GENERATION AND DEVELOPMENT. 299 



Appearance of First Rudiments. The Wolffian duct makes its ap- 

 pearance at an early stage in the history of the embryo, as a cord running 

 longitudinally on each side in the mass of mesoblast, which lies just ex- 

 ternal to the protovertebrae (ung, Fig. 460). This cord, at first solid, 

 becomes gradually hollowed out to form a tube (Wolffian duct) which 

 sinks down till it projects beneath the lining membrane into the pleuro- 

 peritoneal cavity. 



The primitive tube thus formed sends off secondary diverticula at fre- 

 quent intervals which grow into the surrounding mesoblast: tufts of ves- 

 sels grow into the blind ends of these tubes, invaginating them and pro- 

 ducing "Malpighian bodies" very similar in appearance to those of the 

 permanent kidney, which constitute the substance of the Wolffian body. 



FIG. 460. Transverse of embryo chick (third day), m r, rudimentary spinal cord; the primitive 

 central canal has become constricted in the middle : ch, notochord ; uwh, primordial vertebral mass ; 

 m, muscle-plate; dr, d/, hypoblast and visceral layer of mesoblast lining groove, which is not yet 

 closed in to form the intestines; ao, one cf the primitive aortae; un, Wolffian body; ung< Wolffian 

 duct; vc, vena cardinalis; /i, epiblast; h p, somatopleure and its reflection to form a/, amniotic 

 fold; p, pleuro-peritoneal cavity. (Kolliker.) 



Meanwhile another portion of mesoblast between the Wolffian body and 

 the mesentery projects in the form of a ridge, covered on its free surface 

 with epithelium termed "germ epithelium/' From this projection is 

 developed the reproductive gland (ovary or testis as the case may be). 



Simultaneously, on the outer wall of the Wolffian body, between it 

 and the body-wall on each side, an involution is formed from the pleuro- 

 peritoneal cavity in the form of a longitudinal furrow, whose edges soon 

 close over to form a duct (M tiller's duct). 



All the above points are shown in the accompanying figures, 460, 461, 

 462, 463. 



The Wolffian bodies, or temporary kidneys, as they may be termed, 

 give place at an early period in the human foetus to their successors, the 

 permanent kidneys, which are developed behind them. They diminish 

 rapidly in size, and by the end of the third month have almost entirely 

 disappeared. In connection, however, with their upper part, in the male, 



