xm 



PHYLUM CHORDATA 



115 



appear in the segments posterior to the pronephros, and together 

 constitute the mesonephros or Wolffian body (B, ms. nph.}, from 

 which the permanent kidney is formed in most of the lower Craniata. 



FIG. 801. Diagrams illustrating the development of the urinogenital organs of Craniata. 

 A, development of pronephros and pronephric duct ; B, atrophy of pronephros, develop- 

 ment of mesonephros ; C, differentiation of pro- and mesonephric ducts ; D, development 

 of metanephros, male type ; E, female type. al. bl. allantoic bladder ; an. anus ; cl. cloaca ; 

 gon. gonad ; int. intestine ; m. c. Malpighian capsule ; ms. n. d. mesonephric duct ; ms. nph. 

 mesonephros ; mt. n. d. metanephric duct ; mt. nph. metanephros ; nst. nephrostome ; ov. 

 ovary ; p. n. d. and sg. d. pronephric duct ; p. nph. pronephros ; t. testis ; v. e. vasa efferentia. 



The mesonephric tubules open at one end into the pronephric duct 

 (sg. d.), at the other by ciliated funnels (nst.), into the ccelome ; a 



H 2 



