664 MAMMALIA [CH. 



as to be used only for grasping, and locomotion is effected by a 

 series of leaps carried out by the hind-limbs aided by the powerful 

 tail. The sole of the hind-foot is excessively narrow, the second 

 and third digits being represented by bones so slender that they 

 take no part in supporting the body. Macropus giganteus, the 

 Gray Kangaroo or " Old Man," may attain a height of from 4 to 5 

 feet. The fourth toe of the hind-foot has a powerful claw with 

 which when the animal is brought to bay it has been known to 

 rip open a dog. The allied genus Petrogale includes smaller species, 

 called Rock Wallabies, with only a short claw on the hind-foot. 

 As their name implies, they frequent rocky regions. The so-called 

 Kangaroo-rats, Bettongia and others, are nocturnal animals of small 

 size, which live on leaves, grass, and roots, the last of which they 

 dig up with their fore-paws. 



Sub-class III. EUTHERIA. 



The highest division of the Mammalia, the Eutheria, includes 

 all the most familiar animals, Hedgehogs, Rats, Rabbits, Cats, Dogs, 

 Lions, Tigers, Horses, Oxen, Whales, Elephants, Monkeys, up to 

 and including Man himself. In them as in the Metatheria the egg 

 is exceedingly small, in Man and the domestic animals for instance, 

 it varies from T ^j- to ^^ inch in diameter. The upper part of the 

 oviduct, the Fallopian tube, is consequently narrow; the uterus is 

 however enlarged, for the egg not only lies there a long time 

 called the period of gestation or pregnancy but as it is 

 developing into the young Mammal a special organ called the 

 placenta is developed, which grows out and becomes interlocked 

 with folds in the wall of the uterus. This organ is a result of an 

 enormous development of the allantois. In Eutheria, the egg 

 contracts an adhesion with the wall of the uterus over its whole 

 surface, but that part of the surface against which the corrugated 

 and richly vascular allantois impinges is covered with vascular out- 

 growths called villi, which fit into pits on the wall of the uterus. 

 It is this area which is called the allantoic placenta. The rest 

 of the surface of the egg is termed the umbilical placenta. This 

 region is non-vascular, and through it nourishment is absorbed in 

 Metatheria. In the lower Eutheria some nourishment is absorbed 

 through the umbilical placenta for a considerable part of the period 

 of pregnancy, but in the higher Eutheria it is early destroyed by 

 an extension of the allantois round the egg. Both the membrane 



