623 



organ of its owd, but slightly indents the base of the neck of that of 

 the large incisor, the cells of which are specialized and aggregated 

 in the immediate vicinity of this 



small mass of dentine. In sec- ^ p „ 



tion the dentinal cap 9 ) appears ,rv<> \-sjk 



as a narrow loop forming about <?- ■._.'■ 

 3 / 4 of a circle, the remaining 

 fourth being open and gives en- 

 trance to the mesoblast which fills 

 up the cavity of the cup and 

 forms the pulp. The nuclei of the 



V 







.e — 



Pig. 2. Enlarged drawing of the left vestigial incisor. X ^^0. d dentinal cap, 

 p pulp, e epithelium lining mouth, e' base of enamel organ of functional incisor. 



outer layer of cells lining the cup are specialized in relation to the 

 dentine and obviously represent the secreting cells or odontoblasts much 

 degenerated as they bear no dentine processes (see fig. 2). A few 

 cells of the mesoblast have flattered themselves against the outer surface 

 of the dentine where it curves away from the enamel organ. 



This tooth vestige measures in its greatest length 1 / g0 mm and 

 the same in width. The absence of an enamel organ and the fact 

 that the entire structure is often altogether wanting might lead one 

 to doubt the advisibility of referring this body to the category of teeth, 

 but the form and histological reactions of its dentinal cap, its rela- 

 tion to the enamel organ of the large incisor together with the 

 character of the pulp forces on one the conclusion that it is really a 

 very much reduced tooth, one which in fact is on the verge of dis- 

 appearance and which may be altogether wanting in some specimens. 



From my investigations in the Marsupialia I find that the enamel 

 organ is frequently one of the first structure to suffer during the re- 

 duction of a tooth, so that the absence of a distinct organ as in this 

 case would not necessarily imply that this calcified cap of dentine did 

 not represent a tooth; moreover we are here dealing with a with- 

 drawal or a non-independent development of the enamel organ from 



9) This calcification although, not exhibiting the typical structure 

 of dentine viz : — its tubules, nevertheless presents a totally different 

 reaction with stains to that shown by bone, which latter at this early 

 stage invariably stains a much lighter colour than dentine, while the 

 supposed tooth rudiment present the same appearance as the latter. 



