. Anatomischer Anzeiger. Anatomy, Comparative. 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 cap9) 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 p
. Anatomischer Anzeiger. Anatomy, Comparative. 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 cap9) 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-ind
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