DEVELOPMENT. 707 



mass of mesoblast, which lies just external to the intermediate cell-mass 

 (ung, Fig. 495). This cord, at first solid, becomes gradually hollowed 

 out to form a tube (Wolffian duct) which sinks down till it projects be- 

 neath the lining membrane into the pleuro-peritoneal cavity. 



The primitive tube thus formed sends off secondary diverticula at 

 frequent intervals which grow into the surrounding mesoblast : tufts of 

 vessels grow into the blind ends of these tubes, invaginating them and 

 producing " Malpighian bodies " very similar in appearance to those of 

 the permanent kidney, which constitute the substance of the Wolffian 

 body. 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 



FIG. 495. Transverse of embryo chick (third day), mr, rudimentary spinal cord; the primi- 

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

 mass; m, muscle-plate; dr, df, hypoblast and visceral layer of mesoblast lining groove, which is not 

 yet closed in to form the intestines; a, o, one of the primitive aortse; itn, Wolffian body; wngr, Wolf- 

 fian duct; vc, vena cardinalis; h, epiblast; hp, somatopleure and its reflexion to form a/, amniotic 

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



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 (Mullers duct). 



All the above points are shown in the accompanying figures, 495, 

 496, 497. 



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, there are developed from a new mass of blastema, the vasa efferen- 

 tia, coni vasculosi, and globus major of the epididymis; and thus is 



