226 DK WALTER H, GASKELL. 



between this compound retina and the brain, forms the optic 

 nerve of this type, and is clearly homologous, not with the optic 

 nerve of the simple eye, but with the connection between the 

 whole or part of the optic ganglion and the brain in type I. 



The ditfereuce between the development of these two types of 

 eye has led in the most natural manner to the conception that 

 the retina is developed in the animal kingdom sometimes from 

 the cells of the peripheral epidermis, sometimes from the tissue 

 of the brain, two modes of development termed by Balfour 

 peripheral and cerebral. An historical survey of the question 

 shows most conclusively that the origin of the simple retina of 

 the simple eye is universally ascribed to the peripheral method 

 of development, the retina being formed from the hypodermal 

 cells by a process of invagination, while the cerebral type of 

 development has been described only in the development of the 

 compound retina of the compound eye. The natural conclusion 

 from this fact is, that in watching the development of the com- 

 pound retina it is more difficult to differentiate the layers formed 

 from the epidermal retinal cells and those formed from the 

 epidermal optic ganglion cells than in the case of the simple 

 retina, where the latter cells withdraw entirely from the surface. 

 This is the conclusion to which Patten has come, and indeed, 

 judging from the text-book of Korschelt and Heider,^ is the 

 generally received opinion of the day as far as the Appendicu- 

 lata are concerned, viz., the retina in the true sense — the retinal 

 end cells, with their cuticular rods — is formed in all cases from 

 the peripheral cells of the hypodermal layer, the cuticular rods 

 being modifications of the general cuticular surface of the body ; 

 the apparent cerebral development of the retina, as quoted from 

 Bobretsky by Balfour,- being in reality the development of the 

 retinal ganglion, and not of the retina proper. 



It is, I imagine, a universal feeling that the natural mode of 

 origin of a sense organ like an eye must always have been from 

 the cells forming the external surface of the animal, and that 

 an origin direct from the central nervous system is a priori most 

 improbable. It is therefore a matter of satisfaction to find that 

 the evidence for the latter origin has universally broken down, 



^ Ted-Book of the Emhryolorjy of ihc Invertebrates, part iii. 

 - Coiii}^ Emhnjolog)!, vol. ii. p. 397. 



