Jh'id'itiuii r/I)iMrI|.liys. 287 



Icijth sluuild be present, internal to rudiincnts of teeth wliich 

 are subsequently cut, the proof would thus be furnished that 

 the series of teeth whieh arrives at development bclon;^s not, 

 as was hitherto generall}' believed, to the second, but to the 

 first dentition. Thus it would be shown that the milk- 

 dentition is not to be regarded as a new and secondary acqui- 

 sition within the Mammalian class. 



'J'hc very fact that the third milk-preraolar is cut at about 

 the same time as the other premolars, whereupon the molars 

 appear, commencing from the first, and that the third jjremolar 

 which replaces it develops much later than tlie other teeth, 

 especially than its two neighbours *, gives ground for the con- 

 jecture that the third milk-premolar belongs to the same series 

 as the rest of the teeth which are situated in front of it. This 

 difficulty of regarding the third milk-premolar and the other 

 teeth as belonging to two distinct series was felt by Winge, 

 who believed lie was able to remove it by explaining that the 

 other teeth, in spite of belonging to the second dentition, are 

 cut simultaneously with the single milk-tooth because their 

 precursors are wanting. Perfect clearness is naturally attain- 

 able only by means of an embryological investigation. The 

 material at my disposal consisted in the first place of a number 

 of lower jaws of young stages of DideJphijSj for which I am 

 indebted to the kindness of Prof. M. Fiirbringer; my thanks 

 are also due to Dr. Kraepelin, the Director of the Natural 

 History JliJuseum at Hamburg, who afterwards handed over 

 to n-e for treatment a number of well-preserved young speci- 

 mens of Dideljjhys, through the heads of which series of 

 frontal sections were made. The two smallest cubryos 

 examined measured 1 centim. in length from the rump to the 

 nape of the neck. 



J. select the upper jaw for the purpose of description, since 

 the conditions in it are more distinct than those in the lower. 

 Throughout the entire length of the upper jaw there runs a 

 cord of epithelium, the dental fold (" Zalinleiste "), close 

 beneath the epithelium of the cavity of the mouth ; in front 

 it is not sharply separated from the epithelium of the oral 

 cavity, but further back, on the contrary, it lies at a greater 

 depth. The rudiments of the enamel- organs of the five 

 incisors appear as knobbed thickenings of the dental fold. 

 Nothing is yet to be seen of the invagination of the enamel- 

 organ by the dental papilla ; no indication whatever of the 

 latter is as yet presented by the rudiments of the incisors. 

 The connective tissue surrounding the epithelial knob has 



* I'ide Tlioiua.s, he. cit. p. 452. 



