232 COMPARATIVE ANATOMY 



Further than this, the segmental ducts of each side become 

 split longitudinally in the course of development into two 

 ducts. One of these, now known as the Wolffian duct, retains 

 its connection with the mesonephric tubules and serves as the 

 mesonephric duct ; the other,' now known as the Miillerian 

 duct, is converted into the oviducts of the female, but is 

 aborted and represented only by a rudiment in the male. 



Let us now examine the excretory organs and their relations 

 to the generative system in the adult male dogfish (fig. 56, £). 

 The mesonephros of either side is an elongate glandular 

 body extending far forward in the roof of the peritoneal cavity, 

 and continuous posteriorly with the metaphros. The latter is 

 a compact laterally-compressed glandular mass extending back 

 into the pelvic region dorsal of and to the side of the cloaca. 

 The mesonephros is functional in the adult, and its excretion is 

 carried to the cloaca by the Wolffian or mesonephric duct, 

 which is a conspicuous tube imbedded in the ventral surface 

 of the mesonephros and thrown into a number of horizontal 

 loops. The Wolffian duct, however, is not only an excretory 

 duct, but serves also as the sperm duct or vas deferens. The 

 testes are a pair of elongated bodies lying in the dorsal part 

 of the peritoneal cavity and attached to its dorsal wall by 

 folds of membrane, the mesorchia. They are united together 

 posteriorly, but their anterior ends lie on opposite sides of the 

 gullet, and each communicates by a number of very slender 

 ducts, the vasa efferentia, with the anterior end of the 

 mesonephros of its own side. The sperm passes through the 

 vasa efferentia into the mesonephric tubules, and thence is 

 carried by the Woffian duct to the cloaca. The posterior end 

 of each Wolffian duct dilates to form a vesicula seminalis, 

 which runs straight down the ventral face of the metanephros 

 and opens into a cavity known as the urogenital sinus, which 

 in turn opens by a single median aperture situated at the end 

 of a prominent papilla, into the cloaca. The urogenital sinus 

 is formed by the union of the posterior ends of a pair of thih- 

 walled conical sperm-sacs. The anterior ends of these sacs are 

 adherent to the ventral surfaces of the vesiculre seminales and 

 taper forwards to end blindly near the front end of the latter. 

 The urogenital sinus receives not only the openings of the 

 Wolffian ducts, but also those of the ureters or metanephric 

 ducts. The latter are formed by' the union of five ducts on 



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