UROGENITAL ORGANS. 



117 



arise almost entirely from the mesomeric segments (nephro- 

 tomes). The pronephros arises from a few ^ nephrotomes im- 

 mediately behind the head. From the outer wall of each of these 

 an outgrowth occurs, — sometimes solid at first, but usually 

 hollow from the beginning, — its apex directed towards the skin. 

 These outgrowths form the pronephric tubules, each of which 

 opens at the inner end, by means of the remains of the cavity of 

 the mesomere, into the body cavity, the opening being funnel- 

 shaped and, in its full development, ciliated. These openings 



A B 



Fig. 126. Diagrams of the relations of pro- and mesonephros, based on 

 Semon. Mesomeric structures shown with conventionalized cells. A, pronephros ; 

 B, mesonephros. a, aorta ; b. Bowman's capsule ; d, pronephric duct ; g, gonad ; 

 gl, glomerulus; gs, glomus; in, inner nephrostome; m, myotome; nis, formation of 

 mesenchyme; w/, mesonephric tubule ; ;/, cavity of nephrotome; ns, nephrostome; 

 on, outer nephrostome ; /, pronephric tubule. 



are the nephrostomes. Distally the tubules of the successive 

 segments fuse together, thus giving rise to a longitudinal tube, 

 the pronephric duct, which gradually extends backwards be- 

 hind the pronephric segments, until at last it fuses with the 

 cloaca or with the skin immediately adjoining. An opening 

 now forms between the duct and the cloaca, and thus, through 

 the system of tubes leading from the nephrostomes to the vent, 

 the body cavity is placed in connection with the external world. 



1 Two in most urodeles and amniotes ; three in lampreys, some sharks, anura, and some 

 amniotes ; four in some sharks, seven or eight in skates, and a dozen in csecilians. It has 

 been pointed out that in general terms the number of pronephric nephrotomes is roughly 

 correlated to the number of segments in the whole body. 



