THE DEVELOPMENT OF THE UROGENITAL SYSTEM. 107 
The mesonephros (Wolffian body), though largely developed 
in all vertebrates during foetal life, is not a persistent excretory 
organ of adult life. 
Fig. 123.—Diagrams illustrating development of pronephros in the fowl (Haddon). ao, aorta; 
6. c, body-cavity ; ep, epiblast with its epitrichial (flattened) layer ; hy, hypoblast ; m. s, 
mesoblastic somite; n. c, neural canal; nch, notochord; p. t, pronephric tubule; so, 
somatic ; and sp, splanchnic, mesoblast. 
In the fowl recent investigation has shown that the Wolffian 
(segmental) tubes originate from outgrowths of the Wolffian 
Fig. 124. Fia. 125. 
Fic. 124.—Rudimentary primitive kidney of embryonic dog. The posterior portion of the 
body of the embryo is seen from the ventral side, covered by the intestinal layer of the 
yelk-sac, which has been torn away, and thrown back in front in order to show the primi- 
tive kidney ducts with the primitive kidney tubes (a). _b, primitive vertebree ; c, dorsal 
medulla ; d, passage into the pelvic intestinal cavity. (Haeckel, after Bischoff.) . 
Fig. 125.—Primitive kidney of a humanembryo. uw, the urine-tubes of the primitive kidney ; 
w, Wolffian duct : w’, upper_end of the latter (Morgagni’s hydatid); m, Miierian duct ; 
m’, upper end of the latter (Fallopian hydatid); g, hermaphrodite gland. (After Kobelt.) 
duct and also from an intermediate cell-mass, from which lat- 
ter the Malpighian bodies take rise. The tubes, at first not con- 
