I JO PHYSIOLOGY AND MORPHOLOGY OF ANIMALS. 



101, page 163), nor is there any fovea. There are cer- 

 tainly no corresponding points of the two retinae, and 

 therefore no binocular vision ; also, as we shall see pres- 

 ently, no blind spot. 



2. The Vertebrate Eye. — There are two essential 

 differences between the invertebrate and the vertebrate 

 eye. (1) In the former the nerve fibers terminate for- 

 ward in the posterior ends of the rods in the most 

 natural way, as in the case of nerve terminals, in all other 

 sense-organs. The bacillary layer is the innermost 

 layer of the retina, and exposed directly to the action 

 of light. In the vertebrate eye, on the contrary, the 

 bacillary layer is the outermost layer of the retina, and 

 therefore the fibers have to go forward beyond and 

 turn back and terminate in the anterior ends of the rods. 

 This is wholly exceptional not only among eyes, but 

 among special sense-organs. It is this course of the 

 fibers which makes a blind spot, and therefore the in- 

 vertebrate eye can not have a blind spot. 



(2) In invertebrates the whole eye, both the retina 

 and the lenses, is made by infolding of an external epi- 

 thelial surface. In vertebrates, on the contrary, the 

 instrumental part, especially the crystalline lens, is made 

 in this way, but the retinal part is made, as embryonic 

 development shows, from the brain, by an outfolding of 

 the cerebral vesicle. 



The steps of the development of the vertebrate eye 

 are briefly as follows: (1) The brain is developed as 

 three vesicles. The anterior one is the thalamus (Fig. 22, 

 P a ge 37), which is the basal part of the cerebrum, and we 

 shall call this the cerebral vesicle. (2) From the cerebral 

 vesicle by outfolding is formed on each side the optic vesi- 

 cles (OF, Fig. 108, A), which become more and more con- 

 stricted off until they are connected only by a narrow 

 neck, which becomes the optic nerve (Fig. 108, B). 



