RELATION OF MEDIAN TO LATERAL EYES 



373 



ends of which the nerve-fibres emerge. Admitting that in the ancestral 

 eye the rhabdomes were in their usual position, namely, at the outer end 

 of each retinal cell, an inversion of the retina would not only place the 

 optic fibres on the front face of the retina, but the rhabdomeres would 

 come to occupy the deep ends of the cells. The prenuclear rhabdomeres 

 of the median eyes in scorpions must then be secondary structures, 

 developed in such a way as to replace functionally the older post-nuclear 

 structures. The sphaeospheres, as Mark has suggested, may represent 

 the remains of post-nuclear rhabdomeres. These are then to be regarded 

 as disappearing, and the fact that in some species of scorpions they are 

 present while in others they are absent, would favour this view." 



' The possible relation of the median eyes to the lateral eyes in 

 scorpions has already suggested itself, for in pointing out the probable 



Fig. 255. — Section through One of the Developing Lateral Eyes of a 

 Spider, Agelena. (After Kishinouye.) 



The retina is upright, and the nerve-fibres issue from the base of the retinal 

 papilla. 



n.f : nerve-fibres. r. oc. I. : retina of lateral eye. 



nature of the phylogenetic antecedent of the median eyes a condition 

 has been implied which agrees with the essential features of the lateral 

 eyes. Of all the eyes in spiders and scorpions, the lateral eyes in scorpions 

 are undoubtedly the least complicated, and they may be looked upon as 

 deviating least from the ancestral type." 



These considerations, taken in connection with papers published by 

 Mark (1887) and Patten (1889) on the mode of development of the eyes 

 of Apus, Branchipus, spiders, and scorpions, provide a clear explanation 

 of the means by which a pre-retinal cellular and cuticular lens are formed 

 in continuity with a retinal and post-retinal layer, by a simple folding of 

 the hypodermis in such a way that the developing eye appears in transverse 

 section as an S-shaped bend (Fig. 254, A : compare Fig. 255, which 

 shows the position of the nerve-fibres in the upright lateral eyes of the 

 spider Agelena). The middle segment of the bend is inverted and becomes 

 the retina, and when the visual cells become differentiated their outer 



