MUSEUM OF COMPARATIVE ZOOLOGY. 195 



apparently connecting with the external ends of the retinal cells. kX 

 least the fihres disappear here, and cannot be traced into the retina. The 

 optic nerve (PI. III. fig. 16, n. opt.) at this stage emerges from the retina 

 by passing over the rim of the optic cup in a region corresponding to the 

 outer edge of the pocket. The region extends from the dorsal margin 

 half-way down toward the ventral margin of the cup. 



During the further development there is but little change in the 

 point of exit for the optic nerve. It simply shifts from its posterior 

 lateral position in the embryo, to a posterior ventral one in the adult. 

 The change in the course of the intracapsular fibres is much more sig- 

 nificant. 



There is reason to believe that in the embryo the nerve fibres are at- 

 tached to the external ends of the retinal cells (Figs. 14-17). In the adult 

 they certainly emerge from the deep ends of these cells. The steps which 

 connect the earliest with the final condition consist of a migration of the 

 point of attachment for the nerve fibre from the external end of the cell 

 to the deep end. The migration of the fibres takes place at the same 

 time that the nuclei recede into the deeper parts of the eye, and seems to 

 be controlled by the same influence, namely, the growth of the rhabdo- 

 meres. An analogous condition in the eyes of Agelena has been described 

 by Mark ('87, pp. 84-87). There is, however, a difference ; the nerve 

 fibres in Agelena never come to have a post-nuclear attachment to the 

 retinal cell, whereas in the figures of Graber and those of Lankester and 

 Bourne, and certainly in the retina of Centrurus, the nerve fibres emerge 

 from the cell behind the nucleus. Mark ('87, pp. 91, 92) has claimed 

 for thsse facts an important significance, and concludes that they point 

 to a functional condition of the retina before involution. The bearing of 

 this will be further considered under the head of theoretic conclusions. 



The Lateral Eyes. 



The lateral eyes in scorpions, although in some respects more inter- 

 esting than the median eyes, have on the whole received less attention. 

 Grenacher in his two papers previously quoted makes no mention of 

 them ; for our present knowledge of their structure we are indebted to 

 the researches of Graber ('79) and of Lankester and Bourne ('83). The 

 results of these inquiries are in so far unsatisfactory that in several es- 

 sential points they are directly opposed to each other. The points upon 

 which there is a conflict of opinion are (1) the origin of the retina, 

 and (2) the presence or absence of a lentigen. 



