TOOTH-DEVELOPMENT IN ORNITHORHYNCHUS. 157 



only in those regions of the lamina wliicli constitute tlie 

 enamel-organs of the future teeth. They therefore appear to 

 be appended to the enamel-organs in question. The later 

 relationship of the actual nodules to the main cusps of the 

 teeth is evidence that the early relation of these precursors of 

 the nodules to the enamel- organs is not a chance one. 



In fig. 1 there is reproduced a photomicrograph of a section 

 through the enamel-organ of the upper tooth "x." Here 

 there is seen a typical example of the structural arrangement 

 of one of these early " nodular '' differentiations. Imbedded 

 in the neck of the dental lamina, at its junction with the 

 enamel-organ, is a small group of cells occupying an other- 

 wise clear central area. This is surrounded by a zone of 

 enamel-epithelial cells, which show signs of differentiation 

 into layers, and the entire mass appears as an appendage of 

 the main enamel-organ. The group of central cells is meso- 

 dermal, and is in the next section seen to be continuous with 

 the cellular tissue of the surrounding tooth-sac. 



Another example of the structures in question, less striking 

 but essentially identical, taken from the opposite side of the 

 jaw, is represented in fig. 2. Here the continuity of the 

 mesodermal core of the rudimentary nodule with the tooth- 

 sac tissue is more obvious, but even here it is nearly surrounded 

 by enamel-cells. It is plain that were the included pulp-cells 

 seen in fig. 1 to assume an odontoblastic function there would 

 result just such a shell of dentine as we figure in fig. 11 from 

 our older stage, whilst the surrounding- enamel-cells would 

 account for the outer concentric layers of the fully-developed 

 nodules. 



Since no differentiation of the future cusps has occurred at 

 the stage under consideration, no reference to the later con- 

 dition in this respect is relevant. And, since the enamel- 



"y" 



organ of the future tooth 77 — y, has not yet come into 



existence as such, posteriorly, the rudimentary stage of the 

 interesting nodules connected with the upper tooth "y" 

 is likewise in abeyance. 



