594 MUSCLES AND SKELETON CHAP. 



nephros ; int. n. d. metanephric duct ; mt. nph. metanephros ; nst. nephrostomes ; 

 ov. ovary ; p. n. d. Miillerian duct ; /. nph. pronephros ; sg.d. pronephric duct; 

 t. spermary ; v. c. efferent ducts. (From Parker and Haswell's Zoology). 



the mesonephros and is now known as the mesonephric or 

 Wolffian duct (C, ms. n. d) : the other, or Miillerian duct (p. 

 n.d\ has no connection with the nephridia, but opens into 

 the coelome in the region of the vanishing pronephros, and 

 assumes the functions of ah oviduct in the female. In some 

 Vertebrates the Miillerian duct 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 (Reptiles, Birds, and Mammals) 

 a diverticulum (D, E, ////. n. d) is given off from the pos- 

 terior end of the Wolffian duct, which grows forwards and 

 becomes connected with a series of posterior nephridia. In 

 this way is formed a metanephros (mt. npli), which gives rise 

 to the permanent kidney, and a metanephric duct (mt. n. d} 

 which becomes the ureter. The whole mesonephros ceases 

 to discharge a renal function, becomes in the female a 

 purely vestigial organ, and in the male gives rise to the 

 epididymis (p. 467), which receives the efferent ducts 

 from the spermary and from which the Wolffian duct 

 (spermiduct or vas deferens) arises. 



In the specialisation of the hinder part of the mesonephros and in 

 the development of special ureters the kidney of the dogfish (Fig. 127) 

 resembles the metanephros of higher Vertebrates, which, however, 

 possess no nephrostomes. 



The majority of the muscles are developed, as we have 

 seen (pp. 203 and 584), from the mesodermal segments, 

 others arising from the parietal and visceral layers of the 

 mesoderm. 



The first part of the endo skeleton to be formed is the 

 endodermic notochord (pp. 203 and 578), in the mesoderm 



