THE EYES OF SEPIA 165 



suspensory mechanism, ciliary body, iris, anterior- and vitreous-chambers, 

 cornea, and a palpebral fold (Fig. 121). They are, however, of the 

 upright type, and are developed as an ingrowth of the epidermis. It 

 must be realized also that the names which have been applied to the 

 various parts do not in most cases denote structures which are strictly 

 homologous with the structures similarly named in the vertebrate eye. 



The retina resembles the vertebrate retina in having three principal 

 layers of cell-elements, namely, an inner consisting of the sensory- or 

 visual-cells ; an intermediate resembling the bipolar cells of verte- 

 brates ; and an outer or ganglionic layer. The retina of the cuttlefish is, 

 however, of the upright type, the rods or receptive ends of the visual 

 cells being directed inward towards the source of light, and the ganglion 

 cells and nerve-fibre layer being peripheral, i.e. nearest the fibrous 

 capsule. The retina of Sepia thus agrees with the vertebrate retina in 

 consisting of three principal layers of sensory cells, thus being " com- 

 pound " in type ; but it differs in being upright as contrasted with the 

 inverted retina of the vertebrate eye. 



The general structure of the retina will be most easily understood by 

 a reference to its early stages of development, as shown in (Fig. 36, A, B, C, 

 D, E, Chap. 3, p. 50). The eye commences as a simple pit — optic cup ; 

 this is lined by a single layer of cubical cells, continuous at the mouth of 

 the pit with the epithelium covering the surface of the body. The 

 mouth of the pit then becomes constricted, and later an optic vesicle is cut 

 off from the exterior. The deeper cells of the optic vesicle, which will 

 give rise to the retina, become columnar and develop thread-like processes 

 which project into the cavity of the vesicle. The cells of the superficial 

 segment of the wall of the vesicle become flattened and form the inner 

 epithelial layer of the future corpus epitheliale. Superficial to the optic 

 vesicle is a mesodermal layer covered externally by the epithelium of the 

 body- wall. The latter forms the outer epithelial layer of the future corpus 

 epitheliale and the two epithelial layers with the mesoderm between them 

 form the primary cornea. Around this a circular fold rises up and grows 

 inwards over the developing lens ; this is known as the iris fold (C, D, E). 

 The epithelial body consists of a central portion which is primarily 

 concerned in the secretion of the posterior segment of the lens but later 

 forms a septum between the two segments of the lens and a peripheral 

 portion which is composed of large clear cells, some of which by secretion 

 and also by degeneration give rise to both segments of the non-cellular 

 lens ; others which are of small size contribute to the later stages of 

 development of the lens. In the retina, according to Faussek, the primarily 

 tall columnar cells of the inner segment of the optic vesicle, which at first 

 form a single layer resting on the inner surface of the basement membrane, 



