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in the form of numerous cilia-like processes which, in a 

 Mallory-stained section, are at first red in colour. They 

 soon become blue or purple as one passes from the cell, 

 and then lose their individuality in a homogeneous blue 

 stained mass of chitin, which forms the basis of the ribbon 

 and teeth. The fibrous ground can be detected every- 

 where in young teeth stained with Mallory's connective- 

 tissue stain. In the whelk the teeth are absolutely 

 continuous with the basal membrane of the radula, and 

 it is almost impossible to make out any line of junction 

 or to say which cells have participated only in the 

 fabrication of the teeth. The ventral and lateral cells of 

 the blind end have, however, most certainly played the 

 greater part in the formation of the ribbon membrane and 

 the dorsal cells, the teeth. 



One point, however, of great importance is that 

 the ventral and lateral cells are still connected with 

 the radula by the cilia-like tags some considerable 

 distance away from the blind end, and the same 

 applies to the dorsal cells. This probably indicates 

 that when the point of the radular membrane (in 

 this attached region) was at the extreme posterior end 

 of the sac, the cells below it were there too. In 

 other words, just as the radula is secreted and pushed 

 forwards, so do the odontoblasts move forwards with it, 

 and new ones are formed at the blind end of the sac. 

 There are at present two views on this subject. One is 

 that the odontoblasts, very many of which secrete one 

 tooth, remain functional after that tooth has been formed 

 and go on secreting the next, and so on. The other view 

 is that the odontoblasts have performed all they are 

 capable of in secreting one tooth and that they pass 

 forwards to be replaced by new cells. Obviously they 

 have to become much smaller. The sections of the 



