MUSEUM OF COMPARATIVE ZOOLOGY. 201 



ble that one of these five eyes in Androctonns is represented by the rudi- 

 mentary eye in Centmrus, although this can be definitely settled only by 

 a careful comparison. 



In the embryo the fibres of the optic nerve (n. opt.) emerge from the 

 base of the retina (PI. IV. fig. 25). This, moreover, is their position 

 throughout the life of the scorpion (PI. III. fig. 18). 



The further changes which affect the form of the optic depressions 

 before they become matured eyes are unessential modifications of the 

 already established plan. At the time of the production of a lens (PI. 

 III. fig. 21) the lentigenous (perineural) cells stretch over from all sides 

 and overtop the retina. The external ends of the lentigenous cells con- 

 tain no pigment (PI. III. fig. 20). 



The basement membrane, from the time wnen the depressions are 

 formed till the eye is completed, covers the modified hypodermis as it 

 covers a simple hypodermal thickening. There is never any indication 

 of a preretinal membrane, nor, from the structure of the eye, should we 

 expect to find one. In aU stages the basement membrane presents the 

 appearance of a single delicate lamella, and at no time is there an addi- 

 tional sheet of mesodermic tissue, as in the median eyei. 



The evidence derived from the anatomy of the adult eye, the auiso^^^o 

 of a preretinal membrane and permanent lentigen, and the continuity of 

 the retina with the hypodermis, together with the facts derived from a 

 study of the development of the eye, show conclusively that in scorpions 

 the retina of the lateral eye is what Lankester and Bourne have called 

 monostichous, and that this retina, unlike that of the median eyes, ia 

 normal, not inverted. 



Theoretic Conclusions. 



The striking similarity in the structure and development of the median 

 eyes in scorpions and the anterior median eyes in spiders has already been 

 indicated. In both cases the retina by a process of involution has be- 

 come inverted. The question whether the retina was functional during 

 the phylogenetic involution of the eye is, as Mark has maintained, an- 

 swered in the affirmative by the phases noted in the development of the 

 optic nerve. At least, the fact that the fibres of the optic nerve are at 

 first attached to the morphologically deep ends of the retinal cells, and 

 only at a later date come to emerge from the opposite end, is most easily 

 explainable on the supposition that the retina was functional before invo- 

 lution. The primitive eye would, then, consist of a single layer of retin"' 



