THE PINEAL BODY 21 



brates, it will now be possible to consider in detail some of the 

 variations which the region presents in the different classes. 



1 . The pineal region in cyclostomes 



In cyclostomes the absence of the velum transversum causes 

 the paraphysis to pass over into the dorsal sac without sharp 

 line of demarcation. In fact, it is difficult to make out with 

 any degree of certainty a true paraphyseal process. What 

 there is of a paraphysis is a small evagination from the most 

 cephalic portion of the dorsal sac, and the morphological lines of 

 differentiation are such as to leave it still open to doubt whether 

 there is an actual paraphysis in these forms. Studnicka ('99) 388 

 is authority for the statement that such an organ does exist 

 in Petromyzon. In Ammoccetes the epiphysis is more clearly 

 defined. The lamina supraneuroporica in cyclostomes is more 

 specialized than in other vertebrates. In the most dorsal por- 

 tion of this membrane there occurs a thickening which lodges 

 fibers passing in a transverse direction and constitutes a com- 

 missure known as the commissura pallii. The dorsal sac is un- 

 usually high and deflected in a cephalic direction as a result of 

 the pressure put upon it by the pineal and parapineal organs. 

 Its dorsocaudal wall shows a marked invagination as a result of 

 the pressure not only of the epiphyseal complex, but also of the 

 ganglion habenulae. No chorioid plexus or other vascular for- 

 mation appears in direct connection with either the paraphysis 

 or the dorsal sac. The pars intercalaris anterior is absent, but 

 a very massive commissura habenularis is observed in all forms, 

 making its appearance early in the course of development. 



The epiphyseal complex presents a pineal organ and a para- 

 pineal organ. Both of these lie in close apposition to each other 

 extending cephalodorsad in such a direction that their terminal 

 portions come to overlie the dorsal sac. The dorsal wall of the 

 pineal organ lies immediately beneath the frontal region of the 

 skull. The posterior intercalated portion is also absent, but a 

 large posterior commissure occurs in all forms. The pineal, as 

 well as the parapineal organ, possesses a nerve, that connected 



