20 FREDERICK TILNEY AND LUTHER F. WARREN 



portion, and a peduncle. In all probability the proximal por- 

 tion of the epiphyseal complex gives rise to the epiphysis cerebri 

 or what has been called the pineal gland. In some forms nerve 

 fibers have been found making their course through the stalk of 

 this pineal organ and have thus given rise to the term nervus 

 pinealis. The parapineal organ is the second, though less con- 

 stant, portion of the epiphyseal complex. When present, it also 

 consists of an end-vesicle, a stalk, and a somewhat dilated 

 proximal portion. Most of these evaginations contain cavities 

 which are in communication with the third ventricle. The 

 recess which connects the pineal organ with this ventricle is 

 known as the recessus pinealis. 



The pars intercalaris posterior. The caudal wall of the proximal 

 portion of the pineal organ often shows a marked increase in 

 thickness as it approaches the level of the diencephalic roof. 

 This thickening interposes an area between the proximal portion 

 of the pineal organ and the posterior commissure. Often this 

 intercalated part shows considerable dimensions. In the forms 

 in which it is most developed, the fibers of the pineal nerve may 

 be seen to enter this intercalated portion in the roof of the inter- 

 brain. It has been called the pars intercalaris by Burckhardt 

 in 1890, 42 but the necessity of designating it the posterior inter- 

 calated portion becomes obvious in view of the fact that an 

 anterior structure of like character has already been described. 



The posterior commissure. The last and caudalmost struc- 

 ture in the roof of the interbrain is the posterior commissure. 

 This has already been assigned by Minot in 1901 277 to the mid- 

 brain, but the fact that certain fibers from the tractus pinealis 

 and the nervus pinealis may be traced into direct relation with 

 this commissure seems to ally it more with the derivatives of the 

 roof-plate in the interbrain region rather than that of the mesen- 

 cephalon. 



The homology of all of these parts in the roof-plate of the 

 prosencephalon has been given for the different classes of verte- 

 brates by Burckhardt in 1890 42 in his work on Protopterus and 

 again in his work (45) on the structural plan of the brain. With 

 this view of the generalized plan of the pineal region in verte- 



