376 THE PINEAL ORGAN 



In invertebrates they are seen in a very similar position on the lateral margin 

 of the open medullary plate. Later they are swept into the infolding 

 brain, turning the retinas inside out. They then grow out laterally on the 

 end of membranous tubes in much the same manner as the median eyes. 

 In arthropods the lateral eyes usually have a crescentic or kidney-shaped 

 outline ; in vertebrates this shape is retained, giving the retinas their 

 characteristic crescentic outline during the early stages. When the two 

 limbs of the crescent unite a circular retina is produced, giving rise to 

 the choroid fissure and the centrally located optic nerve, that, together 

 with the inverted rods and cones, have long been such inexplicable 

 features of the lateral eyes of vertebrates. 



-i-:? — -jr — .in.cr 



Fig. 257. — Embryo Spider. (After Patten.) 



Showing the primary position of the olfactory placodes, olf., the parietal eye 

 placodes, pa.e 1 , and pa.e 2 , and the lateral eye placodes, le. These are situated on 

 the edge of the neural crest, n. cr. 



ch. : cheliceral ganglion. op. 1. : optic lobe. 



le. g. : lateral eye ganglion. ro. : rostrum. 



m. ch. : median chord. st. : stomodceum. 

 ol. I. : olfactory lobe. 



4. " The parietal eye : all vertebrates possess remnants more or less 

 distinct of a median or parietal eye, which in some forms contains true 

 retinal cells and visual rods, and is connected by several (? four) distinct 

 nerves with as many ganglia. 



5. " There is but one median or parietal eye, consisting, however, 

 of several parts. 



6. " 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.' 



