Morphological Studies. 741 



epithelium on the apex of the odontoblast papilla cannot be a very 

 favourable one for its nutrition. If it is to be sustained , it must 

 obtain nourishment from within, and whence if not from the deve- 

 loping enamel? 



Further, I may mention that I possess preparations of the deve- 

 loping teeth of Fetromyzon (stained with osmic acid) in which several 

 layers of the stratified epithelium above the papilla and many cells of 

 the epidermal part of the papilla itself contain numerous blackened 

 particles. 



I do not know what the real nature of these particles is in either 

 Spee's case or my own, but it is a little hazardous to pronounce 

 them straight away to be half- elaborated enamel. Spee says they 

 are not of a fatty nature, but may they not be none the less products 

 of cell degeneration? 



Returning now to Scott's work. The earliest stage he describes 

 and figures is one in which there is no longer any trace of a „tooth- 

 sac" such as I figure in figs. 3 and 9. In fact, the first tooth is just 

 breaking through the stratified oral epithelium, and a reserve tooth is 

 already in course of formation. His figure looks as though it were taken 

 from one of the posterior teeth, which, as mentioned above, never 

 present much resemblance to true tooth formations. 



As a preliminary to the tooth formation, the „epiblast which 

 lines the cavity of the mouth becomes extremely thick" (Scott 19, 

 p. 731). 



When he continues that „the essential parts of a tooth are here 

 present", it is impossible to agree with him. The most essential part 

 of a developing tooth is absent, viz., the odontoblasts. 



His opinion that the enamel epithelium functions in Petromyzon 

 as a sort of tooth gland, forming horn, harmonises with the views 

 enunciated in this memoir, but Dr. Scott hardly proved his thesis, 

 for he never saw the true tooth-sac as figured in fig. 9, and had not 

 the evidence for his conclusions which the comparison of the Myxi- 

 noid tooth with the developing Petromyzontoid tooth has yielded us. 



Scott concludes (19, p. 732) that his view of the peculiarities 

 of dental development in Fetromyson „implies, of course, that this 

 group of fishes was derived from ancestors possessed of teeth of the 

 ordinary or Selachian type. Further, as it is now very generally ad- 

 mitted that teeth are only modified placoid scales, it follows that the 



49* 



