CLASS MAMMALIA 641 



which opens by short ducts into the urethra, and just behind 

 are a pair of Cowper's glands. The secretions from these glands 

 are added to the spermatozoa, making the seminal mass more 

 fluid. 



The two ovaries of the female are oval bodies exhibiting small, 

 rounded projections on the surface ; these are the outlines of the 

 Graafian follicles, each of which contains an ovum. The ovi- 

 ducts consist of an anterior Fallopian tube and a middle uterus; 

 the uteri unite posteriorly to form the vagina. The anterior 

 end of the Fallopian tube is wide and funnel-shaped; it carries 

 the ova from the ovary to the uterus, where the young are 

 developed. The urinogenital canal, or vestibule, is a wide, 

 median tube. On its ventral wall lies a small rod-like body, the 

 clitoris, corresponding to the penis of the male. 



The ova undergo holoblastic segmentation in the oviduct; they 

 then pass into the uterus, where they receive nourishment from 

 the blood of the mother through a structure called the placenta, 

 which is formed from the fcetal membranes and united with the 

 mucous membrane of the uterine wall. The interval between 

 fertilization and birth, known as the period of gestation, is thirty 

 days. Eight or ten young may be produced at a birth, and a 

 new litter may be born every month for a large part of the year. 

 Young rabbits breed when three months old. 



2. A BRIEF CLASSIFICATION OF LIVING MAMMALS 1 



As stated on page 632, there are about seventy- five hundred 

 species of living mammals, and three thousand or more species of 

 fossil forms known to man. The living mammals may be 

 grouped into two subclasses and eighteen orders. 



Class Mammalia. MAMMALS or " ANIMALS."- -Warm- 

 blooded vertebrates with a covering of hair at some stage in their 

 existence, and with cutaneous glands in the female, which secrete 

 milk for the nourishment of the young. 



1 Modified from Osborn's Age of Mammals. 

 2 T 



