XXIX] EYE 225 



The ganglionic layer is represented by two rows of cells, the 

 nuclei are conspicuous, but with very little cell substance. 



The outer molecular layer is a mere line. 



The nuclei of the outer nuclear layer are elongated. 



The rods and cones are much larger and more easily seen 

 than in the mammal. The outer limbs of the rods will be more 

 deeply stained with osmic acid (if this has penetrated) than the 

 inner limbs ; the longitudinal fluting of the outer limbs will be 

 seen, and probably the transverse breaking into discs. 



The distinct pigment cells and their processes. 



10. Cut out the eye from a freshly killed frog ; holding it 

 up by the optic nerve cut off a small part '5 to 1 mm. of the 

 coats of the eye around the optic nerve. Insert the point of the 

 scissors into the hole thus made and make two radial cuts 

 including about a third of the eye between them, cut off the 

 anterior part, place the posterior part inner surface downwards 

 on a slide, between two strips of thickish paper, take off the 

 sclerotic, fixing the pigment layer at one edge and the rest of the 

 retina at the other \ pull them apart. If this is done quickly, 

 the retina will be seen to be a purplish-red, due to the visual 

 purple in the rods. Cover with a cover-slip having on it a small 

 drop of salt solution, and examine at once. The mosaic formed 

 by the ends of the rods and cones of the retina should be seen ; 

 most of the rods are purplish-red, but a few green ones are pre- 

 sent. The colours soon fade; when they have faded, remove 

 the strips of paper and tease out the piece of retina. Note the 

 large outer limb of the rods, its longitudinal fluting, and trans- 

 verse breaking into discs, the lenticular piece following this, the 

 refractive globule in the cones, and the pigment granules and 

 cells. 



11. Make a similar preparation of the retina of a mammal. 

 Note the small diameter of the rods and cones. 



12. Tear off a small piece of the retina from a 



sheep's eye preserved in Miiller's fluid, probably the 



pigment-layer of the retina will be left adhering to the 



choroid. Take a small piece of this pigment-layer, and 



L. 15 



