58 THE AMERICAN MONTHLY [Apr. 



as the periphery is reached upon either side they gradu- 

 ally assume a nearly horizontal or equatorial direction. 

 They measure about 1-600 inch long and 1-4000 to 1-2000 

 wide. The outer ends of these cells form a beautifully 

 distinct^ regular, unbroken line ; the membrana limitans 

 externa, (Fig. I, M.,) and external to this is found the pali- 

 sade layer which corresponds to the rod and cone layer 

 in vertebrates. This layer is composed of irregular, 

 long, club-shaped, non-nucleated cells which are ar- 

 ranged with their long diameter perpendicular to the 

 membrana limitans externa. In the central portion of 

 the retina these cells are coarser and shorter than upon 

 the sides. The measurements oi these cells are about 

 two- thirds that of the previous layer. The free ends of 

 these cells are distinctly rounded and club-shaped and no 

 external segments are to be discovered but they seem to 

 be surrounded by a homogenous fluid or very finely 

 granular substance that fills the space between this 

 layer and the tapetum which we next consider. 



Tapetum Lucidum. — Of peculiar interest in stuying the 

 eye of Pecten is the tapetum lucidum. It is a bright 

 colored and apparently light-reflecting membrane situa- 

 ted directly back of the rod and cone layer of the retina. 

 In eyes hardened by the various common methods it 

 appears upon cross-section to be thrown into various 

 folds and masses which do not assume any certain order. 

 The best and most striking view of this membrane may 

 be obtained in equa-sections of the eye after hardening 

 in Osmic Acid. It then forms a very beautiful and inter- 

 esting object for the polariscope. It is not necessary 

 however to use a polariscope to view the beautiful color- 

 ing of this object for in posterior equatorial sections of 

 the eye may be seen the broad annular ring of pigment 

 in the center of which the angular, iridescent mass of 

 the tapetum appears. By using transmitted light, rich 

 rainbow colors strongly contrast this part with the sur- 



