338 COMPARATIVE MORPHOLOGY OF VERTEBRATES 



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

 coelom (metacoele). A varying number of these nephrotomes (as 

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

 formation of the pronephros (some teleosts, one; two in most 

 urodeles and amniotes; three in lampreys, anura, some sharks and 

 some amniotes; four or five in some sharks and Lepidosleus; seven 



Fig. 361. — Scheme of origin of pronephric tubules after Felix. A, earlier, B, later 

 stage, c, coelom; d, pronephric tubule and duct; e, epimere; A, hypomere; m, mesomere 

 (lined); wy, myotome; «, nephrostome; so, sp, somato- and splanchnopleure. 



or eight in skates; eight to eleven in Amia; and a dozen in some caeci- 

 lians; while it is claimed that the whole series of nephridial tubules of 

 Bdellostoma is pronephric) . The somatic walls of these nephrotomes 

 (fig. 361) grow out toward the ectoderm, thus forming slender 

 pronephric tubules (amphibia), or solid cords which later become 

 canalized (elasmobranchs, amniotes), but in all cases they eventually 



Fig. 362. — Reconstruction from longitudinal sections of pronephros of Hypogeophis 

 (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. 



form tubes, the proximal end of each communicating freely with the 

 metacoele by way of the cavity of the nephrotome, the opening of 

 the latter into the metacoele being the nephrostome. As will be 

 understood, these tubules, like the nephrotomes, are metameric in 

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

 outward until they are just beneath the ectoderm; when they bend 



