(5) Connective tissue layer
(6) Layer of nerve-fibres.
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.
Fig. 278. Two sections through the developing eye of a Cephalopod to shew the formation of the optic cup. (After Lankester.)
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 growth inwards of the rim the mouth of the pit is gradually narrowed ([fig. 278] B), resembling at this stage the eye of Nautilus, and finally closed. There is thus formed a flattened sack, lined by epiblast, which may be called the primary optic vesicle. Its cavity eventually forms the inner optic chamber. The anterior wall of the sack is lined by a much less columnar layer than the posterior, the former giving rise to the epithelium on the inner side of the ciliary processes, the latter to the retina.
Fig. 279. Transverse section through the head of an advanced embryo of Loligo. (After Bobretzky.)
gls. salivary gland; g.vs. visceral ganglion; gc. cerebral ganglion; g.op. optic ganglion; adk. optic cartilage; ak. and y. lateral cartilage or (?) white body; rt. retina; gm. limiting membrane of retina; vk. ciliary region of eye; cc. iris; ac. auditory sack (the epithelium lining the auditory sacks is not represented); vc. vena cava; ff. folds of funnel; x. epithelium of funnel.