470 



THE PINEAL ORGAN 



__^ „-n h op 



~-~-p orb pi. 



relative to the pair of placodes which give origin to the lateral eyes. 

 Thus, Patten assumes that two pairs of retinal placodes which become 

 incorporated in the roof of the third ventricle and give rise to the 

 parietal eyes of vertebrates lie in front of those for the lateral eyes in a 

 position which he describes as typical, in the development of Arachnids 

 (Figs. 257, 258, 259), whereas Locy in his account of two pairs of 



p pi , n h " accessor y " or pineal eyes in 



Acanthias, figures these as 

 lying behind the placodal pits, 

 which will develop into the 

 optic vesicles of the lateral eyes 

 (Fig. 143). Moreover, the 

 intermediate stages between 

 the first appearance of the 

 two pairs of rudiments for the 

 accessory eyes and the out- 

 growth of the pineal diverti- 

 culum in the later stages do not 

 appear to have been definitely 

 established by Locy. The 

 appearance, however, of sym- 

 metically arranged sensory 

 placodes or pigment spots 

 formed in series around the 

 margin of the medullary plate 

 (Figs. 257, 258, 259), or head 

 region (Fig. 19) in inverte- 

 brates suggests the possibility 

 that one pair of a series of 

 simple eyes being more 

 favourably placed for the re- 

 ception of visual impressions 

 than the others — e.g. at the 



Fig. 319. — Dorsal Aspect of Bothriolepis 

 canadensis, showing the nasohypophyseal 

 Opening, Lateral Orbits, Pineal and Post- 

 orbital Plate, and the Cephalic Appen- 

 dages WHICH HAVE RECENTLY BEEN SHOWN TO 



be True Pectoral Fins. (After Patten.) 



n.h. op. : naso-hypophyseal opening. 



orb. : orbital cavity. 



p.f. : pectoral fin. 



p. orb. pi. : postorbital plate. 



p. pi. : pineal plate. 



antero-lateral margins of the 

 head on each side — becomes more highly evolved than those in front of 

 or behind this pair The more favourably situated pair, it may be assumed, 

 gains the ascendancy over the others and becomes the principal pair, 

 whereas the less favourably situated ocelli retain their primitive simple 

 character and tend to degenerate. If this is the case, and if as is 

 commonly believed both the lateral and median eyes of vertebrates have 

 been evolved from the simple eyes of a lowly organized type of inverte- 

 brate, the discrepancy which exists with regard to the position of the 



