120 ZOOLOGY SECT. 



In sonic forms ilic pionephric duct now becomes (lividecl by a 

 longitudinal partition into two tubes : one retains its connection 

 with the meson ephros and is known as the mesonephric or Wolffian 

 dud (Cms. 01. d.): the other has no connection with the" tubules, but 

 opens into the ccelome in the region of the vanishing ])rone[)hros, 

 and is called the MuUcrian dud (p. n. d.). In some Craniata the 

 Miillei-ian apjiears (|uiie independently of the Wolffian duct: 

 the latter is then simply the pronephric duet after the union 

 with it of the mesonephric tubules. 



In the higher Vertebrata^ from Reptiles to Mammals, a diverti- 

 culum (D, E, rjit. n. d.) is given off from the posterior end of the 

 Wolffian duct, which grows forwards and becomes connected with 

 the hindmost tubules. In this way is formed a metanephros 

 (mt. npli.), which forms the permanent kidney, and a metanephric 

 duct {mt. n. d.), which gives rise to the ureter. The Wolffian 

 body ceases to discharge a renal function, and becomes a purely 

 vestigial organ. 



In many Fishes there is a dilatation of the ureter, the urinary 

 Uaddcr. which serves as a receptacle for the urine. In the higher 

 Craniata the ventral wall of the cloaca sends off a pouch, the 

 aUautoic Uaddcr (ai. Id.), which serves the same purpose, although 

 morphologically an entirely different structure. 



The gonads (f/on.) are developed as ridges ' growing from the 

 dorsal wall of the c«lome and covered by coiilomic epithelium, 

 from the cells of which, as in so many of the lower animals, the 

 ova and sperms are derived. The testis consists of crypts or 

 tubules, lined with epithelium, and usually discharging their pro- 

 ducts through delicate rasa, rffcrentia (D, v. c.) into the Wolffian 

 duct, but in some groups into the coelome. The sperms are 

 always motile. The ovary is formed of a basis of connective- 

 tissue or stroma, covered by epithelium, certain of the cells of 

 which become enlarged to form ova. In the majority of cases the 

 ova are discharged from the surface of the ovary into the coelome 

 and thtis into the open ends of the Miillerian ducts (E,p.n. d.), 

 which thus function simply as oviducts, having no connection in 

 the adult with the urinary system. In some groups the ova, 

 like the sperms, are shed into the ccjelome and escape by the 

 genital pores, and in many bony Fishes, the ovary is a hollow 

 organ, as in Arthropoda, discharging its ova into an internal 

 cavity, whence they are carried off by a duct continuous with the 

 .gonad. 



A few Craniata are normally hermaphrodite, but the vast 

 majority are dioecious, hermaphroditism occurring, however, 

 occasionally as an abnormal it^^ 



In close topographical relation with the urinogenital organs are 

 found certain " ductless glands," the adrenals or inter- and supra- 

 renal bodies. They are developed partly from ridges of the dorsal 



