ORGANS OF VISION. 



393 



y/is 



&.. 



from communication with the exterior. The edge of the fold consti- 

 tutes the cornea while the remainder of it gives rise to the sclerotic. 



The retina is at first a thick layer of numerous rows of oval cells 

 (fig. 279). When the inner segment of the lens is far advanced 

 towards its complete forma- 

 tion pigment becomes de- 

 posited in the anterior part 

 of the retina, and a layer of 

 rods grows out from the sur- 

 face turned towards the cavity 

 of the optic vesicle (fig. 280 A, 

 st): At a slightly later stage 

 the retina becomes divided 

 into two layers (Bobretzky), a 

 thicker anterior layer, and a 

 thinner posterior layer (fig. 

 280 rt and rt"}. The former 

 is composed of two strata, (1) 

 the rods and (2) a stratum 

 with numerous rows of nuclei 

 which becomes in the adult 

 the granular layer with its 

 pigment. The posterior layer 

 gives rise to the cellular part 

 of the posterior division of 

 the retina, while layers of 

 connective tissue around it 

 give rise to the connective 

 tissue of this portion of the 

 retina (layer 6 in the scheme 

 on p. 390). The nervous layer 

 is derived from the optic gan- 

 glion which attaches itself to 

 the inner side of the connec- 

 tive tissue layer. 



The greater part of the 

 choroid is formed from the 

 mesoblast adjoining the re- 

 tina, but the epithelium cover- 

 ing its outer wall is of epi- 



B. 



DEVE- 



(After 



FIG. 280. SECTIONS THKOUOH THE 

 LOPING EYE OF LoLIGO AT TWO STAGES. 

 Bobretzky.) 



Id. inner segment of lens ; vl. outer seg- 

 ment of lens; a and a', epithelium lining the 

 anterior optic chamber; gz. large epiblast cells 

 of ciliary body; cc. small epiblast cells of 

 ciliary body; wis. layer of mesoblast between 

 the two epiblast ic layers of the ciliary body ; 

 (if. and if. fold of iris; rt. retina; rt". inner 

 layer of retina; st. rods; aq. cartilage of the 

 choroid. 



blastic origin. 



It is difficult to decide from development whether the Molluscan 

 eyes, so far dealt with, originated in the first instance pari passu with 

 the supra-cesophageal ganglia or independently at a later period. On 

 purely a priori ground I should be inclined to adopt the former alter- 

 native. 



In addition to the above eyes there occur amongst Mollusca highly 

 complicated eyes, of a very different kind, in two widely separated groups, 



