268 PROCEEDINGS OF THE ACADEMY OP [1884. 



the porta, or passage from the single to the lateral cavities of the 

 Prosencephalon. In Menopoma, however, the posterior division 

 is immediately below the anterior, and it is found in the horizontal 

 sections to be not a true commissural, but a decussational system. 

 At this point, a large number of the fibres composing each of the 

 longitudinal tracts, just mentioned in connection with the optic 

 chiasma, cross each other and pass to or from the base of the 

 opposite hemisphere. In Menohranchus these two tracts are 

 completely separated, the upper division passing independently 

 across the ventricle. 



The Postcomviissura. — Although this commissure is part of a 

 conspicuous fold of the brain-roof separating the Dien- from the 

 Mesencephalon, it really contains in the Amphibia but few fibres. 

 Another interesting fact is that these fibres do not enter into the 

 thalami, but pass obliquely backwards into the region of the 

 longitudinal tracts composing the pars peduncularis of the Mesen- 

 cephalon. This accords with Mihalkovics' ^ observations upon the 

 chick, and tends to confirm Pawlowsky's ^ view that this is not a 

 commissure in the strict application of the word, but is rather a 

 side connection of the longitudinal fibre system. This view 

 accords also with Ahlborn's recent observations upon the 

 lamprey. 



TJie Supracommissura (fig. 8). — In the forward portion of the 

 roof of the diacoelia, and immediately above the optic chiasma is 

 a commissure, which, as far as I can ascertain, has been hereto- 

 fore entirely overlooked in the Amphibia. In Menopoma and 

 AmpMuma it is very large ; in the frog it is much reduced, and 

 lies further forward ; in Menohranchus it is represented by a 

 slender band of fibres immediately in front of the recessus 

 pinealis. In all these forms it lies in front of the epiphysial 

 process, and completely separates this tube from the dia- and 

 supraplexus. It occupies the same relative position as the 

 variously named Commissura habenarum,^ or the commissure of 

 the pineal stalk (Mihalkovics)^ of the mammalian brain, as well 



1 Loe. cit., p. 73. 



2 Pawlowsky, Ueber den Faserverlauf in der hinteren Gehimcommissur. 

 Zeits. fiir wiss. Zool., Band xxiv, 1874, 



3 Wilder, Anatomical Technology, 1882, p. 463. 



* Loc. cit., p. 100. This comparison is sonae^hat doubtful. 



