PLATE 1 



EXPLANATION OF FIGURES 



All figures except figure 2 were drawn from transverse sections of the gill fila- 

 ments of Cyclas. The tissue was fixed with Meves' fluid, cut at 5/^, and stained 

 with iron hematoxylin. The magnification is 1300 diameters, except in figure 5 

 where is it 1500 diameters. In order not to obscure the basal bodies and their 

 cilia, cytoplasmic details, including mitochondria and the cytoreticulum, are 

 added only in figure 5. A xe Leitz oil-immersion lens was employed in this study. 



1 Transverse section showing basal epithelium and portions of two adjacent 

 gill filaments. The cell in the laterofrontal position is of the primitive type with 

 a tuft of long cilia (P) . Interposed between the laterofrontal and lateral epithe- 

 lium is a small non-ciliated cell (Z). The cell of the lateral epithelium which is 

 also ciliated (L) contains two nuclei and is evidently in a phase of amitotic 

 division. The cells of the basal epithelium are roughly pyramidal in shape and 

 are non-ciliated (A). 



2 Area from the ciliated epithelium of the intestine. The cells are tall 

 columnar and have a tuft of cilia, a double row of basal granules, and a cone of 

 rootlets extending into the cytoplasm. One of the cells is undergoing mitosis, 

 but this cell has no cilia. 



■3 Transverse section showing basal epithelium and a small portion of 

 the filament. A mitotic figure (M) is seen in a cell of the non-ciliated basal 

 epithelium. 



4 Section of a complete filament. The frontal epithelium (A) has short, 

 broken-off cilia. Two pyknotic nuclei of degenerating cells are seen close to the 

 basement membrane (D). The cell of the laterofrontal epithelium shows a 

 nuclear indentation, the initial step in amitosis. The cell of the lateral epithe- 

 lium (L) lies next below the laterofrontal epithelium. 



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