SEXUAL ORGANS AND TISSUES. cy 



its breast-pouch, and Rhinoderma darwinii, with its enlarged croaking 

 sacs, — it is the male which undertakes the brooding office. "When 

 the young are born alive, the internal female ducts become developed 

 in this connection to form uteri. The ovary appears to serve as a 

 womb in the genus Girardinus among fishes, but it is usually the 

 median portion of the female duct which has this function. In pla- 

 cental mammals, where the young are born at an advanced stage, 

 and where the maternal sacrifice is at its maximum, the uterine 

 adaptations become more important and complex. The organs of 

 lactation will be afterwards discussed. 



