738 Dr. J. BEARD, 



Most of the individuals were already too old, and the teeth 

 were so far advanced that a reserve tooth was being formed be- 

 hind and within the first tooth. One or two however showed 

 the very earliest stages in the tooth formation. In 

 passing, I may remark that in even the oldest Ammocoetes no trace of 

 tooth development could be detected. The teeth are among the latest 

 developments in the metamorphosing Fetromyson. 



Their development takes place in individuals which have appar- 

 ently acquired all the Petromyzontoid characters, and the mouth has 

 lost its Ammocoetoid horse-shoe shaped character and become oval. 



Transverse sections shew best the evolution of the teeth. In 

 fig. 3 the appearance presented in such a section is depicted, a 

 smaller portion of one such section is figured in fig. 9. In the first 

 place, it must be noticed that the mouth is lined by a thick strati- 

 fied epithelium (s e). Lying in the deeper layers of this epithelium, 

 that is, forming its basis, one notices a somewhat symmetrical row of 

 what at first sight look like true tooth sacs. In fact, these sacs would 

 be in all respects comparable to those of ordinary developing teeth but 

 for one fact. Though they possess a dental papilla {dp) 

 they have no odontoblasts. Even in younger stages before 

 traces of horn are present no odontoblasts are developed. 



The papilla is mesoblastic (of reticulate connective tissue) and 

 upon it there rests an „enamel epithelium", which passes over, as in 

 other cases, into what I have termed the outer epithelium. The 

 curious circumstance is that from the start the enamel epithelium and 

 the circular horn groove into which it passes externally produce horn 

 (7î), and, beyond the imitation of a true tooth sac in which, if ever 

 present, the odontoblasts never become functionally active, there is no 

 trace of true tooth structure ever produced in Fetromyzon planeri. 

 The first horny tooth formed is a very thin one, and even before that 

 has cut its way through the overlying stratified epithelium, a reserve 

 tooth is in course of formation. 



Not all the teeth in the mouth of the fresh-water form are de- 

 veloped in this way. It is only in the case of a certain number of 

 teeth near the anterior margin of the mouth in which these dental 

 sacs are developed. The teeth more inwardly situated are formed 

 simply in the basal layers of the stratified epithelium, without the 

 intervention of a tooth sac. There are two ways in whkh this mo- 

 dification might be explained. 



