476 CEPHALOPODA. 



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

 is somewhat obscure. The following is Lankester's account of 

 it 1 : "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 sub- 

 sequently by the deposition of concentric layers of a horny material 

 round this cone. No cells appear to be immediately concerned 

 in effecting 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, /if). 



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 Lankester 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 membrane continuous with the ciliary region of the optic 

 vesicle. 



Grenacher 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 primitive optic chamber, and projects into the second or 

 anterior optic chamber where the iridian folds lie closely upon it." 



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



