THE TEETH 265 



diphyodont, that of other vertebrates polyphyodont. However, 

 it is probable that the two sets of teeth of the mammal are not 

 to be regarded as simply an abridgement and reduction of the 

 many sets of teeth of, say, the crocodile, for the following 

 reason. The ectoderm, which sinks down beneath the 

 surface of the skin of the mouth to produce the enamel, forms 

 a long band extending parallel to the edge of the jaw, known 

 as the dental lamina. The rudiments of the teeth appear on 

 the outer side of the dental lamina in two families ; one from 

 the middle of the side of the lamina, and the other from its 

 base. The teeth formed by one family of rudiments grow up 

 and are intercalated between the teeth formed by the other 

 family. When, in the crocodile, for example, a tooth has been 

 formed, another tooth arises beneath it from the same rudiment, 

 and this second tooth will eventually push out and replace the 

 first. But any given tooth is only replaced by a tooth belong- 

 ing to its own family, and which has arisen from the same part 

 of the dental lamina. In the mammal there are the same two 

 families of tooth- rudiments, but each rudiment gives rise to 

 one tooth only. Further, owing to the reduction in size of 

 the jaw, there is not room for both families of teeth at the same 

 time. One family appears first, and gives rise to the lacteal 

 or " milk " dentition. Later on, the other family appears 

 and forms the permanent dentition which pushes out and 

 replaces the lacteal teeth. In the mammal, therefore, replace- 

 ment is effected by a tooth of one family displacing a tooth of 

 the other family ; in the other vertebrates replacement is 

 brought about by the displacement of a tooth by another 

 tooth belonging to the same family. It is probable that the 

 molars belong to the permanent family, the corresponding 

 lacteal teeth having been suppressed. 



The Marsupials have a peculiar mode of reproduction in 

 that the young are born very early and in a very undeveloped 

 condition. They are attached to the nipples of the mother 

 and continue their development in her pouch, or marsupium. 

 During this period no teeth are required, and it is found that 

 in Marsupials the lacteal dentition is reduced ; in fact only one 

 tooth (the third premolar) is replaced. That this is a secondary 



