Eyes of Molluscs and Arthropods. 603 



As soon as the latter could be clearly distinguished, they were seen to 

 contain an axial nerve fibre. 



The chauges, by which an eye as complete as that represented in 

 fig. 17 is trausformed into the adiüt conditiou, are of no great morpho- 

 logical value. and are easily comprehensible without the use of figures. 

 The lens increases in size, while between it and the cornea, the psendo- 

 cornea is formed by an ingrowth of connective tissue. The increasing 

 convexity of the lens causes a space to be formed between it and the 

 retina. The nucleated septal membrane is produced by the flattening 

 and elongation of the connective tissue cells beneath the lens. The 

 fibrous layer becomes more sharply defined, while the small deeply 

 stained bodies (fig. 17 y) disappear. 



The gauglionic cells increase in number, fig. 10, and form a double 

 layer, from the periphery of which severa! small cells subsequently 

 become separated and attach themselves to the ingrowing retinophorae 

 (fig. 10, f/. c. c). In this manner, the two layers of ganglionic cells be- 

 come definitely established. 



The argentea undergoes no great change after the condition 

 represented in fig. 17. The nuclei simply decrease in size until they 

 finally disappear, with the exception of those of the inner layer where, 

 in the adult condition , one or two aborted nuclei may rarely be ob- 

 served. 



With the appearance of the rods, a fourth layer, the vitreous 

 network, is produced, either by a secretion, or transformation of the 

 outer argentea. The viti'eous network , in contrast with its subsequent 

 condition, forms a thick homogeneous and structureless layer (fig. 10, i) . 

 The innumerable, isolated fibres, which even in the earlier stages 

 innervated the eye (fig. 4 and 5), subsequently unite to form a single, 

 loose bündle of nerve fibres, the primitive optic nerve, which 

 later divides into the more sharply circumscribed, axial branches of the 

 definite optic nerve. All the nerve fibres supplying the optic vesicle 

 are not collected to form the optic nerve, for many (fig. 10, :r), terminat- 

 mg in the base of the vesicle , retain their primitive arrangement and 

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

 rods. Mostof these fibres, in'the opposite direction, are turned towardthe 

 Shell side of the stalk, where they become loosely united with each other 

 and connected with the ganglionic swelling of the circumpallial nerve 

 from which the optic nerve originates (fig. 10, cp.n.). Even in the fully 

 grown animals. these nerves, already described, may be seen as rather 



