160 ELEMENTARY BIOLOGY. [CHAP. 



(rod- and cone-fibres), each of which swells out 

 and has a nucleus (the granule) developed in 

 the enlargement. 



0. The external limiting membrane. A thin 

 homogeneous layer like a. 



t. The fibres of Mutter. These are highly re- 

 fracting fibres which can be traced with ease 

 from the internal limiting membrane to the 

 fenestrated layer. They probably run beyond 

 the latter and end on the external limiting 

 membrane, but are difficult to trace through 

 the outer granular layer. 



K. The rod- and cone-layer. The main thing 

 which will be noted here is the huge rods for 

 the most part distorted by the treatment 

 to which the retina has been exposed. In 

 favourable bits it can be seen that each rod 

 is divided transversely into an inner and an 

 outer segment. The cones are few and small, 

 and generally completely concealed by the 

 rods. 



d. Take a fresh .frog's eye: prick its cornea and 

 collect 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 shewing the 

 boundary line between their two segments very 

 plainly. At first both segments are homoge- 

 neous, but very soon they begin to alter ; the 



