Humphrey, Brain of the Snapping Turtle. 105 



into the cephalic portion of the diacoele, reaching in the adult 

 specimens far into the infundibulum. As this plexus, though 

 in the diaccele, is not the homologue of the diaplexus of Amphibia 

 I propose for it the name of infundibular plexus. In adult spec- 

 imens the folds of the plexuses were found united by the thick- 

 ening of the pia layers as is shown in Fig. 17 for the metaplex- 

 us. 



Crista. — Wilder (91) first called attention to this structure 

 in the brain of the cat where it projects into the aula as a semi- 

 oval elevation of a peculiar pellucid, gelatinous appearance in 

 the fresh brain. He has also observed the same in the brain of 

 the sheep and the human fetus. He suggests the possibility of 

 its marking the limit of the primitive terma. No one else seems 

 to have obsrved it as such until Mrs. Gage announced its pres- 

 ence in the Diemyctylus and Amia. In these animals it is small 

 but well marked. In Desmopiat/ius, Fish has found it to be 

 simply membranous. In the turtle it is present and shows an 

 interesting variation in forms that are in general structure very 

 similar. Its location is on the meson projecting into the aula 

 immediately dorsad of the callosum (Figs. 6 and 33). In Chel- 

 ydra it consists simply of an invagination of the membrane as 

 it spans across from one hemisphere to the other. This condi- 

 tion was found in both embryos and adults. In Chtysemys ex- 

 actly the same formation was found. In Cliclone it presents a 

 very important modification. The whole structure is well de- 

 veloped and appears to the naked eye as a pyriform mass with 

 the stem projecting dorsad (Fig. 33). This structure is best stud- 

 ied in a series of frontal sections. Figure 34 shows a section 

 through the stem of this pear shaped body. Here the structure 

 consists mainly of endyma and pia. The endymal cells are pe- 

 culiarly modified, being very narrow and having proceses that 

 project toward the pia and are so closely interwoven with the 

 pial processes that they seem continuous and any clear demar- 

 cation between the two membranes is impossible. At the sides 

 of the crista where they join the hemispheres the walls are made 

 up of endyma and a layer of neuroglia. Nowhere does the pia 

 seem to lie close to the walls of the structure except at its ex- 



