734 I>r- J- BEARD, 



Hum which rests upon the enamel. And in order to create no con- 

 fusion by the introduction of a new term, I may describe it under the 

 old name without prejudice as to its taking part in the enamel for- 

 mation or not. The arrangement of the epithelium under the horn 

 including the enamel epithelium, is somewhat complicated. It will be 

 best understood by first examining fig. 2, which is a diagramm showing 

 the sole way in which the Myxinoid tooth differs in its mode of develop- 

 ment (as deduced from the adult conditions) from the tooth of any 

 ordinary Vertebrate. To illustrate this I have taken the diagram of 

 a developing tooth as given by Wiedersheim (24, p. 484, fig. 367), 

 and from it I have constructed the diagram (fig. 2) for comparison. 

 It is important to notice at once that the horn is formed from the 

 epidermis just outside the involution which gives rise to the enamel 

 epithelium. Here a horn-groove is formed at hg, and within 

 the area thus circumscribed the epidermis is doubly folded -in over 

 the developing tooth, — over the odontoblast papilla. The odontoblast 

 papilla — and it is the same in the developing tooth of any Verte- 

 brate, — is thus surrounded by two layers of epiblast: next it, and 

 resting on it, is the enamel epithelium ; outside this is a second layer 

 of epithelium (o e) which is usually named the external epithelium of 

 the enamel organ: I shall here simply call it the outer epithelium. 

 So far, in speaking of these two layers of epithelium, I have been 

 describing such appearances in the developing tooth of Myxine or 

 Bdellostoma as one can deduce from the anatomical facts to be de- 

 scribed. 



As no one has yet been fortunate enough to obtain an embryo of 

 either of these fishes, it is, of course, not likely that the development 

 of the teeth could be described from actual preparations. But we do 

 not know at what stage of their life-history the teeth are developed in 

 the Myxinoids, and it may just happen that those stages in which the 

 teeth are first developed are obtainable. As yet I have not had 

 very young Myxine at my disposal. The semi-diagrammatic fig. 7 

 shows the actual state of these two layers of epithelium {de and oe) 

 in my specimen ^), which was probably a fairly young individual. 



On comparing the figure with the diagram just described , it is 

 easy to recognise the two layers of the epidermis, which were originally 

 involuted over the papilla. Owing to the growth of the tooth the enamel 



1) As a matter of fact the enamel epithelium was continued further 

 into the pulp cavity than represented in the figure. 



