508 THE HUMAN EMBRYO. 



DEVELOPMENT OF THE URINARY ORGANS. 



The general history of development of the urinary organs in 

 man is the same as in the rabbit. Paired Wolffian ducts and 

 Wolffian bodies appear first ; these form the excretory organs of 

 the early stages, and attain a considerable size during the second 

 month, after which time they gradually shrink, ultimately 

 losing their excretory function, and becoming modified to form 

 accessory parts of the reproductive system. 



The permanent or adult kidneys arise, as in the rabbit, as 

 outgrowths from the hinder ends of the Wolffian ducts : from the 

 third month onwards they replace the Wolffian bodies as the 

 functional excretory organs. 



A pair of Miillerian ducts is formed, independently of the 

 Wolffian ducts, and in the female becomes modified to form the 

 oviducts, uterus, and vagina. The head-kidney, if present at 

 all, is in a very rudimentary and evanescent condition. 



1. The Wolffian Duct and Wolffian Body. 



According to Kollmann, the Wolffian ducts appear, about the 

 fourteenth day, as a pair of longitudinal grooves of the external 

 epiblast, just below the level of the myotomes (Fig. 248, Kc). 

 By the middle of the third week the ducts are tubular, and lie 

 embedded in the mesoblast of the intermediate cell mass. It is 

 not yet certain, however, whether the tubular duct is formed by 

 closure of the lips of the groove, or by splitting oif of a rod of 

 cells from the thickened floor of the groove, which subsequently 

 acquires a lumen, and becomes tubular : while the observations 

 recorded in the case of rabbit embryos render it possible that 

 the origin of the Wolffian duct from the epiblast may prove 

 to be apparent rather than real (cf. p. 421). 



The Wolffian ducts at first end blindly behind, but about the 

 end of the third week or beginning of the fourth week they grow 

 back to the cloaca, and open into its sides '(Pig. 243, kc). 



The Wolffian bodies appear about the eighteenth day as a 

 pair of longitudinal ridge-like thickenings of the dorsal wall of 

 the body cavity, one on each side of the mesentery. These soon 

 become more prominent, and by the beginning of the fourth 

 week extend from about the sixth cervical to the last lumbar 

 somite. 



