144 ELEMENTARY BIOLOGY. [CHAP. 



/?. Their free ends; fimbriated and prolonged 



down among the rods and cones. 

 /. The fibres of Mutter; highly refractive sup- 

 porting fibres, often traceable with ease from 

 the internal limiting membrane to the fenes- 

 trated layer. 



iii. Obtain, if possible, similar sections which shall 

 pass through the entrance of the optic nerve. 

 Examine under a high power, and follow the 

 course and relations of the nerve-fibres, 

 iv. Take a fresh frog's eye : prick its cornea and col- 

 lect the aqueous humour on a slide. Then open 

 the eye, remove a bit of the retina and tease it 

 out in the aqueous humour, mount and examine 

 with a high power. 



a. Numerous rods will be seen floating about, 

 many broken but some intact and showing the 

 boundary line between their two segments very 

 plainly. At first both segments are homoge- 

 neous, but very soon they begin to alter; the 

 outer layer frequently then gets transversely 

 striated and shows a tendency to split up into 

 several pieces : gradually these rods curl up, 

 swell out, and entirely disintegrate. 



13. The olfactory epithelium. 



Open the nasal sac of a frog ; remove a portion of 

 the epithelial lining and transfer to i p. c. osmic 

 acid solution for 2 3 hours; tease up in weak 

 glycerine and examine with your highest power. 



Numerous mutilated cells will be found, and, 

 among them, more or less perfect representatives 

 of the following. 



