RELATION OF MEDIAN TO LATERAL EYES 



377 



7. " 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. 



8. " They were ultimately forced into or carried into the brain 

 chamber by the same forces which produced the brain infolding. The 

 placodes are carried on the crest of the brain, infolding towards the 

 median line, meanwhile shifting from the outer to the inner limb of the 

 fold (Fig. 246, p. 349). 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 (Figs. 247^ 



st- 



abd.op 



Fig. 258. — Embryo Spider seen from the Side, showing the Position of the 

 Placodes which will give rise to the Median and the Lateral Eyes. 

 (After Patten.) 



abd. ap. : 2, 3, 4 : abdominal appendages. 



an. pi. : anal plate. 



ch. : chelicera. 



c. nv. : cephalic navel (do. dorsal organ). 



co. : commissure. 



ht. : heart. 



/. 1, 2, 3, 4 : legs. 

 Lb.: lung book. 

 I.e.: lateral eyes. 

 m. e. : median eyes. 

 ped. : pedipalp. 

 st. : stomodseum. 



p. 350, and 248, p. 360). This vesicle with its four placodes is the parietal 

 eye. 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 ' ectoparietal eye,' containing the two most highly 

 developed placodes, and an inner posterior one or ' entoparietal ' eye con- 

 taining the remaining two placodes completely united into one organ and 

 with greatly reduced structural details. 



9. " The membranous tube or epiphysis may disappear in whole or 

 in part, leaving the terminal eye-sacs either isolated or united by distinct 

 nerves with the parietal eye-ganglia or habenular ganglia (Fig. 250, p. 362). 



10. " The parietal eye of vertebrates is homologous with the parietal 



