THE MOLLUSCAN RADULA. 



131 



part of the membrane on which they are seated. They show 

 lamination, the laminae running parallel to the matrix cell 

 (Rossler's). Soon, as we pass forwards along the ribbon, 

 there is a sharp contrast in staining properties between the 

 young teeth and the underlying part of the basal membrane 

 (text-fig. 3), but this contrast is not maintained, the increased 



Text-Figure 3. — Longitudinal sections of t.lie radula of Helix 

 aspersa, stained with lisematoxvlin, showing four stages in the 

 development of the teeth. ^/, the youngest tooth of tlie radula; d, 

 a tooth from the mouth-cavity ; h and c, intermediate stages. 

 (Drawn with Zeiss camera lucida, Zeiss obj. D., eyepiece 4.) 



staining power spreading downwards through the thickness 

 of the membrane but always leaving a thin layer which 

 preserves the original resistance to stains. At the same 

 time that the membrane darkens the teeth, which nearer the 

 origin were uniformly darkly stained, acquire a lighter peri- 

 phery, the core remaining darkly stained; finally the surface 

 layers become quite colourless and only some scattered small 

 spheres of darkly staining matter remain in the interior. 

 These spheres are arranged with a certain definiteness in rows 

 which converge towards the posterior basal part of the tooth. 

 Such are the appeai-ances seen in sections treated with a 

 protoplasmic stain onl}^, -whether heematoxylin, safranin, 

 borax carmine, or carmalnm. If we first apply Bethe's stain 

 and then safranin or carmalum, the whole section will be 

 coloured pink with the exception of the cores of the adult 

 teeth, which are green, and the surface layers of the same, 

 which are colourless. These outer layers are Sharp and 

 Rossler's enamel. Rossler made the interesting note that 

 they are not doubly refracting, while the other parts of the 



