104 



THE COCKROACH : 



outermost ganglion is undeveloped. The fibres connecting the 

 second ganglion with the eye take a straight course in the 

 young Cockroach, but partially decussate in the adult. 



All the parts between the crystalline cones and the true optic 

 nerve are considered by Hickson to compose the retina of 

 Insects, which, instead of ending at the fenestrated-membrane, 

 as has often been assumed, includes the ganglia and decussating 

 fibres of the optic tract. The layer of retinulae and rhabdoms 

 does not form the whole retina, but merely that part which, in 

 the vertebrate eye, is known as the layer of rods and cones. 



brrv 



N.Op 



Fig. 55. Diagrammatic section of Compound Eye. The references as above. 



As to the way in which the compound eye renders distinct 

 vision possible, there is still much difference of opinion. A 

 short review of the discussion which has occupied some of the 

 most eminent physiologists and histologists for many years 

 past will introduce the reader to the principal facts which have 

 to be reconciled. 



The investigation, like so many other trains of biological 

 inquiry, begins with Leeuwenhoeck (Ep. ad Soc. Reg. Angl. iii.), 

 who ascertained that the cornea of a shardborne Beetle, placed 



