576 MUSCLES AND SKELETON cttAt. 



nephros ; ;///. ;/. d, mtlanephric duct ; mt. iij>/t. mctanephros ; list, nephrostomes ; 

 (^'. ovary ; 6. n. d. I\Iiillerian duct ; /. nph. pronephros ; sg. d. pronephric duct ; 

 /. speriiiary ; v, c. efferent ducts. (Krom Parker and Haswell's Zoology), 



Miillerian duct appears quite independently of the WoMan 

 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, i/if. 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 nietaiiephros {int. nph)., which gi\'es rise 

 to the permanent kidney, and a metanephric duct [int. n. d) 

 which becomes the ureter. The AVolffian body ceases to 

 discharge a renal function, becomes in the female a purely 

 \estigial organ, and in the male gives rise to the epididymis 

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

 the spermar)' and from which the M'olffian duct {spermiduct 

 or vas deferens) arises. 



The homology t.)f Lhu hindi^r pail uf Ihc kitlncy in the dogii.sh (p. 451) 

 i^ difl'erenlly inlcrprutcd by z.oologlsli. It is usually considered as 

 corresponding to pari of the mesonephros, bill in ils differentiation from 

 ihc anterior part of tliis organ and in the development of special iirclcrs 

 il resembles the melanephros of higher Vertebrates. 



The majority of the muscles are dexeloped, as we have 

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

 others arising from the parietal and visceral layers of the 

 mesoderm. 



'I'he 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 vertebra, the notochord bc- 

 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 



