ox THE TEETH OF PETEOMYZOX AND MYXINE. 633 



The cells of the youug tooth gTadually cornify, the process 

 proceeding- from the apex downwards. Cornification may 

 continue for a considerable time around the base of the 

 tooth, and the nuclei which resist the process longer than 

 the rest of the cell may sometimes be seen embedded in the 

 horn near the growing edge. The tooth breaks through the 

 cells above, and soon projects above the surface^ and like a 

 hair it is purely ectodermal in origin (fig. C). Even before 

 the tooth has broken through the surface a new tooth may 

 be seen developing below on the same site as was occupied 

 by the first. 



A pulp-cavity cannot be found to all the teeth. Those 

 situated at some distance from the edge of the mouth become 

 differentiated simply out of a thickening of epidermis. 



Myxine glutinosa (L.). — Fig. II represents the tooth of 

 Myxine in vertical section. Some little distance beneath the 

 horny tooth can be seen a cone of peculiar cells with large 

 nnclei. This cone must be the "^odontoblast cone" of Dr. 

 Beard. The large cells stain yellowish, just like the cells of the 

 developing tooth of Petromyzon. The granules tend to be 

 arranged in lines, and this gives the margin of the cone a 

 distinctly striated appearance. As in the case of Petromyzon, 

 the base of the tooth is firmly fixed in a deep groove of the epi- 

 dermis, and cornification continues here until the tooth is shed. 



The main mass of the tooth projecting out of the epidermis 

 is formed by the cornification of the cone of large granular 

 cells with large nuclei, while the remainder is produced in 

 the horn-producing groove out of comparatively flat unaltered 

 epidermal cells. In thin sections through the horny sub- 

 stance of the functional tooth this difference in origin can be 

 made out. In the upper part of the tooth transformed large 

 nuclei, and also hollow spaces, can be observed. Towards 

 the base the transformed nuclei are small and flat. The 

 hollow spaces just mentioned gradually develop as the large 

 cells of the cone become converted into horn, and in a 

 partially cornified tooth they are very conspicuous. 



The epidermal cells lining the pulp-cavity are distinctly 



