132 ICxERNA B. J. SOLLAS. 



radula are so. But they are evidently not foi'med, as Rossler 

 suggested, as a special secretion apposed to the outer surface 

 of the tooth as first formed, but rather by intussusception. 



With regard to other doubtful points, Rossler assumed that 

 the same odontoblasts secrete all the teeth that are produced 

 during life ; but later writers have thought that these cells 

 must be joeriodically replaced, for, as Bloch points out, ''unless 

 we suppose the odontogenous cells to be replaced from time 

 to time we cannot understand how larger teeth are formed 

 with the growth of the animal, for it is unlikely that cells 

 continually engaged in active secretion can also grow " 

 (p. 381). The new odontoblasts, according to Bloch, are 

 formed out of the same cell complex from which the upper 

 epithelium is renovated. This assumption, he thinks, con- 

 tains no improbability '' since the upper epithelium and the 

 odontoblasts have the same task, namely, the secretion of 

 chitin." But from what has preceded it is clear that the 

 functions of the roofing epithelium and of the odontoblasts 

 are very different ; this need not, however, impair Bloch's 

 main argument that the odontoblasts are replaced ; there is 

 no reason why a single cell complex should not give off 

 chitin-secreting cells on the one hand and cells secreting 

 mineral matter on the other. The single apical cell of the 

 stems and roots of various plants does more than this. 



Again, Rossler considered that the radula must glide forward 

 to a certain extent over the basal epithelum, which he said is 

 delayed by the action of the retractor muscle (tensor superior 

 muscle of Amaudrut). Though I do not agree with his view, 

 Rossler, in alluding to the action of the muscle, seems to me to 

 have touched on an important problem which has been quite 

 neglected by some authors. To this I shall return presently. 

 Bloch objects to Rossler's view : " Ich kann mir nicht denken 

 wie die Basalplatte, die' die Ziilinchen tragt, und nach den 

 Praparaten mit ihrem Epithel in innigen Verbindung zu sein 

 scheint, iiber diesen Zellschicht hinweggleite. Da ist nur 

 eine Moglichkeit namlich die Zelle bewegen sich mit der 

 Basalplatte nach vorn." In the paragraph which follows he 



