390 CEPHALOPODA. 



of capsule, which may be called the sclerotic. Posteriorly the sclerotic abuts 

 on the cartilaginous orbit, which encloses the optic ganglion (G. op) ; and 

 in front it becomes transparent and forms the cornea Co, which may be 

 either completely closed, or (as represented in the diagram) perforated by 

 a larger or smaller opening. Behind the cornea is a chamber known as the 

 anterior optic chamber. This chamber is continued back on each side 

 round a great part of the circumference of the eye, and separates the 

 sclerotic from a layer internal to it. 



In the anterior optic chamber there are placed (1) the anterior part of 

 the lens (I 1 ) and (2) the folds of the iris (Ir}. The whole chamber, except 

 the part formed by the lens, is lined by the epidermis (hit 1 and Tat 2). 

 Bounding the inner side of the anterior optic chamber is a layer which is 

 called the choroid (Int 1) which is continued anteriorly into the fold of the 

 iris (Ir}. The most superficial layer of the choroid is the epithelium 

 already mentioned, next conies a layer of obliquely placed plates known as 

 the argentea externa, then a layer of muscles, and finally the argentea 

 iiiterna. The argentea interna abuts on a cartilaginous capsule, which 

 completely invests the inner part of the eye. 



The lens is a nearly spherical body composed of concentric lamella? of 

 a structureless material. It is formed of a small outer (I 1 ) and large inner 

 (I) segment, the two being separated by a thin membrane. It is supported 

 by a peculiar projection of the wall of the optic cup, known as the ciliary 

 body (Oo.ep), inserted at the base of the iris, and mainly formed of a continua- 

 tion of the retina. This body is however muscular, and presents a series 

 of folds on its outer and inner surfaces, which are especially developed 

 on the latter. 



The membrane dividing the lens into two parts is continuous with the 

 ciliary body. Within the lens is the inner optic chamber, bounded in front 

 by the lens and the ciliary body, and behind by the retina. 



The retina is formed of two main divisions, an anterior division adjoin- 

 ing the inner optic chamber, and a posterior division (N.S) adjoining the 

 cartilage of the choroid. The two layers are separated by a membrane. 

 Passing from within outwards the following layers in the retina may be 

 distinguished : 



(1) Homogeneous membrane. ) . . v . . ,. 

 / a \ T f j ' Anterior division of 



(2) Layer of rods. 



(3) Layer of granules imbedded in pigment. ) 



(4) Cellular layer. | 



(5) Connective tissue layer. ' Posterior layer of retina. 



(6) Layer of nerve-fibres. ) 



At the side of the optic ganglion is a peculiar body, known as the Avhite 

 body (not shewn in the figure), which has the histological characters of 

 glandular tissue. 



The first satisfactory account of the development of the eye is 

 due to Lankester (No. 365). The more important features in it 

 were also independently worked out by Grenacher (No. 363), and 

 are beautifully illustrated in Bobretzky's paper (No. 362). The eye 

 first appears as an oval pit of the epiblast, the edge of which is 

 formed by a projecting rim (fig. 278 A). The epiblast layer lining 

 the floor of the pit soon becomes considerably thickened. By the 



