344 ELEMENTARY ZOOLOGY. 



pelvic region, and are nourished here until they are 

 mature enough to run about (opossum, Fig. 149, or 

 kangaroo) . The third and highest division of the mam- 

 mals consists of the placentals, in which there is a close 

 connection between the young and the mother. The 

 period of internal nourishment is longer, the young being 



FIG. 145. Diagram of a trunk vertebra of a mammal, c, centrum; ch, position 

 originally occupied by notochord; h, head of rib (r); h.c., haemal cavity; n.a., neural 

 arch; n.c., neural canal; st, sternum; s.c., sternal cartilages, uniting ribs to sternum; 

 t.p., transverse process of vertebra against which the rib works. 



Questions on the Figure. Compare all the parts here with cor- 

 responding ones in Fig. 106. What is gained by the attachment 

 of ribs to a sternum? What is lost? What is the value of having 

 the attachment cartilaginous ? Which vertebrae bear ribs in mam- 

 mals? Why do not all the trunk vertebrae bear them? 



much more mature and like the adult when born. This 

 period varies from three weeks in the mouse to twenty 

 months in elephants. It is related in some degree to the 

 size of the animals. 



It will be seen that the two principal points mentioned 

 the mammary glands and the development in the uterus 

 are both devices to care for young. There is no group 



