THE UROGENITAL SYSTEM 95 



the anterior end of the uterine horn to the dorsal body- 

 wall near its union with the diaphragm. 



The ovary of most mammals is not inclosed in a capsule. 

 The ova which break through the wall of the ovary pass 

 directly to the opening {ostium ahdominale) of the uterine 

 tube, and are then carried by cilia in the tube to the uterus. 

 The following events, however, are said to accompany ovu- 

 lation in the rat. The periovarial cavity is filled with fluid 

 at the time of ovulation. The bursa ovarica exerts pres- 

 sure upon this fluid as the result of the contraction of the 

 muscles located near the infundibulum tubae and the at- 

 tachments of the uterine tube. Simultaneously muscular 

 contractions expand the opening (ostium) of the infundi- 

 bulum tubae. Thus the fluid within the bursa ovarica, 

 with the contained eggs, is forced into the uterine tube, 

 where muscular contractions, possibly assisted to some ex- 

 tent by ciliary action, carry them to the uterus. 



The fertilized egg is implanted in the wall of the uterus, 

 where it develops. Examine a female rat with well ad- 

 vanced fetuses in the uterus. Note the large size of the 

 uterus, its abundant blood supply, and segmental char- 

 acter. Each segment contains a fetus. Slit a segment 

 lengthwise along its ventral surface and expose the fetus. 

 This is covered with a thin membrane, the amnion. Care- 

 fully open the amnion along the back of the fetus. The 

 fluid in the amniotic cavity protects the embryo from sud- 

 den jars. Note the discoidal placenta on the dorsal side 

 of the uterus. The umhilical cord connects the belly of 

 the embryo with the center of the placenta. The blood 

 of the fetus passes through the umbilical cord to and from 

 the placenta, where by osmosis, it receives nutriment from 

 the maternal blood and surrenders waste materials to the 

 mother's blood. The blood of fetus and mother do not mix 

 in the placenta, but are everywhere separated from each 



