TEETH OF PHYSETER 31 



each (or each two) to a tooth germ. In the anterior half of the 

 jaw the tooth rudiments also possess two labial outgrowths, 

 of which the first, i. e. that nearest to the tooth germ, is longer 

 than in the posterior series of tooth germs, while the second, 

 i. e. that nearest to the origin of the dental lamina, arises partly 

 from the dental lamina and partly from the oesophageal 

 epithelium, and forms a short pyramidal process. I have not 

 seen intermediate conditions. 



There is also a differentiation into two series of the teeth 

 of the lower jaw, but the anterior teeth seem to differ merely 

 through greater age. 



In both the teeth of the upper and of the lower jaws the 

 permanent tooth rudiments (i.e. those outgrowths from, and 

 on the lingual side of, the dental lamina) are not isolated 

 outgrowths of the dental lamina but are connected successively 

 by a continuous outgrowth of the dental lamina, as follows : 



In both lower and upper teeth the individual tooth germs 

 arise from a marginal thickening of the dental lamina, but the 

 subsequent course of the development differs in the two series. 

 In the teeth of the lower jaw the thickening is shifted to the 

 lingual side of the dental lamina by the freeing from it of 

 a residual lamina on the labial side. In the upper jaw the 

 corresponding thickening at the distal edge of the dental lamina 

 is bent over to the hngual side, while a later formed residual 

 lamina continues at intervals the straight line of the dental 

 lamina; thus the tooth germ has grown to he laterally 

 instead of being formed in situ. 



As to the homologies of the various regions of the embryonic 

 teeth, it has been attempted to show that there are reasons 

 for believing that the teeth of the adult correspond to the 

 permanent dentition of other mammals, that there are also 

 rudiments of precedent milk dentition, and that a residual 

 lamina succeeds the rudiments of the permanent dentition. 



