THE KIDNEY AND URETER. 591 



of the Wolffian bodies. The ridge lies close to the Wolffian 

 duct, and extends along its whole length, but is quite independent 

 of this. 



Early in the fifth week, the Miillerian duct is formed in this 

 ridge ; it is a narrow straight tube, lying along the outer side 

 of the Wolffian duct, but distinct from this. Its anterior end 

 opens into the body cavity by an elongated slit-like mouth, 

 situated in a patch of thickened peritoneal epithelium, a little 

 way in front of the anterior end of the Wolffian body. Poste- 

 riorly, the Miillerian duct ends blindly. 



By the eighth week the Miillerian duct has undergone some 

 changes. It commences in front with a wide funnel-like mouth, 

 the margins of which are already slightly fimbriated. Behind 

 this mouth, the duct runs straight backwards for some distance, 

 along the outer side of the Wolffian body, then turns sharply 

 inwards, crosses ventral to the Wolffian duct, and continues back- 

 wards in close contact with the Miillerian duct of the opposite 

 side ; it still ends blindly behind. 



In the male, the Miillerian ducts begin to atrophy shortly 

 after reaching this stage. In the female, they undergo further 

 development, and give rise to the oviducts, uterus, and vagina, 

 as will be described in the section dealing with the accessory 

 organs of reproduction. 



4. The Head-kidney. 



Janosik has described, in an embryo eighteen to nineteen 

 days old, what he thinks may prove to be a rudimentary pro- 

 nephros, in the form of a couple of peritoneal funnels just in 

 front of the anterior end of the Wolffian duct ; the anterior 

 funnel having close to it a structure not unlike an external 

 glomerulus. The early development and subsequent fate of 

 these structures have not yet been determined. 



THE DEVELOPMENT OF THE REPRODUCTIVE 

 ORGANS. 



1. The Essential Reproductive Organs, 



These have already been described, in the introductory portion 

 of this chapter (pp. 449 to 457) ; but a few further details may 

 conveniently be added here. 



In embryos thirty-two days old (cf. Fig. 205), the genital 



