l86 THE PINEAL ORGAN 



parietal eye. The posterior organ (pineal organ or Epiphysis) remains 

 entirely within the cranial cavity, and though as in Sphenodon it may 

 develop a terminal vesicle (pineal sac) (Fig. 252, Chap. 24, p. 369) it is 

 cut off from all access of light and cannot function in any way as a visual 

 organ. The same objection to the use of the terms " anterior organ " 

 and " posterior organ " may be raised on developmental and morpholo- 

 gical grounds as was made in discussing the terms Epiphysis I and 

 Epiphysis II. The two organs in Sphenodon are developed from a 

 single median diverticulum which afterwards is subdivided into two parts, 

 an anterior vesicle which is separated off as a closed vesicle, the parietal 

 organ or pineal eye and a posterior part which consists of the original 

 stalk and an end-vesicle, the pineal sac. The stalk and pineal sac together 

 form the pineal organ or epiphysis. On both developmental and morpho- 

 logical grounds Dendy regarded the anterior part of the diverticulum 

 which becomes detached, to form the parietal sense-organ in Sphenodon 

 as the representative of the primary left pineal eye, and the pineal sac 

 as representing the primary right pineal eye and it would seem that the 

 stalk which is common to both has arisen by approximation and fusion 

 during phylogeny of the bases of a pair of bilaterally placed median eyes. 

 If this interpretation is correct, the parapineal organ does not arise 

 ontogenetically as a separate and distinct outgrowth from the roof of the 

 interbrain, and does not spring from a separate cephalic metamere lying in 

 front of the metamere which gives origin to the pineal organ. The separate 

 anterior diverticulum which arises in front of the pineal diverticulum gives 

 rise to the paraphysis and never develops a terminal sensory vesicle. 



C. The connections of the parietal organ and the parapineal organ 

 with the central nervous system will be described separately with reference 

 to different types of animals. 



6. The Intercalated Segment is a part of the roof plate which lies 

 between the base of the pedicle of the pineal body and the posterior 

 commissure. It is characterized by the absence in it of commissural 

 nerve-fibres and it varies greatly in its extent in different types of animal. 



7. The Posterior Commissure. This lies immediately above the 

 anterior end of the aqueductus cerebri, and receives some fibres from the 

 pineal organ, in addition to its nuclear connections, and the connections 

 with the median longitudinal bundles. It varies greatly in its size and 

 extent in different animals, Figs. 130 and 208, Chap. 22, p. 303 (rabbit 

 embryo). In some it is spread out in a series of bundles, while in others 

 it is compressed into a rounded cord. The fibres which it receives from 

 the parietal organ in Petromyzon are said by Studnicka to divide in a 

 T-shaped manner into branches which go to either side, but he was unable 

 to trace their ultimate destination. 



