Chap, xii.] EYES OF CHORDATA. 453 



cup. The difference, then, in the position of the rods and 

 nervous elements of a chordate as compared with that 

 of an invertebrate eye is due to the primary alteration 

 in position, caused by the mode of formation of the 

 central nervous tube, and by the fact that the retina 

 is an outgrowth from an anterior enlargement of this 

 tube. The stalk of the vesicle forms the optic nerve. 



The other or anterior half of the eye has a his- 

 tory which is essentially similar to that of the eyes 

 of most invertebrates ; the epiblast of the surface 

 thickens and gives rise to the lens and cornea, while 

 the mesoblast forms supporting and protective tissues. 



The simple eyes of the Tunicata agree in the 

 important point of the position of the sensory layers, 

 with the typical eye of the Vertebrata ; Ampliioxus 

 has no well-developed eyes, but eye spots have been 

 observed in the larva ; the eyes of the Cyclostomes 

 remain in a condition which is embryonic as com- 

 pared with that of higher forms ; a lens, for example, 

 is absent. Throughout the rest of the Yertebrata the 

 eye has essentially the same structure as in man (see 

 " Elements of Histology," chap, xxxvi., and " Human 

 Physiology," chap, xv.) ; such differences as obtain are 

 of importance and interest as bearing on the adapta- 

 tion of the different parts of the eye to the different 

 media in which it is placed, or to certain differences 

 in its duties. 



In Fishes, where the aqueous and vitreous hu- 

 mours have but little effect in bringing to a focus 

 the rays of light that have already entered the water, 

 the antero-posterior axis of the eye is short, and the 

 lens is very large and convex, while the cornea, owing 

 to the small amount of vitreous humour that is present, 

 is much flattened, and, inasmuch as it is without 

 eyelids, it is thereby less liable to friction than if it 

 projected outwards. The pupil is large, so as to admit a 

 large quantity of light ; in Anableps, which swims with 



