UROGENITAL SYSTEM. gir 
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 © 
coelom (metaccele). A varying number of these nephrotomes (as 
they are called) lying a little behind the head are concerned in the 
Fic. 316.—Scheme of origin of pronephric tubules after Felix. A, earlier, B, later 
stage. c, ccelom; d, pronephric tubule and duct; e, epimere; h, hypomere; m, mesomere 
(lined); 2, 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 and Lepidosteus; seven or eight in skates; eight to 
eleven in Amia; and a dozen in some cecilians; 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 
Fic. 317.—Reconstruction from longitudinal sections of pronephros of Hypogecphis 
(cecilian), after Brauer. Pronephric duct (fd) and primary pronephric tubules light; 
the rest of the somites (nephrotomes) black; glomeruli between tubules 2-8. The three 
trunk somites in front of 1 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 
