66 ARACHNIDA. 



of the nuclei. The whole of this thickened layer represents the 

 retina, while the thin lower wall of the invagination (pr), the so- 

 called post-retinal layer, perhaps yields the posterior enveloping layer 

 of the eye (Locy). A displacement of the nerve, in relation to the 

 retinal elements, must take place here, as in the eye of Scorpio, 

 since the former, in the adult eye, is connected with the posterior 

 ends of the retinal cells (Fig. 10 A and 0, p. 14, and Fig. 34 A). 



The posterior median eyes and the two pairs of lateral eyes 

 (accessory eyes) also arise through infoldings of the ectoderm, which 

 are directed posteriorly, and become applied to the ectoderm (hypo- 

 dermis). The lens is then secreted above the infolding (Locy, Mark). 

 Up to this point the formation of these eyes appears to agree with 

 that of the anterior median eyes. 



In the anterior lateral eyes the invagination is, according to Mark, directed 

 anteriorly. Perhaps this fact accounts for the number of the optic folds as 

 described by Claparede, the anterior eyes arising separately, while the posterior 

 eyes originate from a common fold. This is, indeed, merely a conjecture, which 

 we are led to make by the difficulty of gaining, from the statements before us, 

 any exact idea of this important process. 



The development of the posterior median eyes and that of the 

 lateral eyes further differ in that the formation of the rods occurs 

 not in the outer, but in the inner parts of the retinal cells. The 

 nuclei take up an external position, and the cells lengthen in 

 the direction of the cavity of the optic vesicle, and secrete the 

 rods on their inner ends. In consequence of this, the rods here 

 lie behind the nuclei, a position similar to that found in the eye 

 of the adult (Fig. 34 B). 



"While the outer wall of the invagination is transformed into the 

 retina, as in the anterior median eyes, the inner part seems to 

 undergo another process of folding (Mark). The new fold extends 

 into the original cavity of the first invagination, and represents the 

 rudiment of the tapetum. Its cells secrete later the small crystals 

 which cause the glitter of the adult eye. The post-retinal layer 

 behind the tapetum-fold here also bounds the eye posteriorly (Mark). 



According to Locy, the tapetum owes its origin to a continuation of the 

 outer chitinous covering of the body into the primitive eye-fold. But since 

 the tapetum is of a cellular nature, as was shown by Bertkau's researches 

 (No. 52), this does not appear very probable. Another view concerning the 

 origin of the tapetum, which has been repeatedly suggested, is that it consists 

 of migratory mesoderm-cells laden with a silvery pigment. 



A displacement of the nerves and of the retinal elements need 

 perhaps not be assumed in these eyes, for, according to Bertkau, 



