INTERNAL ANATOMY. 321 



imiltiiilicci. In Plate 9, fig. 8, the spine has reached nearly its full size, but is 

 imbedded so deep in the common cuticula that its apex is some distance below 

 the free surface. In fact but very few spines retain their connection with the 

 basal matrix-cell when the wearing away of the general surface exposes their 

 tips. When each spine reaches a length of approximately 0.5 mm. the further 

 deposition of basal layers by the matrix-cell ceases, and the spine becomes 

 separated from it and is simply imbedded in the surrounding cuticle. The 

 continual formation of the cuticle below and its erosion above brings the tip 

 of the spine to the surface and successively the remaining portions (Plate 9, 

 fig. 7), the spines being evidently harder and more resistant than the cuticle 

 surrounding them. The base of the spine is carried farther and farther away 

 from the epithelium by the continued formation of new layers of the general 

 cuticle, while its upper portion is progressively more and more exposed above, 

 finally becoming entirely free, and falUng away. The basal cell appears to 

 enter upon the formation of a new spine in a short time, as frequent instances 

 are found in which a very young spine is in process of development immediately 

 beneath the base of a mature one, which has evidently been produced by the 

 same matrix-cell. 



This relation of the palatal spines to the epithelium and to the common 

 cuticle is shown in Plate 9, fig. 7, under low magnification. The free ends of 

 the spines here are relatively few in number in the thin section, while others 

 are seen at various levels. This appearance is not due to any obliquity of the 

 section, as might be readily supposed. The plane of the section is nearly trans- 

 verse to the long axis of the fold, c is its free margin, a its dorsal surface, and b 

 its ventral one. At e there is found the typical thickness of the cuticle of the 

 general Uning of the buccal cavity. It increases very rapidly on the ventral 

 surface of the palatal fold, but abruptly thins away to a very thin layer at the 

 free margin. In this region a deep gland-Uke infolding of the epithelium occurs 

 in all the sections, characterized by the presence of a very great number of large 

 unicellular gland-cells extending down below the basement-membrane. Simi- 

 lar cells are found continuing around the free margin and over the dorsal surface 

 of the fold. One of these gland-cells is shown in Plate 8, fig. 2. Here the epi- 

 theUum had been stripped away accidentally, so that the apex a, showing the 

 secretion discharge from the cell, appears at the level of the basement-membrane 

 d. The nucleus of the cell is not contained in this section, but is relatively 

 poor in chromatin. The cytoplasm in the lower proximal portion of the cell 

 shows a fine reticulum with deeply staining nodal granules. These gradually 



