436 TRANSACTIONS LIVERPOOL BIOLOGICAL SOCIETY. 
substance staining rather darkly with Iron-llaematoxylin 
(fig. 30, S.m.2.) 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 Rawitz (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 (82) 
has calculated in Pecten jacobacus 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 compact 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 eylindrical 
columns might have in consequence of the pressure of 
adjoiing 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, Aw. f.). This can be seen excellently after 
fixation in Flemming and staiming with IHlaematoxylin, 
and is distinct, both in transverse and longitudinal 
sections. It was, however, best followed after treatment 
by Apathy’s “ Nachvergoldung Method.” or: by the same 
author's Haematein IA. ‘This fibre can be seen distinctly 
in all rods extending from the base of the rod cell to the 
base of the rod (fig. 30, Aa. f.) Patten (35) described it 
as dividing at the base into two branches, which became 
connected with other nerve twigs outside the rod. This is 
