38 MICROSCOPIC ANATOMY OF THE TEETH 



posterior (mesio- distal) direction, which would be a fusion 

 of teeth of the same series, and fusion in a transverse or 

 bucco-lingual direction, which would mean the fusion of 

 teeth of different series. Professor Bolk, in his first paper (3 a) 

 on the relationship of the mammalian dentition to that of 

 the reptiles, considers that there is an intimate relation 

 between the diminution of mammalian tooth generations 

 and the complication of their crown surfaces. He says that 

 the so-called single tooth row of the reptiles is really double, 

 consisting of an outer and an inner row, and that their 

 dentition only secondarily becomes what he terms ' mono- 

 stichic ', consisting of an apparently single row. He even 

 describes a third row, but this is resorbed before eruption. 

 He considers that the diphyodont dentition of the Mam- 

 malia represents the two rows of the reptilian ancestor, thus 

 concluding that each tooth in the primates has arisen from 

 the concrescence of two tooth generations. ' The complica- 

 tion of the teeth in longitudinal (mesio -distal) direction was 

 initiated among the reptiles and inherited by the Mammalia. 

 The complication of the crowns in a transverse (bucco- 

 lingual) direction is the result of the concrescence of two 

 t^oth generations, whereby the origin of the mammalian 

 tooth from the reptilian was completed, and ' by this con- 

 crescence the multiplicity of the tooth generations was 

 suppressed'. In other words, the polyphyodont dentition 

 of the reptiles is represented by the complication of the 

 crowns of the mammalian tooth in a transverse direction. 



In his second paper (3&), Bolk describes the results of 

 the microscopic examination of early tooth germs in man 

 and other primates. He also made use of the Born system 

 of modelling employed by Rose in his work on tooth develop- 

 ment (see p. 16). In this paper he describes, in the germs 

 of both deciduous and permanent teeth, the following 

 structures in connexion with the enamel organ : ( 1 ) a lateral 

 enamel ledge, (2) an enamel crypt, (3) an enamel septum, 

 (4) the enamel navel. 1 



The lateral enamel ledge is a buttress or outgrowth arising 

 from each tooth on the lateral margin of the enamel organ. 



1 A good summary of Professor Bolk's views is given in a review in the 

 Dental Ccsmos for 1913, vol. 55, pp. 103 and 1058. 



