THE NERVOUS SYSTEM. 455 



(Fig. 402) extending from this fold to the optic recess indicates the location of a 

 fold in the side walls in some forms and is taken by some as the boundary be- 

 tween two subdivisions of the fore-brain, the end-brain or telenccphalon and the 

 inter-brain or diencephalon. Cranial to the epiphysis proper, is a commissure 

 in the dorsal wall (commissura habemdaris^ connecting two structures which 

 develop in the crests of the side walls, the ganglia habenula. 



From the dorsal part of the telencephalon is developed the pallium. The 

 ventral anterior part evaginates toward the olfactory pit, its end receiving the 

 olfactory fibers. This region is often termed the rhinencefhalon. Thickenings 

 of the basal lateral walls of the telencephalon form \h^cozpora striata. 



FIG. 402. Scheme of a median sagittal section through a vertebrate brain showing 



the five-fold division of the brain, von Kupfler. 



T., Telencephalon; D. : diencephalon; M., mesencephalon; Mt., metencephalon; Ml., myelence- 

 phalon; c. y cerebellum; cc., cerebellar commissure; ch., habenular commissure; cp., posterior 

 commissure; cw., chiasma eminence; e., epiphysis; e 1 ., paraphysis; J., infundibulum; //., 

 lamina terminalis; />., processusneuroporicus; pr., rhombo-mesencephalicfold; pv., ventral 

 cephalic fold; ro. } recessus (prae-) opticus; si., sulcus intraencephalicus posterior; tp., tuber- 

 culum posterius. The lines aa., dd and ft indicate the boundaries between four divisions. 



The roof of the mesencephalon finally develops the "optic lobes" The 

 thickened part of the roof lying immediately caudal to the rhombo-mesen- 

 cephalic fold develops into the cerebellum. The part of the tube of which this 

 forms the roof is often called the hind-brain or metencephalon, while the rest of the 

 rhombencephalon is then termed the after-brain or myelencephalon. The roof of 

 this portion, which has become very thin in the course of its development, forms 

 the epithelial part of the tela chorioidea of the fourth ventricle. The con- 

 stricted portion of the tube between the rhombic brain and mid-brain is the 

 isthmus. 



The above subdivisions of the three primary expansions into five parts 

 (end-, inter-, mid-, hind- and after-brains), especially the subdivisions of the 

 rhombic brain, do not have the morphological value of the three primary 



