468 WM. A. KEPNER AND JOHN S. LAWRENCE 



shape of the retinula is that of a bent spindle. The proximal 

 third of the cell is bent almost at right angles to the distal two- 

 thirds. This proximal third of the cell is that part of the cell- 

 body which projects beyond the contour of the pigment cup. 

 The parts of the two retinulae that do not lie within the cup 

 leave the pigment cup in opposite directions (fig. 3). This 

 proximal third of the retinula represents one of the three regions 

 which can be recognized in this cell. The nucleus lies within 

 this region. The cytoplasm of this nuclear-bearing portion of 

 the retinula shows no striation (fig. 1, M). The part of the retin- 

 ula, enclosed by the pigment cup, presents two other regions. 

 The distal of these is closely applied to the inner surface of the 

 surrounding pigment cell. The cytoplasm of this region dis- 

 plays a homogeneous, or better, a uniformly finely granular tex- 

 ture. It is only on the slides that show the best general fixation^ 

 of tissues that we find the uniform texture of this part of the cyto- 

 plasm. In slides that show a little, but no great shrinkage it 

 is suggested that there may be lines of less density present in 

 this region of cytoplasm. These lines, due to slight shrinkage 

 in some fixing, break, and a striated condition of part or all of 

 this part of the cell results. That this variation, due to the 

 method of fixing, expresses some specific condition of the rhab- 

 dome is indicated by the fact that the striae are always directed 

 from the inner surface of the surrounding pigment cell towards 

 the middle region (ellipsoid) of the retinula (fig. 1, R). In this 

 distal part of the retinula we have the end organ of the visual 

 cell — the rhabdome (figs. 1, 2, 3, R). Striated rhabdomes have 

 been described for Turbellaria. There is, however, a third 

 region in the retinula of this rhabdocoele, which is well differ- 

 entiated by Mallory's connective tissue stain, by which the 

 basal portion of the cytoplasm and the rhabdome take a blue 

 color, while the third region of the visual cell has an affinity 



' We have used Carnoy's fluid, chrom-aceto formalin, aceto-sublimate, and 

 Flemming's stronger solutions. These are given in the order in which we found 

 them yielding results — the poorest fixing resulting from Carnoy's and the best 

 from Flemming's stronger mixture. All staining was with Mallory's connective 

 1;issue stain. The sectioning was done at five microns and serially. 



