TOOTH-DEVELOPMENT IN ORNITHOEHYNCHUS. 143 



sections from the left side of the head^ staiued in borax- 

 carmine (PL lOj fig. 4). It is also well seen in the upper 

 jaw in the hasmatoxylin-stained series from the opposite side 

 (fig. 5). It is^ however, only doubtfully present in the lower 

 jaw of this series. Where it is visible the vestigial denticle 

 in each case projects partly into the mouth-epithelium, partly 

 into the labial aspect of the neck of the slender dental laniiua, 

 so that it is partially surrounded by the epithelial cells of 

 these structures, which form a cap for its more superficial 

 portion. Its deeper or root-portion is simply imbedded in the 

 connective tissue. The dentine forms a shell partially enclos- 

 ing a more irregularly arranged dentinal mass (fig. 4). 



It is in relation with this vestigial tooth that the attenu- 

 ated anterior segment of the dental lamina shows itself slightly 

 enlarged and swollen (" kolbig '') on cross-section. This some- 

 what bulbous character is not well illustrated in the sections 

 reproduced, since it only reaches its maximum in the sections 

 immediately in front of those figured. In the figures, how- 

 ever, there is some indication of a capsular arrangement of 

 the connective tissue around the lamina. The enlargement 

 of the lamina is too insignificant in amount to permit of its 

 accurate proportional representation in the schematic text- 

 fig. 1. 



Behind the large posterior enamel-organ the dental 

 lamina is continued backwards for some little distance as 

 shown in the text-fig. 1. This posterior segment of the 

 lamina is relatively plumper and more massive tlian the 

 anterior segment, in accord with its more fertile and pro- 

 gressive character; for, as we shall subsequently show, this 

 segment gives rise to one of the large teeth of the later stage. 

 Already the advent of this tooth is heralded by a more 

 localised bulbous swelling, which is illustrated, in the lower 

 jaw in Fig. 2. This swelling shows a slight flattening of its 

 fundus (though not in the section depicted) which represents 

 the commencement of the process of cupping or papilla- 

 formation. In these respects the condition is practically 

 identical in both jaws of this specimen. 



