of the Tooth- Rudiments in Rodents. ;:J17 



SciURUS. 



Of til is very interesting form, as it proved to be, I suc- 

 ceeded, in spite of a considerable expenditure of trouble and 

 expense, only in obtaining one pregnant female with seven 

 embryos. However, I reserve to myself the working out of 

 older and younger stages, since 1 have at any rate learnt the 

 way in which these maybe obtained with certainty next year. 

 Fortunately the stage which I have before me corresponds 

 fairly accurately with that to which the most important state- 

 ments of Chabry and Pouchet refer. Since I differ from them 

 very considerably, it will be well to give a summary of the 

 statements of the French authors, which are somewhat diffi- 

 cult to understand. They run as follows : — " The dental fold 

 commences in the upper jaw at the level of the incisors, and 

 extends to beyond the molars, ... In front of the canals of 

 Stenson the dental fold is entirely wanting, ... In the 

 mandible the dental fold is continuous throughout the entire 

 extent of the diastema,^' 



Now follows a sentence which does not seem quite intelli- 

 gible after what has gone before. It runs : — " Consequently 

 the dental fold in all Kodents which we have studied is corn- 

 loosed of two halves, which are not united in the middle. In 

 the squirrel in particular the incisors arise from the same 

 epithelial invagination (involution) as the molars." 



According to what was previously stated, this would be 

 correct only for the lower jaw. For in the upper jaw the 

 dental fold is said to be completely wanting even before the 

 canals of Stenson. 



The explanation of the figures shows that the statements 

 of Chabry and Pouchet refer to embryos of the length of 

 4 centini. The embryos studied by me were 2 centim. in 

 cephalic length and 4'7 centim. in all, and were consequently 

 only slightly larger than those examined hy the French 

 authors. 



The following are the results which I obtained : — 



(1) Like the rabbit, the squirrel possesses in front of the 

 large incisor a rudimentary tooth, which seems to have been 

 overlooked by Chabry and Pouchet. 



(2) There is found in the lower jaw, in the diastema be- 

 tween the incisors and molars, and extending only for a very 

 short space, about as far as may be seen in fig. 15, in con- 

 tinuation of the dental fold of the incisors, a small epithelial 

 cord, which is detached from the epithelium of the oral cavity, 

 and is to be regarded as a remnant of the dental fold. In the 

 whole of the large space behind this, as far as the molars, no 



A7in. & Mag. X. Hist. Ser, 6. Vol. x'l. 23 



