164 Journal of Comparative Neurology. 



ever, a question for comparative and developmental study. It 

 has been discussed somewhat by Wilder ('84) and by Stroud. 



MeiencepJial. — This segment may be regarded provision- 

 ally as co-extensive dorsally with the membranous roof, the 

 metatela. In Nectunis this portion of the brain is extremely 

 long, equalling indeed, the length of the entire brain cephalad 

 of it, and accepting the limit here given, is not contained with- 

 in the cranium but extends caudad into the first vertebral 

 arch. The transition in structure to the myel is gradual, as 

 usual. The myelocoele expands gradually to form the meta- 

 coele which widens cephalad to the region of the fifth nerve. 

 On each side of the cerebellum well marked lateral recesses 

 extend cephalad. The analogy of these to the lateral recesses 

 or parepicoeles of higher forms has been suggested, and a com- 

 parison with the similarly situated restiform bodies of sharks 

 and rays may perhaps be made. There is a mesal sulcus in 

 the floor of the metacoele, beginning near the exit of the 9th 

 and becoming most marked in the region of the 5th nerve, 

 cephalad of which point it soon disappears. This and the 

 sulcus at the point of junction of the wall and the floor appear 

 to be the only depressions which might be regarded as of 

 structural importance. There is no sulcus just laterad of the 

 fibre bundle on each side of the meson, such as occurs in many 

 other forms. 



The metaplexus occupies but a portion of the roof of the 

 metencephal, the remainder being a simple tela. It (the 

 metaplexus) is highly pigmented and extends from the region 

 of the tenth nerve to slightly cephalad of the 7th, and laterally, 

 cephalad over the lateral recesses, overlapping slightly the 

 parts to which it is attached. Its relations and the regular ar- 

 rangement of the folds which project into the metacosle can be 

 seen from Figs, i and 6. The remainder of the roof is a 

 simple tela ; no metapore, or foramen of Magendie such as 

 occurs in Diemyctylus (Mrs. Gage '93) was observed. In its 

 caudal portion, however, the metatela was very thin and often 

 ruptured. 



Opposite the exit of the tenth and caudad of the meta- 



