TOOTH-DEVELOPMENT IN OENITHORHTNCHTJS. 149 



but^ ns we have just seen, no moi'e matni*e product of tliis 

 enlargement appears in the later stage of development. This 

 is doubtless to be accounted for by the advanced position in the 

 jaw occupied by the tooth-germ in question. 



If we now take account of the combined conditions set 

 forth in the schematic text-figs. 1 and 2, it will be evident 

 that they tend to establish the view that representatives of 

 five quasi-permanent teeth are developed in each jaw daring 

 the phases of tooth-development under consideration. These 

 include the recognised teeth of the adolescent animal. (The 

 most posterior member of the series in the upper jaw {" z") 

 is not indicated in the scheme.) In addition to these there 

 have also been indicated vestigial representatives of deciduous 

 predecessors to the most anterior of these five tooth-elements. 



We shall presently adduce evidence to prove that the 

 deciduous vestiges already described are not the sole repre- 

 sentatives of an earlier tooth-generation occurring in our 

 specimens. Meanwhile we may observe that the dentitional 

 characters above set forth seem to afford, evidence of the 

 operation of factors which determine early suppression and 

 abortion of the more anterior segment of the dental lamina 

 and its derivatives in Ornithorhynchus. 



(5) Concerning the Epithelial Nodules formerly described 

 BY Professor Poulton in connection with the Enamel- 

 Organs OF Ornithorhynchus. 



In the course of the account given by Professor Poulton 

 of the developing teeth in Ornithorhynchus, he described a 

 number of epithelial nodules situated "almost immediately 

 over the apex of each calcified cusp of the second and third 

 tooth." He states that "nothing of the kind could be made 

 out in the case of the first upper tooth" (i.e. our tooth "w"). 

 In these nodules he found that "the inner cells appear to be 

 corneous and collected into a dense central mass, between 

 which and the outer fusiform cells is a space containing 



