106 FRANK HELVESTINE, JR. 



and four relatively large basal granules (fig. 6, T). The amitotic 

 division involves two vertical cell-planes at right angles to each 

 other. Whether this portion of the epithelium be viewed in 

 sections cut through the gill filament vertically, as in the illus- 

 tration, or in a plane at right angles to this, that is, in a plane 

 that cuts the filament longitudinally, these cells, thus cut lon- 

 gitudinally, present in either case two basal bodies and two cilia. 

 This condition demonstrates that these daughter cells of the 

 amitotically dividing mother cell are of columnar shape and 

 possess a double pair (that is, four) of basal bodies and ciUa. 

 Amitotic division in the laterofrontal epithelium is not always 

 in the same plane (fig. 5, N), nor are the daughter nuclei of 

 uniform size (fig. 7, H). Neither does the development of the 

 cells of the lateral and laterofrontal epithelium always take 

 place in the order that I have described. In some cases the 

 primitive cell of the lateral epithelium undergoes amitosis before 

 reaching the laterofrontal position (fig. 6, T), and the columnar 

 cells which form a transition' between the laterofrontal and the 

 frontal epithelium may be both in the lateral and laterofrontal 

 position (fig. 6, T). 



d. Frontal epithelium. The frontal epithelium is derived from 

 the transitional cells of the laterofrontal epithelium. These 

 transitional cells as they are needed are pushed into the frontal 

 position. Here a transformation occurs. First, there is a par- 

 tition of the basal granules and a splitting of the cilia, so that 

 the cell, instead of having two pairs of cilia and two pairs of 

 basal granules, now has a tuft of cilia and many basal gran- 

 ules (fig. 5, R). The ciha of these cells are originally long, 

 but due to their exposed position on the crest of the filament, 

 where they are constantly in contact with grit and other abrasive 

 materials, the cilia are broken off and give the appearance of a 

 brush-like border (figs. 4, A, 5, A, 7, A). The nuclei of the fron- 

 tal cells that are adjacent to the laterofrontal region are of the 

 same character as the nuclei of the latter region (fig. 5, R). As 

 the cells are moved crestward the nuclei become for a time more 

 vesicular (fig. 5, A). When the cells of the frontal epithelium 

 become injured or worn out the nucleus of the cell becomes 



