98 PROF. DR. L. BOLK 



My hypothesis regarding the origin of the dental formula of 

 the catarrhine Primates explains in a very simple manner the 

 otherwise incomprehensible fact that the diminution of the pre- 

 molars could occur without a gap in the dental arch. And in the 

 post-canine portion of the set of teeth of the higher Primates a 

 diastema is never found. That the continuity of the set of teeth 

 by the above hypothesis was never interrupted, surely does not 

 tell against the justice of it. The foregoing, however, are mere 

 theoretical considerations, let us now proceed to some more 

 practical arguments. 



Regarding the embryological evolution of our dentition and 

 the succession of the eruption of our teeth, I believe our first per- 

 manent molar was, in an earlier stage of phylogenetical evolu- 

 tion, a deciduous tooth, belonging to the first or milk dentition. 

 In I'eality this tooth appears (in man, as in all other catarrhine 

 Primates) before the first permanent incisor. And during the 

 nearly two years between the eruption of our first permanent 

 molar and that of our permanent first incisor, the structure 

 of our dentition is identically the same as in young platyrrhine 

 monkeys. During this period of the platyrrhine phase of our 

 dentition, there are three molars immediately behind the canine 

 tooth. 



The affinity of our first permanent molar to the set of milk 

 teeth is more clearly shown the moment the first anlage of this 

 tooth appears in human embryos. According to the investiga- 

 tions of Rose, the anlage of milk teeth commences in the ninth 

 week of embryological development. Immediately afterward the 

 germ of the second milk molar is produced by the dental lamina, 

 the latter is prolonged backwards, and the enamel-organ of the 

 first permanent molar is formed. This happens in the sixteenth 

 week of embryological development. Therefore, there is no dis- 

 continuity in the succession of the first anlage of the enamel- 

 organs of our deciduous teeth and that of our first permanent 

 molar. A relatively long time follows before the formation of 

 the other teeth, especially of the second permanent molar. The 

 individual is six months old before the dental lamina recom- 

 mences to grow further l)a('kwards and pi'oduces the germ of this 



