104 



substance staining rather darkly with Iron-Haematoxylin 

 (fig. 30, S.'rn.i.) which separates them from the under- 

 lying argentea. There has been some disagreement as to 

 whether this is a secretion of the argentea or an artifact. 

 It is certainly not the latter, nor have we to do with single 

 rod mantles as stated by Eawitz (36). It is simply a 

 homogeneous mass in which the rods are imbedded, 

 and I propose the name Basement Membrane instead of 

 Schreiner's Inner Sieve Membrane, owing to the 

 difference from the outer sieve membrane. Hesse (32) 

 has calculated in Fecten jacohaeus there were present 

 24,000—27,500 rods to the sq. mm. That is about 2,400 

 rods as the average for a medium-sized eye. 



The internal structure of these elements, both rods 

 and rod cells, has caused much confusion. The rod cells 

 have rather more comjjact protoplasm than the rods, which 

 appears in sections to be condensed rather in the middle 

 (fig. 32). In transverse section the rod cells appear rather 

 irregular in shape — some circular, others rectangular or 

 triangular ; the appearance, in short, that cylindrical 

 columns might have in consequence of the pressure of 

 adjoining cells. The rods are not so irregular in shape, 

 and the cell contents are not so conspicuous, but running 

 down through the middle is a well-defined axial fibre 

 (fig. 32, Ax. /.). This can be seen excellently after 

 fixation in Flemming and staining with Haematoxylin, 

 and is distinct, both in transverse and longitudinal 

 sections. It was, however, best followed after treatment 

 by Apathy's " Js^achvergoldung Method " or by the same 

 author's liaematein lA. This hbre can be seen distinctly 

 in all rods extending from the base of the rod cell to the 

 base of the rod (fig. 30, Ax. f .) Patten (35) described it 

 as dividing at the base into tw^o branches, which became 

 connected with other nerve twigs outside the rod. This is 



