576 MUSCLES AND SKELETON chap. 



nephros ; mi, n, d. metanephric duct ; int. nph. metanephros ; nst. nephrostomeb ; 

 (rj. ovary ; i. n. d. Mtillerian duct ; >. npk. pronephros ; sg. d. pronephric duct ; 

 t. sperinary ; v. e. efferent ducts. (From Parker and Haswell's Zoology). 



Mullerian 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, mt. n. d) is given off from the pos- 

 terior end of the ^\^olffian duct, which grows forwards and 

 becomes connected with a series of posterior nephridia. In 

 this way is formed a metanephros (int. nph), which gives rise 

 to the permanent kidney, and a metanephric duct {mt. n. d) 

 which becomes the ureter. The Wolffian body ceases to 

 discharge a renal function, becomes in the female a purely 

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

 (pp. 451 and 521), which receives the efferent ducts from 

 the spermary and from which the Wolffian duct (spermiduct 

 or vas deferens) arises. 



The homology of the hinder part of the kidney in the dogfish (p. 45 1 ) 

 is differently interpreted by zoologists. It is usually considered as 

 corresponding to part of the mesonephros, but in its differentiation from 

 the anterior part of this organ and in the development of special ureters 

 it resembles the metanephros of higher Vertebrates. 



The majority of the muscles are developed, as we have 

 seen (pp. 203 and 565) from the mesodermal segments 

 others arising from the parietal and visceral layers of the 

 mesoderm. 



The first part of the endoskeleton to arise is the endo- 

 dermic notochord (pp. 203 and 560), in the mesoderm 

 surrounding which cartilage appears and undergoes seg- 

 mentation, giving rise to the vertebrse, the notochord be- 

 coming constricted by the ingrowing cartilage, and eventually 

 disappearing more or less completely (compare pp. 425 

 and 565) : it at first extends into the head as far as the 



