TOOTH-DEVELOPMENT IN ORNITHORHYNCHUS. 151 



or strands (fig. 13) in relation to the tips of cusps in tlie 

 vicinity of nodules in the position indicated by Poulton. We 

 find, however, that where they are present their connection 

 with the tip of the cusp is the constant and characteristic 

 relation, whilst their relation to the concentric epithelial 

 nodule, if the latter be present, is inconstant and variable. 

 We find (a) that there may be a cyliudriform or strand-like 

 prolongation from the tip of a cusp, either inner or outer, 

 without any nodule being present in relation with it; and (b) 

 such an epithelial strand is absent in some cases in Avhich a 

 nodule is present in the vicinity of the apex of a cusp. Tlius 

 there are no epithelial cylinders in connection with the cusps 

 to which the nodules marked "dx^" and "dx~" in text-fig. 2 

 are related. Nor has the presence or absence of the epithelial 

 strand anything to do with the extent to which the nodule is 

 imbedded in or recessed into the main enamel-organ; in the 

 case of ''dx^" the nodule lies largely outside of the enamel- 

 organ in a very shallow bay of its surface-contour (figs. 6 and 

 7). On the other hand, the nodule "dx- " is largely surrounded 

 by the tissue of the main enamel-organ, but here, again, no 

 epithelial cylinder is present. We therefore believe that the 

 relation of the epithelial cylinder to the nodule is a mere 

 accident of the common relationship Avhicli both these 

 structures seem to possess to the prominence of the cusp. 

 But while the relationship between the nodule and the point 

 of the cusp is not necessarily an intimate one, as text-fig. 2 

 will show, the relationship between the point of the cusp and 

 the epithelial cylinder is fundamental. The cylinder appears 

 to be, in fact, nothing but a prolongation of the inner enamel- 

 epithelium of the apex of the cusp, accompanied by a sheath, 

 composed of those cells of the stratum intermedium which 

 form a more condensed layer in contact with the inner enamel- 

 epithelium. In certain cases it is true that the epithelial 

 strand so constituted does appear to reach and come in 

 contact with the outer shell of the concentric nodule, but 

 this relationship is not an invariable one, and in all probability 

 is of no essential significance. In fig. 13 there is reproduced 



