INTERNAL ANATOMY. 315 
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 fibrillae 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 
redlets 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 with 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 f, and from thence ramifying between the bases of the epi- 
thelial cells e. Since the figure was drawn with a minimum change of focus, 
