THE UROGENITAL SYSTEM 21 



mesonephros to the dorsal anterior wall of the cloaca. 

 The ducts are those white tubes running in this tissue 

 which do not break or cut as readily as the connective tissue. 

 They extend medially and forward and enter a chamber, 

 the urinary sinus, situated on the dorsal surface of the 

 anterior end of the cloaca. Remove the connective tissue in 

 this region. The two sinuses unite to form a common cham- 

 ber, the urinary vesicle (it is not a true bladder or homolo- 

 gous with that organ since it is the enlarged termination 

 of the mesonephric ducts rather than an evagination of the 

 ventral wall of the cloaca). Make a small incision in the 

 sinus and run the probe into its entrance to the vesicle. 

 Probe through the vesicle and into the opening in the mid- 

 dorsal wall leading into the cloaca. The Wolffian ducts 

 (degenerate and functionless for the most part in the 

 female) of the cranial mesonephros are slender tubes which 

 extend anteriorly from the urinary vesicle lying on the 

 dorsal surface of the strong white portion of the mesen- 

 teries. The remainder of the excretory system (cloaca and 

 cloacal aperture) is in common with the digestive system. 

 Cut open the extended cloaca along the median ventral line 

 and also open the intestine a short distance. Note the open- 

 ing of the intestine, the anus, into the ventral part of the 

 cloaca and the horizontal fold which separates this from 

 the dorsal or urogenital portion. Cut into the latter by cut- 

 ting forward through this fold. The urogenital opening m 

 the median dorsal wall is half way between the openings 

 of the oviducts which are on either side. A probe may now 

 be run from the opening made in the urinary sinus through 

 the urinary vesicle and into the cloaca by way of this 

 urogenital opening. 



The ovaries are two soft bodies (masses about 25 mm. x 

 40 mm ) which contain large ova or eggs and are situated 

 in the back portion of the anterior half of the pleuroperi- 



