458 CHORD AT A. 



in the Monotremata^ temporary teats only in the Mefatheria 

 and permanent ones in the Eutheria. 



The evolution of mammary glands probably commenced before 

 that of the viviparous habit. We can see how the ventral surface of 

 the parent lent itself first for the incubation of the eggs and later for the 

 tending of the young. The desire of the young for fluid would naturally 

 be satisfied by the local glands, and if we may suppose that the duties 

 of incubation and nurture were shared by both sexes, we can to some 

 extent understand how the males of many mammals still have mammae 

 and functionless mammary glands. After the viviparous habit was 

 developed the male, divested of his share in the incubation, would 

 gradually give up the mammary function as well. 



The mammae in modern mammals vary much in posi- 

 tion, though all situated upon the ventral surface. They 

 may be pectoral^ axillary^ abdominal or inguinal according 

 to their position on the breast, under the armpit, along the 

 abdomen and in the groin respectively. 



Teeth. — The teeth are well developed in most mammals, 

 though some types, such as whales, ant-eaters, Monotremata 

 and others, appear to have lost them. The characters of 

 mammalian teeth may be summed up in the expressions — 

 thecodont^ heterodont, diphyodont ; to which we may add a 

 limitation to a single row on the premaxillae and maxillae 

 above and to the dentary below. In a thecodont dentition 

 the teeth are held in definite sockets in the bone ; a hetero- 

 dont dentition is one in which the teeth differ markedly 

 amongst themselves in size and shape ; and, lastly, in a 

 diphyodont dentition there are two sets of teeth succeeding 

 one another in the life of the individual. [We may recall 

 that the teeth of most reptiles are fused to the bone 

 {acrodont or pleurodont)^ they are usually of the same size 

 {homodont), there are several series of teeth (^polyphyodont)^ 

 and the teeth often occur upon the palatines, pterygoids or 

 vomers in addition to the premaxillae and maxilla. The 

 crocodile, as in other anatomical features, approaches the 

 mammal in having one row of thecodont teeth which are 

 slightly heterodont] 



Development of a Tooth. — A typical mammalian tooth arises 

 from an enamel-organ consisting at first of a protrusion of the mucous 

 layer of the epidermis downwards into the dermis. This becomes 

 pushed in on the under side and the dermis thus protrudes into it 

 as a small "dentine-germ." The mucous epithelium, bordering the 



