pronephros is present in 

 the larva, and the kidney duct probably represents the pronephric 

 duct. 



In many Fishes the kidneys extend far back into the root of 

 the tail. 



A close examination of the organ, which appears to the naked eye as the 

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

 — more particularly the anterior part — consists of an adenoid or lymphoid 

 substance. 



In the Dipnoi the kidneys also undoubtedly correspond to 

 the mesonephros. They are relatively longer in Protopterus 

 (Fig. 288) than in Ceratodus, extending through a considerable 

 portion of the body-cavity : as in Elasmobranchs, a narrower 

 anterior can be distinguished from a broader posterior part, and 

 the whole is largely invested by lymphoid and adipose tissue. 

 Nephrostomes are wanting. Until their develojoment is known, 

 it is uncertain to what extent the renal ducts correspond to the 

 primary mesonephric ducts: each opens into the cloaca inde- 

 pendently, behind the genital aperture. The cloacal csecum (p. 262) 

 probably functions as a urinary bladder. 



Amphibia. — The pronephros is well developed in the larva, 

 and is particularly large in the Gymnophiona, in which as many as 

 12 or 13 nephrostomes may be present. 



In adults, the most primitive condition is met with in the 

 Gymnophiona, in which the kidney (mesonephros) consists 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 

 metamerically, and in each of them a glomerulus, a nephrostome, 

 and an excretory duct can be distinguished (Fig. 291). This con- 

 dition sometimes persists in the anterior portion of the kidney. 



