580 THE DUCTLESS GLANDS EXDOCEIX GLANDS 



epiphysis has been known to anatomists as a probable gland since the 

 time of Galenus (200). It has excited interest and been the subject 

 of. much speculation for centuries. Descartes (1649) regarded it as 

 the seat of the 'soul.' Notwithstanding much study, it is still but im- 

 perfectly understood, and even its histological makeup is to some extent 

 in dispute. 



Development. The epiphysis cerebri arises as an ependymal diver- 

 ticulum from the roof of the diencephalon. The apical cells proliferate 

 and undergo differentiation into neuroglia and interneuroglia cells. The 

 latter may perhaps assume secretory activities, but no convincing cyto- 

 logical evidence appears indicative of such function apart from numerous 

 melanic granules, which, however, are present in all of the cells in early 

 stages of development and certain lipoid granules and spherules. In the 

 sheep numerous cysts, lined by tall columnar (ependymal) cells appear at 

 half term (21 cm. stage). The cysts contain a coagulum. The cysts pro- 

 gressively disappear, apparently through pressure from proliferating cells 

 without, only a few persisting to birth and occasionally "after. In the sheep 

 the cysts arise through accumulations of fluid, compelling a cellular 

 arrangement simulating acini, not as tubular outgrowths from the original 

 lumen, as is apparently the case in birds. Both melanic. granules and 

 alveoli (cysts) are more probably to be interpreted as of ancestral signifi- 

 cance. As far as is known an epiphysis is present in all vertebrates, with 

 the exception of Myxinoides and Crocodilia. No trace of an epiphysis is 

 said to appear even in embryos of Crocodilia. The pineal eye of certain 

 reptiles is commonly regarded as the homologue of the pineal body of 

 mammals. However, in the lower groups of animals the true homologue is 

 generally a double structure, the pineal eye developing terminally on a 

 secondary anterior evagination from the base of the primary one. This 

 secondary process is not developed in mammals. In Hatteria (a Xew 

 Zealand 'lizard') the pineal eye comes to the surface in the middle of the 

 head, and consists of an optic cup with a lens covered by transparent epi- 

 dermal scales, forming a cornea. It is believed to function only as a light 

 or warmth perceptive organ. An additional structure, the parapliysis, 

 found in certain lower forms and in the marsupialia, anterior to the pineal 

 body, and arising similarly as an ependymal evagination, is commonly 

 regarded as a choroid plexus, evaginated instead of being invaginated, as 

 is usually the case in mammals. In sheep the pineal undergoes its greatest 

 development during the first year of life, approximately fivefold. 



The pineal body is attached to the posterior portion of the roof 

 of the third ventricle by means of the pineal stalk. This contains the 

 pineal recess, a cavity continuous with the ventricle. The anterior wall 



