MORPHOLOGY OF THE FOREBRAIN 417 



the hemisphere, both of the latter being relatively small. The 

 medial olfactory tract divides into a dorsal part passing into the 

 massive dorso-medial wall and a ventral part for the medial wall 

 below the septum ependymale. The former joins the fimbria 

 complex. The lateral olfactory tract is also divided into dor- 

 sal and ventral parts, the former {tr. olf. lat.) running separately 

 to the pars dorso-lateralis of the hemisphere and the ventral being 

 confused with the lateral forebrain tract and ending in the pars 

 ventro-lateralis. The tract marked tr. o. is a mixture of the 

 ventral parts of both the medial and lateral olfactory tracts and 

 the medial and lateral components of the basal forebrain bundles, 

 all of which are separate in the frog. 



Midway of the hemisphere the septum ependymale is inter- 

 rupted by the interventricular foramen and from this point cau- 

 dad the ventriculus lateralis communicates widely with the ven- 

 triculus impar of the forebrain (fig. 3) The wide membranous 

 roof over the ventriculus impar is a choroid plexus which is ex- 

 tended laterally to form the plexus lateralis Farther caudad this 

 membrane is evaginated dorsally to form the paraphysis and back- 

 ward into the third ventricle as velum transversum and dienceph- 

 alic plexus, both of which at this age are very small. The line of 

 contact of the roof membrane with the massive dorso-medial wall 

 of the hemisphere is the taenia fornicis. The caudal border of 

 this membrane is attached to the thalamus along the taenia 

 thalami, where for a short distance the taenia fornicis and the 

 taenia thalami come together (fig. 4). At the line of contact of 

 the hemisphere and the thalamus there is a membrane stretching 

 across from the taenia fornicis et thalami of one side to the other. 

 This is the locus of the velum transversum (fig. 4). Passing for- 

 ward from this point are three separate membranes: (1) the 

 forebrain roof and the plexus lateralis already described; (2) the 

 paraphysis (figs. 2 and 3); (3) the plexus chorioideus ventriculi 

 tertii or diencephalic plexus. For further details regarding these 

 membranes I refer to the excellent account of Necturus by War- 

 ren ('05). 



At the levels of figs. 2 and 3 the pars ventro-medialis is very 

 small with no recognizable pre-commissural body (nucleus media- 



