60 BLIND VERTEBRATES AND THEIR EYES. 



(12) An arrest in the ontogenetic development has taken place in so far as the 

 number of cell multiplications concerned in forming the anlage of the various 

 parts of the eye have decreased in number, and in the lack of union of the lips of 

 the choroid fissure. 



(13) It is possible that the absence of cones or rods is due to an arrest in the 

 histogenesis of the retina, but since these structures are normally formed in the 

 young of Typhlotriton and disappear with age, it is possible that their absence in 

 the adult eye of Rhineura is also due to ontogenetic degeneration. 



(14) The irregularity in the structure and existence of the lens and the great 

 reduction of the vitreous body offer evidence in favor of the idea of the ontogenet- 

 ically and the phylogenetically earlier disappearance of the ontogenetically and 

 phylogenetically newer structures. 



(15) Horizontal nuclei found between the pigment epithelium and the outer 

 limiting membrane are probably derived from the proximal layer of the optic cup. 



(16) The different layers of the retina have reached a degree of differentiation 

 out of proportion to the great reduction of the dioptric apparatus and general 

 structure of the eye. 



TYPHLOPS. 



(1) The dermis and epidermis over the eye differ from the same over neigh- 

 boring parts, by being thinner, more compact, and free from pigment and glands. 



(2) The conjunctival sac is present and has a width at least as great as the eye. 



(3) Harder's gland surrounds all but the distal part covered by the conjunc- 

 tival sac. 



(4) Eye muscles present, but their number and structure could not be made out. 



(5) A large lens with capsule is present. 



(6) The various layers of the normal snake retina are present but the com- 

 parative thickness is different. 



