474 



CEPHALOPODA. 



(/) 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 (Co.ep), inserted at the base of the iris, and mainly formed of a 

 continuation 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 adjoining 

 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. 



(2) Layer of rods. 



(3) Layer of granules imbedded in pigment. 



(4) Cellular layer. \ 



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



(6) Layer of nerve-fibres. 



Anterior division of 

 retina. 



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

 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 



A 



FlG. 278. TWO SECTIONS THROUGH THE DEVELOPING EYE OF A CEPHALOPOD 



TO SHEW THE FORMATION OF THE OPTIC CUP. (After Lankester.) 



layer lining the floor of the pit soon becomes considerably thick- 

 ened. By the growth inwards of the rim the mouth of the pit 



