8o fatten: [Vol. I. 



colorless granules, which subsequently acquire a characteristic 

 red color. The nuclei of the retinophorae, which at first are 

 situated in a peripheral thickening of the retina, gradually push 

 their Avay towards the centre of the eye. It is not till quite late 

 in the devolopment, after the appearance of the rods, that their 

 cell-walls become visible. 



The only difference between the rods, when first seen, and 

 those of the adult, was the large size of the axial core, and 

 the extremely thin shell, or sheath, scarcely visible except at 

 the tips of the rods. As soon as the rods could be clearly dis- 

 tinguished they were seen to contain an axial nerve-fibre. The 

 nuclei of the argentea decrease in size until they finally disap- 

 pear, with the exception of those in the inner layers, where, in 

 the adult even, one or two aborted nuclei may occasionally 

 be seen. 



The vitreous net-work^ in contrast with its subsequent condi- 

 tion, forms at first a thick homogeneous and structureless 

 layer. 



The innumerable isolated fibres which, even in the earlier 

 stages innervated the eye, subsequently unite to form a single, 

 loose bundle of nerve-fibres, — the primitive optic nerve, which 

 divides later into the axial and ganglionic branches of the de- 

 finite optic nerve. All the nerve-fibres supplying the optic 

 vesicle are not collected to form the optic nerve ; for many en- 

 tering the base of the vesicle retain their primitive arrangement, 

 and appear to penetrate the sclerotica, tapetum, and argentea, as 

 far as the rods. The circumpallial nerve contains as many 

 ganglionic swellings as there are optic nerves. In many, if not 

 all, of these ganglia there is a peculiar infolding dividing them 

 into halves. 



The free edge of the ophthalmic fold contains, at regu- 

 lar intervals, large ova-like cells, which may be seen in 

 preparations of the whole mantle edge, as well as in sections. 

 In the neighborhood of the hinge the branchial wall of the 

 mantel of younger specimens is thrown into a variety of thick 

 ciliated folds, the nuclei of which are, in most cases, several 

 rows deep. In some cases one of the folds becomes especially 

 enlarged at its extremity, the walls thickening to form a kidney- 

 shaped body with a great many small, deeply-stained nuclei. 

 The surface is covered with a cuticula, provided with minute 



