TOOTH-DEVELOPMENT IN ORNITHORHYNCHUS. 



141 



side are tliin (11^) and were cut in paraffin and stained in 

 borax-carmine. 



(2) Description of the Phase op Tooth-Development seen 

 IN the Younger Specimen ("Delta"). 



The phase of tooth-development which we find to be 

 manifested in our younger specimen is one of which no 

 account has hitherto been available for Ornithorhynchus. It 

 is characterised by the presence of a continuous dental lamina 



Text-fig. 1. 



throughout a considerable extent of both jaws. The lamina 

 begins, in the upper jaw, about 5*3 mm. behind the tip of the 

 snout, and extends backwards for a distance of 2*6 mm. The 

 lamina in the lower jaw begins about half a millimetre in 

 front of the plane of its commencement in the upper jaw, and 

 extends backwards to end at a distance of 2'8 mm. further 

 back. 



Text-fig. 1 represents a scheme of the organisation of the 

 dental lamina in the two jaws at a magnification of about 

 18 diameters. From this it is evident that the lamina in 

 each jaw shows two definite papillated enamel-organs. These 

 are in the early stage at which practically the entire thickness 

 of the lamina is involved in their production, prior to the 



