302 COMPARATIVE ANATOMY. 



The Miillerian duct (Fig. 236, B, MG} of the female gives rise 

 to the oviduct, and in Mammals becomes distinctly differentiated 

 into three portions, a Fallopian tube, a uterus, and a vagina, 

 the latter of which opens to the exterior (Fig. 236, A, Ot, Ut, Vg]. 

 The Fallopian end of the oviduct always opens into the abdominal 

 cavity by a ciliated funnel-shaped aperture. This abdominal aper- 

 ture represents the only connection between the body-cavity and 

 the exterior in the Amniota, where the nephrostomes no 

 longer appear in the course of development. 1 



In the male, the Miillerian duct is always developed, but 

 plays no important part, coming into the category of rudiment- 

 ary organs. It will be further treated of together with the 

 generative organs in a later chapter. 



URINARY ORGANS. 2 



Fishes and Dipnoi. The Myxinoids retain throughout 

 life a functional pronephros which is provided with numerous 

 peritoneal apertures and a limited number of glomeruli. 



In Petromyzon slight rudiments of the pronephros of the 

 larva (Ammoccetes) alone persist, and the mesonephros with its 

 (segmental) duct becomes the functional urinary organ. 



In the Teleostei the pronephros may possibly persist in 

 some cases, but further researches are necessary on this point. The 

 mesonephros constitutes the main, if not the entire, excretory 

 organ of the adult, and consists of a narrow band varying in size, 

 which lies on the dorsal side of the body-cavity, between the verte- 

 bral column and the air-bladder. 3 Secondary fusions between the 

 organ of either side often occur, and this is also true of Ganoids. 

 The urinary duct in both groups probably represents the primary 

 segmental duct, and may lie more or less freely, or be embedded in 

 the substance of the kidney. Posteriorly the two ducts usually fuse 

 together and become expanded to form a kind of urinarybl adder, 

 which has evidently nothing to do with the similarly-named organ 

 (allantoic bladder) of Amphibia and Amniota (comp. p. 273). The 

 "bladder" usually opens behind the anus, either independently, 

 or together with the genital ducts, by a simple pore, or on the 

 summit of a urinogenital papilla. 



The splitting of the segmental duct into a Wolffian and a 

 Miillerian duct is not known to occur in Teleostei; in Elasmo- 

 b ranch s this differentiation does take place, and at the same time a 



1 For a different view see Mikalovics in the notes on pp. 296 and 300. 



2 No urinary organ is at present known to exist in Amphioxus. 



3 The delicate glistening threads from which the nest of the Sea-Stickleback 

 (Spinachia vulgaris) is made are formed as a secretion of the urinary tubules, which 

 undergo a change of function at the breeding-season. The secretion is much), which 

 becomes hard in water (Mobius). 



