120 ZOOLOGY SECT. 



In some forms the pronephric duct now becomes divided by a 

 longitudinal partition into two tubes : one retains its connection 

 with the mesonephrosand is known as the mesonephric or Wolffian 

 duct (C, 7ns. n.d.): the other has no connection with the' tubules, but 

 opens into the coelome in the region of the vanishing pronephros, 

 and is called the MilUerian duct (p. n. d.). In some Craniata the 

 Miillerian appears quite independently of the Wolffian duct : 

 the latter is then simply the pronephric duct after the union 

 with it of the mesonephric tubules. 



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

 culum (D, E, rat. 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 

 hladder, which serves as a receptacle for the urine. In the higher 

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

 allantoic lladdcr (cd. hi.), which serves the same purpose, although 

 morphologically an entirely different structure. 



The gonads {gon.) are developed as ridges growing from the 

 dorsal wall of the cffilome and covered by ccelomic 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 vasa efferentia (D, v. e.) 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 thus 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 coelome 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 abnormality. 



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 



