RELATION OF MEDIAN TO LATERAL EYES 375 



2. " The parietal eye : there are probably two pairs of ocellar placodes 

 that for a short time occupy this marginal position (Fig. 257). Later 

 they are caught in the pallial overgrowth and carried on the inner limb 

 of the closing neural crests to the median line. There they form a group 

 of one, two, or three placodes lying in the membranous roof of the brain. 

 During or after the closing of the cerebral vesicle the brain roof is evagi- 

 nated at the place where the ocelli are located, thus forming a sac or 



sens 



vitrch. 



Fig. 256. — Section through a Developing Lateral Eye of Ammoccetes, 

 showing the inversion of the optic cup ; and the lens vesicle, super- 

 FICIAL to which is a Thick Layer of Mesenchyme. (After Beard.) 



/. ves. : lens vesicle. sens. l.r. : sensory layer of retina. 



mes. : mesodermal layer of cornea. vitr. ch. : vitreous chamber. 



pig. l.r. : pigment layer of retina. 



tube in the blind end of which the ocellar placodes lie. The development 

 is essentially like that of the parietal eye of Limulus and the scorpion. 

 This fact demonstrates that the parietal eye of crustaceans and arachnids 

 is a true cerebral eye in the vertebrate sense, and is identical with the eye 

 of vertebrates. 



3. " The lateral eyes of vertebrates : these represent the compound or 

 convex eyes of arthropods that have been transferred to the interior of 

 the cerebral vesicle. In the arthropods the lateral eyes lie near the margin 

 of the cephalic lobes on the outer edge of a deep ganglionic infolding. 



