DEVELOPMENT OF ORGANS 



657 



KIDNEYS AND GONADS 



The renal organs begin to be formed during the second half of the 

 second day. They arise, in the three sections which are described 

 on p. 615 — pronephros, mesonephros, and metanephros — as groups 

 of tubules formed from the nephrotome or intermediate cell mass 

 (Fig. 511 i.c.m.) which unites the epimere (' mesoblastic somite ') 

 to the lateral plate. The pronephros is never functional. The outer 

 end of each of its tubules turns backwards and joins the next one. 



Fig. 513. — A transverse section through the middle of the mesonephros of a chick 



embryo of thirty-six hours, — From Lillie. 



Ao., Aorta ; cael., coelom ; col.t., collecting tubule ; germ.ep., germinal epithelium (rudiment of gonad) ; 

 glom., rudiment of glomerulus ; mst., mesentery ; n.t., nephrogenous tissue from which tubules are 

 formed ; T.i, T.2, r.3, tubules in three stages of formation ; V.c.p., posterior cardinal vein ; W.d., 

 Wolfl&an duct. 



SO that there is formed a longitudinal duct, the archinephric or 

 segmental duct, which grows back to join the cloaca. The meso- 

 nephric tubules come to open into this duct, and then, from the 

 point where the first of them joins, it is known as the Wolffian duct. 

 The ureter grows forwards from the hinder end of the Wolffian 

 duct to receive the tubules of the metanephros (Fig. 473). Rudi- 

 ments of the gonads appear on the fourth day on the peritoneum 

 at the base of the mesentery (Fig. 513). At about the seventh 

 day sexual differentiation begins to appear in them. The oviducts 

 (Miillerian ducts), of which a pair appear but that on the right 

 degenerates, begin, on the fourth day, as a thickened strip of 

 peritoneum overlying the mesonephros. The anterior part of the 



