INTERNAL ANATOMY. 3l6 



only the most anterior of the rodlets are exposed to the attrition of use, the 

 remainder being covered by the projecting layer of cuticle. 



Under a high magnification the innermost portion of the mandibular sulcus 

 is shown in fig. 2, Plate 7, and merits somewhat closer examination. The epi- 

 thelial cells, e, are of a high columnar form, and each one bears upon its distal 

 end a single rodlet, in successively later stages of formation from the bottom 

 of the groove outward. The distal end of the cell is nearly flat or but slightly 

 convex while the first layers of chitin (conchin) are produced, but becomes more 

 arched in the later stages, until it reaches a high conical form, as shown at the 

 right side of the figure. The corresponding layers of each rodlet are similarly 

 arched, the outermost ones but slightly, while those more recently formed are 

 more and more curved. The margin of each successive layer forms a ridge 

 upon the surface of the rodlet, as is more clearly seen in fig. 3 and 4 of Plate 7, 

 in which a fully formed rodlet with its basal cell is contrasted with a much 

 younger one. The volume of the basal cell in the older stage is very strikingly 

 less than that of those in the earlier stages. The nuclei of these basal cells, 

 or rhabdoblasts, are large, oval in outline, and deeply staining, with many 

 irregular chromatin granules. The cell-protoplasm is longitudinally fibril- 

 lated, a great complex of fine fibrUlae being readily made out, extending through- 

 out the cell and continuous, indeed, through the substance of the rodlet itself. 

 This is shown (Plate 8, fig. 3a), in three basal cells from the younger portion 

 of the mandible. But two layers of the rodlet have here been laid down, and 

 the fine fibrillae of the cytoplasm may be followed out into them for varying 

 distances. The same may be noted, Plate 8, fig. 1, in a group of seven basal 

 cells from an older portion of the mandible, only the innermost part of their 

 rodlets being shown. 



Plate 7, fig. 2, and Plate 8, fig. 1, 3, also show the strikingly intimate 

 manner in which the ends of the underlying muscle-fibres are related to the 

 epithelium of the mandible. Immediately below the epithelium is a strongly 

 developed faintly striated layer of compact connective-tissue, with scattered 

 cells, and below this the connective-tissue is more loosely arranged. Numerous 

 smooth muscle-fibres pass up through this compact layer, branching in it and 

 continuing up among the epithelial cells, where they ramify still more, and 

 terminate in close contact mth the bases of these cells. In fig. 3 of Plate 8 

 two such muscle-fibres are represented, penetrating and branching in the con- 

 nective-tissue layer /, and from thence ramifying between the bases of the epi- 

 theUal cells e. Since the figure was drawn with a minimum change of focus, 



