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 
Fic. 229.—Diagrammatic transverse sections through 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; @.c, 
alimentary canal; a.d, archinephric duct ; g, glomus; gi, glomerulus ; 7.m, inner 
nephrostome ; mb, Malpighian body ; md, Miillerian duct ; mnd, mesonephric 
duct ; mnt, mesonephrie tubule ; myc, myocoele ; myt, myotome ; 7, notochord ; 
np, nephrotome ; nt, nephrostome ; 0.n, outer nephrostome ; px.f, pronephric 
tubule ; s.c, spinal cord ; ¢, 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 
mesonephrice 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. 
