MUSEUM OF COMPARATIVE ZOOLOGY. 201 
ble that one of these five eyes in Androctonus is represented by the rudi- 
mentary eye in Centrurus, 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 (Pl. 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 (Pl. III. fig. 20). 
The basement membrane, from the time when 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 all 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 eyes. 
The evidence derived from the anatomy of the adult eye, the absence 
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, is 
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 retinal 
