ANATOMY OF A 17.8 MM. HUMAN EMBRYO 47 



Miilleriau dud. The Miillerian duct {D.M.) lies parallel with 

 the Wolffian, being a little ventral to it. Its cephalic end opens 

 freely into the abdominal cavity. The opening which occurs 

 near the cephalic end of the Wolffian body, shows something of a 

 fimbriated condition {Fim.). In the caudal direction the Miil- 

 lerian duct seems to terminate in a blind pointed end close to the 

 Wolffian duct. The point of termination is about on a level with 

 the junction of the middle and caudal thirds of the Wolffian body. 



Bladder. The ventral segment of the cloaca is divisible into 

 two parts, a cephalic portion [B) which will ultimately form the 

 bladder and a caudal portion (S.u.-g.) the urogenital sinus. 



The region of the bladder anlage adjoining the urogenital sinus 

 is expanded on either side to produce a dorso-lateral ridge. Into 

 each ridge at the caudal extremity of the bladder anlage opens the 

 corresponding ureter. The portion of the bladder remote from 

 the urogenital sinus is continued as the allantois into the caudal 

 wall of the umbilical cord. The proximal portion of the allantois 

 consists of a solid cord of cells, the urachus, but distally isolated 

 portions of the original lumen are found. 



Urogenital sinus. The cephalic region of the urogenital sinus 

 receives the Wolffian ducts. The orifices of these are placed on 

 either side, a short distance from the median hne. The remainder 

 of the sinus extends into the median caudal part of the genital 

 tubercle (Pa.gen.) as a laterally compressed structure. In profile 

 view this part of the sinus appears somewhat triangular in outline. 

 Its dorsal region encloses a cavity, but its latero-ventral walls 

 are approximated to form an incomplete raphe, the urogenital 

 membrane (Mem.u.-g.). This membrane is broken down in its 

 extreme caudal part so that the urogenital sinus opens to the 

 exterior. 



The lower part of the sinus becomes the vestibule in the female, 

 while in the male it forms the main portion of the urethra. The 

 female urethra and the proximal part of the male urethra being 

 formed by a canal differentiated later between the bladder anlage 

 and the urogenital sinus. 



Metanephros. The metanephros (Met., pi. 2; see also pi. 1) is 

 situated dorsal to the middle part of the Wolffian body, extending 



