1884.] NATURAL SCIENCES OF PHILADELPHIA. 2*ll 



small and spherical, and folding over, form a single-layered much 

 flattened sac, the lumen of which retains its connection with the 

 diacoelia by a narrow slit. This is the only adult trace of the 

 processus pinealis in Menopoma. In Rana (fig. 5) I find the 

 same elongation of the ependyma cells, and similar cells forming 

 the processus, but in a double row. Here the supracommissura 

 is much smaller, and more widely separated from the postcom- 

 missura, this interval is bridged b}'^ a delicate single row of cells 

 which appear to turn up and form the anterior border of the re- 

 cessus, although this point is not very clear. There is also some 

 doubt whether the lumen of the processus retains its communi- 

 cation with the diacoelia. The processus itself is a long, flattened, 

 two-layered sac, circular in section, extending anteriorly so as to 

 overlap the supracommissura. The pia mater overlaps the pro- 

 cessus upon all sides, indicating that it primitively was directed 

 upwards. Extending from above the postcommissura, forwards 

 to the base of the epiphysial stalk, are numerous fibres, which 

 appear to enter into relations with the cells of the stalk. In 

 Menobranchus and Amphiuma we find a nearer approach to the 

 frog than to the Menopoma condition, the processus forming an 

 elongated flattened sac, completely constricted off from the brain 

 cavity. 



The Flexi choroidei. — There is a singularly simple and beau- 

 tiful display of the relations of the intra-ventricular blood-vessels 

 in the brain of Menopoma (fig. 4). The thrusting in of the 

 ependj^ma extends from the supracommissura to the upper por- 

 tion of the terma. The arterial supply is apparently derived 

 from the median arteria carotis cerebralis, and the venous return 

 is at the sides of the supraplexus. The division into supra-, dia- 

 and proplexus is a somewhat artificial one here, but is not so 

 when applied to the Amphiuma brain, where the supraplexus 

 is very prominent, and the diaplexus extends well back into 

 the Mesencephalon. The lateral wings of the diaplexus are shown 

 passing through the porta in fig. 9. The nature of the ependyma 

 cell-lining of these vessels is very constant ; small and large, the 

 cells have the same elongated, bead-like appearance. 



The Encephalic Segments. — Stieda,^ following general usage, 

 considers that portion of the median brain-floor lying behind the 



Loc. cit. 



