292 THE PINEAL ORGAN 



The first indication of the development of the pineal organ in the 

 chick is a thickening of the epithelium forming the roof of the third 

 ventricle, which soon grows forward in the form of a hollow diverticulum. 

 It rises a short distance in front of the posterior commissure and is approxi- 

 mately mesial in position. Rarely two epiphysial outgrowths have been 

 observed. Thus in 1897 Saint Remy found on each side of the neural 

 tube, before this had become closed, a small evagination of the epithelium 

 in the pineal region. Hill in 1900 observed two small diverticula close 

 to each other in the same region of a chick embryo in which the brain 

 was already closed. Cameron in 1903 saw similar outgrowths in chick 

 embryos, which he considered to be an indication of the primary bilateral 

 origin of the pineal organ. It is possible that some of these cases may be 

 explained on the assumption that they are simply due to an abnormal 

 development, but their occurrence in birds, considered along with the 

 evidence of similar indications of the bilateral development of the pineal 

 organs in other classes of vertebrates, does lead one to think that the 

 pineal, like other sense-organs, is a bilateral structure. Thus important 

 evidence of its bilateral origin has been found in cyclostomes, Dendy ; 

 in Selachian and Teleostean fishes (Locy, Hill) ; in Amia (Kingsbury) ; 

 besides important embryological and geological evidence in Amphibia 

 and reptiles. 



The pineal diverticulum at the fourth day in the chick embryo is a 

 conspicuous hollow, club-shaped organ (Fig. 200). Its wall consists 

 of an, as yet, undifferentiated ependymal layer, which is continuous at 

 its base with the ependyma forming the roof of the brain. Capillary 

 vessels lie in the receding angle between it and the roof of the brain. The 

 superior commissure has not yet appeared. The posterior commissure, 

 on the other hand, is well-developed and forms a broad band some dis- 

 tance behind the pineal outgrowth. A low post- velar arch and velum are 

 already recognisable. There is no evidence of any connection with the 

 cutaneous ectoderm, a fairly thick stratum of loose mesenchyme lying 

 between the diverticulum and the epidermis. Its distal end already 

 shows a slight indication or lobulation, which increases as age advances 

 and is most marked on its anterior surface and sides. 



In an 8-day chick embryo, Fig. 196, A and B, the pineal organ, which 

 is still directed forward over the superficial aspect of the dorsal sac, 

 is seen to have budded out a number of hollow vesicles, lined by ependyma 

 (Fig. 197), and separated from each other by a loose mesenchyme. The 

 cavities of the vesicles are at first continuous with the central lumen or 

 with branches which are given off from this, but already some are con- 

 stricted off and form closed, independent vesicles which are surrounded 

 on all sides by mesoderm. The mesoderm at this stage has condensed 



