UROGENITAL SYSTEM. 31 1 



becomes segmented, and later, when the epimere separates to form 

 the myotome, the dorsal end of each mesomere becomes closed, the 

 whole then forming a sac, opening below into the ventral, undivided 

 ccelom (metaccele). A varying number of these nephrotomes (as 

 they are called) lying a little behind the head are concerned in the 



FIG. 316. Scheme of origin of pronephric tubules after Felix. A, earlier, B, later 

 stage, c, coelom; d, pronephric tubule and duct; e, epimere; h, hypomere; w, mesomere 

 (lined); n, nephrostome; my, myotome; so, sp, somato- and splanchnopleure. 



formation of the pronephros (two in most urodeles and amniotes; 

 three in. lampreys, anura, some sharks and some amniotes; four or 

 five in some sharks andLepidosteus; seven or eight in skates; eight to 

 eleven in Amia; and a dozen in some caecilians; while it is claimed that 

 the whole series of nephridial tubules of Bdellostoma is pronephric). 

 The somatic wall of these nephrotomes (fig. 316) grow out toward 



FIG. 317. Reconstruction from longitudinal sections of pronephros of Hypogecphis 

 (caecilian), after Brauer. Pronephric duct (pd) and primary pronephric tubules light; 

 the rest of the somites (nephrotomes) black; glomeruli between tubules 2-8. The three 

 trunk somites in front of i develop no tubules. 



the ectoderm, thus forming slender pronephric tubules (or solid cords 

 which later become canalized), the proximal end of each communica- 

 ting freely with the metaccele by way of the cavity of the nephrotome, 

 the opening of the latter into the metaccele being the nephrostome. 

 As will be understood, these tubules, like the nephrotomes, are meta- 

 meric in character, equalling the somites in number. The distal ends 



