XIII.J THE FEOG. 251 



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

 Tvhicb 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 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 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 

 outer layer frequently then getting a trans- 

 versely striated appearance and shewing a 

 tendency to split up into corresponding pieces : 

 gradually these rods entirely disintegrate, first 

 curling up, swelling out, &c. 



i. The skin. 



1. Cut out a piece of skin from the back of the thigh of 

 a recently killed frog : spread it out in water, cover, 

 and examine with a low power : note — 

 a. The pigment cells; seen as black irregularly 



shaped patches; some compact, others more or 



less branched. 

 h. The mouths of the cutaneous glands; seen as 



clear round spots, although their openings are 



really triradiate : their number. 



