DEVELOPMENT OF TEETH IN HATTERIA PUNCTATA. 165 



Stage Q. 



Embryo Q, 52a, in about the thu-cl month of incubation. 

 The skeleton is mainly cartilaginous, but along the upper 

 border of Meckel's cartilage, and in several other regions, a 

 considerable amount of ossification has taken place. Length 

 of head as measured from sections 5 mm. 



The epithelium along the upper and lower jaws has grown 

 down into the mesoderm in the form of a well-marked 

 dental lamina, consisting* of an outer limiting layer of 

 columnar cells, and a central core of more irregular ones 

 (fig. 1, d. I.). The line of ingrowth is marked on the 

 surface of the epithelium by a shallow groove, the dental 

 furrow (fig. 1, cl.f.), such as has been described in many 

 other vertebrates. The dental lamina is on the whole less 

 developed in the lower than the upper jaw, and in the 

 former is quite indistinguishable for some little distance in 

 the middle line anteriorly ; in the latter, also, it is not con- 

 spicuous in this region. 



The most interesting feature of this stage is the presence 

 of minute teeth, which are perhaps homologous with the 

 early (embryonic) dentition described by Rose (12, 13) in 

 the Crocodile, by Leche (14) in Iguana tuberculata, and 

 by others in various fishes and Amphibians. 



The best developed tooth of this first series is shown in 

 fig. 2. Here we see that the tooth is formed just below the 

 epidermis, and has a well-defined enamel organ, although I 

 have been unable to detect any enamel. The presence of 

 the tooth produces a prominent papilla. The dentine is 

 formed by the activity of odontoblasts, which do not be- 

 come so elongated as do those of the next set of teeth, but 

 the process of dentine formation is apparently the same 

 in the two cases. In fact, except for the minute size and 

 the position immediately below the epidermis, the members 

 of the first series of teeth are very similar to those of the 

 functional set. Referring to fig. 1, we see that at the labial 

 attachment of the dental lamina to the epithelium there is 



