SUCCESSION OF MAMMALIAN TEETH. 443 



The greatest contention has raged round the molars ; 

 Lataste (20), maintaining that they belonged to the second 

 dentition, was supported by Magitot (21), whereas Beaure- 

 gard (22) believed them, as Owen had previously done, to 

 belong to the milk dentition. 



Nearly all these views were based on the study of dried 

 skulls or of the relative position of the calcified teeth in the 

 gum of the young animal and not on microscopic study of 

 the relation of the enamel organs to one another, by which 

 alone these points can be cleared up. It is true that Tomes 

 (23) had done a great deal in the microscopy of teeth and 

 that Pouchet and Chabry (15), who had worked in this way 

 at quite a number of mammals, had definitely shown 

 amongst other things the relation of the minute milk 

 incisors of the rabbit to the functional ones ; but the last- 

 named investigators did not realise all that their specimens 

 show, for they certainly figure the molar of a squirrel with 

 what would be now regarded as a rudiment of a successional 

 tooth. 



Not until the last three years has the microscopic study 

 of the developmental relationship of the sets of teeth 

 occurring in mammals been seriously taken up. Now, how- 

 ever, we are almost flooded with literature on this subject, 

 and in many instances we now are familiar with the develop- 

 ment of the teeth from their earliest indication until the 

 distinct sets are fully formed. 



The earliest worker in this direction was Kukenthal 

 (26, 27). His chief work (26, 28) deals with the hitherto 

 supposed monophyodont dentition of the whales, which he 

 has shown to be in reality diphyodont, the functional or 

 most developed set being in the Mystacoeti, the remains of 

 the milk dentition. In certain Odontocoeti {Phocoena) he 

 considers this to represent a fusion of the two dentitions, 

 the milk set, however, predominating. He confirms the 

 observations of earlier writers as to the primarily hetero- 

 dont nature of the cetacean dentition and comes to the con- 

 clusion that the homodont dentition of these animals is 

 secondary and largely due to a splitting apart of the com- 

 ponent conical cusps of the double teeth. In like manner 



