THE EYb] OF PECTEN. 93 



Below the rod-matrix which underlies the rods is a limiting' 

 membrane, thebasement membrane (PL 7, fig. 13, B.m.), 

 which extends completely across the eye. It corresponds to 

 Schreiner's ^' Innere Siebmembran," but is a perfectly con- 

 tinuous thin sheet. It is stained by hfematoxylin similarly 

 to the matrix but darker, and since the rods terminate a little 

 distance above it it is obvious that they cannot pass through 

 it. It occupies a similar position to Patten's "vitreous net- 

 work," but his description also refers to a thin layer of hyaline 

 substance perforated by large holes into which the inner ends 

 of the rods fit, and Schreiner states that the points of the 

 rods come to lie against the tapetum. No traces of any cell- 

 structure have been made out in this bounding membrane, 

 which, as noted above, is not perforated by the rods. 



Eeference has already been made to the marginal area 

 of the retina (PI. 6, fig. 1, M. Ret.). This is best studied from 

 specimens fixed in Carnoy's fluid. The rods remain practically 

 similar in size until about the tenth from the margin of the 

 rod-bearing region, and then follows a rapid decrease in size, 

 leading to the apparently fibrous lateral parts where no rods 

 are present. Careful examination will reveal the fact that 

 the so-called outer sieve-membrane can be traced to the very 

 edge of the retina, but the space between it and the basement- 

 membrane is exceedingly small. This corresponds, howevei-, 

 to the space occupied by the rods in the middle part of the 

 retina. The axial fibre or neurofibril can be seen more dis- 

 tinctly in these marginal rod-cells, which for a little distance 

 are similar in diameter to the much longer ones in the centre 

 of the retina. They next become much less in diameter until 

 finally the boundaries become difficult to detect, and the axial 

 fibril is the most distinct part of the cell. It can also be seen 

 extending below the line of the pseudo sieve-membrane, 

 though without any rod. Between these modified rod-cells 

 are more supporting cells. 



The mai-ginal region differs, therefore, from the central 

 part of the retina in being composed of rod-cells which are 

 far shorter than those of the latter region, whose diameter is 



