of the Tooth-Rudiments in Rodents. 309 



Chabry the enamel-organ of the large incisor is also connected 

 by a cord of epitlieliuiu with the mur 'plonyeant, but with its 

 posterior and upper border. The authors referred to found a 

 similar little rudimentary tootli in the upper jaw only in 

 embryos which measured not less than 32 to 40 millim. in 

 length. " ]t lies immediately in front of the large incisor, 

 and its tip ajipears in the midst of the epithelium of tiie mur 

 plongeant''^ (tig. 21). 



The earliest stages which I was able to examine tlioroughly 

 consisted of embryos of Lepiis cuniculus measuring 1-3 millim. 

 in total length, of which the head occupied 9 millim. For 

 earlier ones I had at my disposal only the series of sections 

 from an embryo with a head 6 millim. in length. Since this 

 shows only the dental fold, which is apparently continuous in 

 the up])er jaw, but in the lower one on the contrary is inter- 

 rupted behind tiie region of the incisors, while the enamel- 

 organs are as yet entirely absent, 1 shall defer the investiga- 

 tion of these youngest stages until I have more abundant 

 material at my disposal. 



First of all I deal with the region of the incisors in embryos 

 with heads measuring somewhat more or somewhat less than 

 1 centim. The length of these embryos from the crown of 

 the head to the root of the tail, which, for reasons which will 

 be readily understood, is much more variable, fluctuates be- 

 tween 1"5 and 3 centim. Fig. 13* gives a representation of 

 the model which I have prepared of the e])ithelium of the 

 oral cavity, with the tooth-rudiments of the upper jaw of the 

 right bide, of an embryo measuring "U centim. in cephalic 

 length and 2'1 centim. in all; the model is seen from above 

 and somewhat from outside ; I have not figured a second 

 model showing the rudiments of the incisors in the upper and 

 lower jaws of an animal of about the same size. 



In the lower jaw the rudimentary tooth discovered by Chabry 

 and Pouchet may be seen. Fig. 1 gives a distinct view of this, 

 from which it appears that the enamel-organ of the tooth is 

 not closed on all sides, as asserted by Chabry and Pouchet, 

 but is open on the underside. Through the aperture a richly 

 nucleated pulp enters the interior of the tooth. The centre of 

 the pulp is occui)ied by a wide blood-vessel, while its peripheral 

 cells foim a well-developed layer of odontoblasts, which has 

 already secreted a solid hook-shaped cap of dentine. The 

 latter, in the preparation upon which the drawing was based, 

 has withdrawn a little from the well-developed enamel-epi- 



• The figures referred to in this paper will be found in vol. 39 of the 

 * Archiv fiir mikroskopische Anatomie.' 



