696 MAMMALIA. 



apertures on the skin. From the white blood corpuscles of 

 the abundant vascular supply, and from a degeneration of 

 the cells lining the glandular tubes, the milk is produced. It 

 begins to be produced when the young are born, when, in 

 Placental Mammals, the demand upon the mother through 

 the placenta has ceased. 



In Monotremes the simple glands, compressed by 

 muscles, open by many pores on a bare patch of skin. 

 This is depressed into a slight cup, from which the young 

 lick the milk. In Marsupials the glands open by teats or 

 mammae, generally hidden within a marsupium ; and again 

 the action of surrounding muscles forces the milk into the 

 mouths of the young, which do not seem to be able to suck. 

 An anterior prolongation of the larynx to meet the posterior 

 nares, establishes a complete air passage, and enables the 

 young to continue breathing while they are being fed. In 

 Cetacea the milk ducts are dilated into large reservoirs, the 

 contents of which can be rapidly injected into the mouth of 

 the young. In all other Mammals the young suck the 

 milk from the mammae. 



Dentition.- -The teeth of Mammals are developed in the 

 gum or soft tissue which covers the borders of the pre- 

 maxillae, maxillae, and mandibles. As in other animals, 

 they are in part of epidermic, in part of dermic origin. In 

 the course of their development their bases are usually 

 enclosed in sockets formed in the subjacent bones. 



In most teeth there are three or four different kinds of 

 tissue. The greater part consists of dentine or ivory (of 

 which about a third is organic matter) ; outside of this 

 there is a layer of very hard glistening enamel (practically 

 inorganic) ; in the interior there is a cavity which in grow- 

 ing teeth contains a gelatinous tissue or pulp supplied by 

 blood vessels and by branches of the fifth nerve, and con- 

 tributing to the increase of the dentine ; lastly, around the 

 narrowed bases or roots of the tooth, or between the folds 

 of the enamel if these have been developed, there is a bone- 

 like tissue called the crusta petrosa or cement. 



The development of teeth begins with the formation of 

 a dental ridge, an invagination of the ectodermic epithelium. 

 From this ridge a number of bud-like "enamel germs" are 

 next differentiated. Beneath each germ a papilla of the 



