PARIETAL EYE. 147 



In the scorpion and other arachnids (Limulus, Galeodes and Phalangium) 

 a transformation takes place in the arrangement of the retinal cells, shortly after 

 the eye assumes its definite form. In the scorpion, owing to the method of in- 

 folding, the retinal cells are inverted, the nerves being distributed over the outer 

 surface of the sac, and the rods turned toward the lumen of the vesicle. Later, 

 however, the nerves, entering from the side, appear to penetrate the retina about 

 midway between the inner and outer surfaces. In the adult, the rods are located 

 on the sides of the cells, near their outer ends, and the nerves then enter the oppo- 

 site, or inner end. Just how this apparent, or actual, reversal of the retinal 

 cells takes place, I have not been able to determine. 



In the scorpion, Limulus and Phalangium, the rods lie in isolated groups, on 

 the sides of the cells, just below the outer surface of the retina. But in the parietal 

 eye of Galeodes and of spiders, where the same method of development prevails, 

 the rods form in the adult a continuous layer outside the retinal cells, and there 

 is no indication as to what was the nature of the of the post-embryonic trans- 

 formation that brought the rods and nerve ends into that position. 



I. Summary. 



We may summarize our conclusions in regard to the parietal eye as follows: 



1. All vertebrates possess remnants, more or less distinct, of a median or parie- 

 tal eye which in some forms contains true retinal cells and visual rods, and is 

 connected by several (4 ?) distinct nerves with as many ganglia. 



2. There is but one median or parietal eye consisting, however, of several 

 parts. 



3. The eye proper consists of three or four sensory placodes, each one 

 representing the retina of a simple ocellus of the arthropod type. The placodes 

 form the walls of a sac on the end of a membranous tube projecting from the 

 roof of the tween-brain. 



4. The placodes have a paired arrangement and probably represent two pairs 

 of ocelli, located originally in the ectoderm, just outside the lateral margins of 

 the open medullary plate. 



5. They were ultimately forced into, or carried into, the brain chamber by the 

 same forces that produced the brain infolding. The placodes are carried on the 

 crest of the brain infolding toward the median line, meantime shifting from the 

 outer, to the inner, limb of the fold. When the crests unite, the four placodes 

 form a compact group on the membranous roof of the brain. At that point a 

 tubular outgrowth of varying length is formed which has a vesicle or dilatation at 

 its distal end, in the walls of which the placodes lie. This vesicle with its four 

 placodes is the parietal eye. 



6. The primary vesicle may now be constricted, forming two unpaired lobes, 

 or the lobes may separate, forming two separate sacs, a larger, anterior and outer 

 one, the ecto-parietal eye, containing the two most highly developed placodes, 



