630 CHORD ATA. 



species, forming an unpaired blind sac (vJ), which may even open 

 into the urogenital sinus as a third vagina. This partial fusion of 

 the vaginae of the marsupials is completed in the placental mam- 

 mals, the single vagina and the sinus forming a single canal (fig. 

 654). Here the uterine portions may remain distinct (uterus 



A 



FIG. 651. A, uterus duplex; B, uterus bicornis; C, uterus simplex. (From Gegen- 

 baur.) od, oviduct; i/, uterus; v, vagina. 



duplex of rodents, A), or they may fuse partially (uterus bicornis 

 of insectivores, whales, ungulates, and carnivores, J9), or they may 

 be completely fused (uterus simplex of apes and man, C). 



Thus there are three different types of the female genitalia, in 

 which the vagina is not differentiated (Ornithodelphia), or is 

 double (Marsupialia), or is single and unpaired (Monodelphia). 

 To these correspond three types of development. The Ornitho- 

 delphia are oviparous, the others viviparous, but are distinguished 

 by the duration of pregnancy. The eggs of the viviparous forms 

 are so small (about .01 inch) that they have a total, nearly equal 

 segmentation. Such eggs require nourishment from the mother 

 in order to produce an animal with the complicated structure of a 

 mammal. Since in the Didelphia the uterine nourishment is 

 usually very incomplete, the period of pregnancy is very short, 

 in comparison with the Monodelphia, in which a placenta, a com- 

 plicated apparatus for the nourishment of the young, appears; 

 hence the marsupials, with their small imperfectly formed young, 

 are often called Aplacentalia; the Monodelphia, Placentalia. 



All mammals care for the young, this being chiefly or wholly done by 

 the mother, who not only supplies them with milk but protects them in 

 warm if rude nests. Most mammals are monogamous, some polygamous, 

 while in others there is no permanent association of the sexes. The body 

 temperature is constant and ranges from 36 to 41 C. (98 to 106 F.) ; in 

 Echidna it is only 26 to 34 C. (79 to 83 F.). In most, continual feeding 

 is necessary for existence; from this rule there are a few exceptions, like 

 the bears, marmots, badgers, etc., which hibernate during the winter^ 

 taking no food. At this time there is a fall in the temperature due to the 

 diminished metabolism. 



