424 .F. (IRA HAM KKKi;. 



an actual bodily involution of ectoderm such as is ordinarily 

 associated with the term stomodteum. On the contrary, the 

 perfectly gradual transition between the " stomoda^nl '" cells 

 and the typical yolk-laden endoderm cells shows quite con- 

 clusively that the former are being derived from the latter. 

 The buccal rudiment retains its solid character till about 

 Stage 31. About this time the cells in its interior begin to 

 degenerate and break down^ and so give rise to the cavity of 

 the mouth. 



The tooth germs begin to appear long before there are any 

 traces of lumen in the buccal cavity.^ Already in Stage 32 

 they may be detected. 



Development of the Teeth. 



One of the most striking points brought out by Professor 

 Semon^s researches on the development of Ceratodus has 

 been the way in which the so characteristic tooth plates are 

 formed by the joining together, by dermal bony trabeculae. of 

 originally separate denticles. On coming to consider the 

 tooth development of Lepidosiren I not unnaturally ex- 

 pected to find a similar state of affairs, and I was accordingly 

 much astonished on using appropriate macerating media to 

 fail completely to discover separate denticles. I then turned 

 to young specimens of Ceratodus, and had no difficulty in 

 completely confirming Semen's description. In Lepido- 

 siren the only possible reminiscence of such a stage in tooth 



' In Urodele Amphibians tlie tcetli similarly develop before a lumen is 

 formed, and tlie lining of the buccal cavity appears to arise in lliem just as 

 in Lepidosiren and Protopterus. Rose (Scliwalbe's ' Morpliologisclic 

 Arbeiten,' Bd. iv, S. 182), in describing the development of the teeth in 

 Urodeles, talks of the buccal cavity being " mit Dotterpliittchcn imd abgestos- 

 seneu schollcnformigen Epithelzcllen ausgefiillt." On the contrary, I should 

 say, from a study of my own sections of Urodele embryos (Amblystoma 

 and Triton), that the buccal cavity has not yet arisen at the stage of which 

 Rose is speaking. In fig. 6b I figure a section of the mouth region of an 

 Amblystoma of tiic stage in question for purposes of comparison with the 

 corresponding section from Lepidosiren. 



