4 THE PINEAL ORGAN 



The work on the pineal region in the latter part of the nineteenth 

 century and the commencement of the present century was attended by a 

 marked advance in our knowledge of the true nature of the pineal apparatus 

 in all its aspects — geological, zoological, embryological, and phylogenetic. 

 It is to this period that we owe the conception, which was generally held 

 at that time, of the vestigial nature of the pineal organ. It was thought 

 that the " parietal organ " or " pineal eye," which was found to be most 

 highly developed in certain living reptiles, amphibia, and cyclostomes, 

 was the vestige of an unpaired median eye or the persistent member of a 



n ect 



n P-"- 



~-ch.op. 



ect — 



Fig. 2. — Sagittal Sections through the Brain of Acanthias Embryos. (After 



Kupffer.) 



A : 33 mm., showing the open neuropore. B : io mm., showing the cutaneous 

 ectoderm still adherent to the neural ectoderm, at the site of the closed 

 neuropore, which lies in front of the pineal diverticulum, pin. 



ch. op. : optic chiasma. n.ch. : notochord. 



ect. : cutaneous ectoderm. n. ect. : neural ectoderm. 



inf. : infundibulum. np. : neuropore. 



M. : midbrain. Rh. : rhombencephalon. 



pair of median eyes one of which was rudimentary or had completely 

 disappeared. 



Before proceeding to the discussion of the more recent work on the 

 pineal, we shall give a short account of some of the problems which arose 

 during the period 1870-1915, and we shall mention in an approximately 

 chronological order the names of a few of the principal authors, giving at 

 the same time a brief note of the special contribution which each made 

 tc the general knowledge of the subject. 



Among the first of these was Leydig (1872), who discovered the parietal 

 organ in the embryos of Lacerta agilis, L. muralis, and L. vivipara. He 

 described its general position above the interbrain in the region of the 

 third ventricle ; the external appearance of the pineal plate ; the disposi- 

 tion of the pigment ; and the microscopical structure of the vesicle ; but 



