226 ON THE DEVELOPMENT OF THE TEETH 



In the Frog the same relations essentially hold good ; the young 

 teeth are here developed in minute sacs, which lie at the bottom of 

 the dental groove in the upper jaw. I could never detect any free- 

 projecting pulps (nothing, therefore, corresponding to the papillary 

 stage in the human tooth), but the smallest and youngest rudiments 

 of the teeth I found were oval or rounded sacs, i-i8oth of an inch 

 long, containing an oval papilla, about one-fourth shorter. Externally, 

 these were bounded by a strong structureless basement membrane, 

 which enclosed a homogeneous substance, containing nuclei in its 

 cavities. These were rounded, and very close together, next to the 

 basement membrane, but became transversely elongated in the inner 

 layers and next to the pulp. This last was bounded by a structureless 

 membrane, which at its narrow base became continuous with the 

 basement membrane of the capsule. 



In the Frog, then, the relations of the pulp and of the capsule are 

 the same as in the Mackerel. 



In the Skate, as is well knovvn,^ the young teeth are developed in 

 longitudinal rows within a deep fold of the mucous membrane of the 

 mouth, behind the jaw. So far as my examinations go, however, I 

 find that this is not a mere simple fold, such as it has been described to 

 be ; but its two walls behave just in the same manner as those of the 

 primitive dental groove in man — that is, they become closely united 

 in lines perpendicular to the direction of the jaw, so that partitions are 

 formed between every two rows of teeth — transverse partitions again 

 stretch between the separate teeth of each row, but these did not 

 appear to me to be complete, terminating by an arcuated border 

 below {fg. ii). Each longitudinal canal therefore answers to a single 

 elongated mammalian follicle, or to that prolongation of the alveolar 

 groove from which the posterior permanent molars are formed in man 

 {see Goodsir), only the process does not go so far as in this case, 

 the separate capsules remaining imperfect anteriorly and posteriorly. 

 The lateral walls of the capsule, however, seem to me to have as much 

 (or as little) '' organic connexion with the pulp and attachment to its 

 base " as in man, and the process seems to correspond with something 

 more than the " first and transitory papillary stage of the development 

 of the mammalian teeth." ^ 



Each pulp is invested by a very distinct basement membrane 

 whose continuity with that of the mucous membrane of the follicle is 



' See Blake's 'Essay,' &c. iSoi, in which the essential pecuHarities of the development 

 of the teeth in the shark and skate, and their mode of advance, are very well pointed out. 

 He refers to Herissant and Spallanzani as having anticipated him. 



- See Owen's ' Odontography,' p. 15. 



