622 



Mus musculus. 



The development of the Mouse' dentition has been investigated 

 by Freund 6 ) with especial reference to the presence or absence of 

 vestigial teeth in the immediate vicinity of the incisors and as a result 

 he states that he was unable to find any trace or suggestion of these 

 structures. 



My own observations have led me to a different conclusion. 



Having carefully examined a complete series of frontal sections 

 of the head of a young mouse with a head length of 9 mm, I found 

 a pair of very minute calcified tooth rudiments in connection with the 

 two large upper incisors, one on each side of the jaw. 



4^i 



pi'—-- - .'■ -.: .,\ 



Fig. 1. Frontal section of the upper jaw of a foetal Mouse (head length 9 mm) 

 passing through the two upper incisors. X 100. pi' tip of permanent left incisors. 

 e.o. enamel organ of ditto, di' the two vestigial milk incisors. 



Each vestigial tooth, which is present in the form of a cup like 

 structure composed entirely of dentine, is situated just external (labial- 

 wards) to the point of attachment of the enamel organ of the 

 large incisor to the epithelium lining the mouth (see Fig. 1 di'). 

 It does not at the stage I have examined present a distinct enamel 



