392 CEPHALOPODA. 



by epiblast. The posterior segment of the lens arises as a structureless 

 rod-like body, which is shewn in fig. 279 depending from the inner 

 side of the anterior wall of the optic vesicle. Its exact mode of origin 

 is somewhat obscure. The following is Lankester's account of it^: 

 "It is formed entirely within the primitive optic chamber, and at first 

 depends as a short cylindrical rod from the middle point of the 

 anterior wall of that chamber, that is to say, from the point at which 

 the chamber finally closed up. It grows subsequently by the depo- 

 sition of concentric layers of a horny material round this cone. No 

 cells appear to be immediately concerned in affecting the deposition, 

 and it must be looked upon as an organic concretion, formed from 

 the liquid contained in the primitive optic chamber." 



The lens would thus appear to be a cuticular structure. It 

 gradually assumes a nearly spherical form; and is then composed of 

 concentrically arranged layers (fig. 280 hi). 



While the lens is being formed, the ciliary epithelium of the optic 

 vesicle becomes divided into two layers, an outer layer of large cells 

 and an inner of small cells. Both layers are at first continuous across 

 the anterior wall of the optic chamber in front of the lens, but soon 

 become confined to the sides (fig. 280 A, cc and gz). The inner layer 

 is stated by Lank ester to give rise to the muscles present in the 

 adult. The mesoblast cells also disappear from the region in front 

 of the lens, and the outer epithelium is converted into a kind of 

 cuticular membrane. By these changes the original layers of cells 

 in front of the lens become reduced to mere membranes, — a change 

 which appears to be preparatory to the appearance of the anterior 

 segment of the lens. The formation of the latter has not been fully 

 followed out by any investigator except Bobretzky. His figures 

 would seem to indicate that it is formed as a cuticular deposit in 

 front of the membrane already spoken of (fig. 280 B, vl). The two 

 segments of the lens appear at any rate to be separated by a mem- 

 brane continuous with the ciliary region of the optic vesicle. 



Grenadier believes that the front part of the lens is formed in a pocket- 

 like depression of the epiblastic layer covering the outer side of the optic 

 cup; and Lankester thinks that the lens "pushes its way through the 

 median anterior area of the ])rimitive optic chamber, and projects into the 

 second or anterior optic chamber whei'e the ii'i(Uan folds lie closely 

 upon it." 



While the lens is attaining its complete development there 

 appears a fresh fold round the circumference of the eye, which 

 gradually grows inwards so as to form a chamber outside the parts 

 already present. This chamber is the anterior optic chamber of the 

 adult. In most Cephalopods (fig. 277 c) the edges of the fold do not 

 quite meet, but leave a larger or smaller aperture leading into the 

 chamber containing the iris, outer segment of the lens, etc. In some 

 forms however they meet and coalesce, and so shut off this chamber 



1 "Devel. of Cephalopoda." Q. J. Micro. Scien. 1875, p. 44. 



