398 FISHES CHAP. 



tomes lose their connexion with the myocoeles, although they 

 still retain their " nephrostomes " or apertures through which they 

 communicate with the ventral coelom. When fully developed the 

 pronephros consists of a few tubules, more or less convoluted, open- 

 ing at their inner extremities into the coelom by means of their 

 ciliated nephrostomes, and at their outer ends communicating 

 with the exterior through the archinephric duct. In relation 



mnf 



Fig. 229. — Diagrammatic transverse sections througli an embryo Craniate to show the 

 mode of development of the pronephros (A) and of the mesonephros (B). The right 

 side of each figure shows an earlier stage than the left. In B (left side) the con- 

 nexion of a vas efferens with a mesonephric tubule, and the division of the archi- 

 nephric duct into Miillerian and mesonephric ducts are shown, a. Aorta ; ax, 

 alimentary canal ; a.d, archinephric duct ; g, glomus ; gl, glomerulus ; i.n, iainer 

 nephrostome ; mb, Malpighian body ; vid, Miillerian duct ; mnd, mesonephric 

 duct ; mnt, mesonephric tubule ; mt/c, myocoele ; myt, myotome ; n, notochord ; 

 np, nephrotome ; nt, nephrostome ; o.n, outer nephrostome ; pn.t, pronephric 

 tubule ; s.c, spinal cord ; t, testes ; v.c, ventral coelom ; v.ef, vas efferens. (After 

 Kingsley and Semon. ) 



with the pronephros a branch from the dorsal aorta forms a tuft 

 of capillary blood-vessels or " glomus," opposite the nephrostomes, 

 which projects into the ventral coelom on each side. Later, a 

 second series of much more numerous tubules is formed behind 

 the pronephros, which constitute the mesonephros. In forming 

 mesonephric tubules the nephrotomes become disconnected from 

 the myotomes and their myocoeles, and curving outwards they 



epiblast ; hence the suggestion that in the primitive Vertebrates the duct was a 

 longitudinal groove in the superficial skin into which the pronephric tubules 

 opened externally. 



