BRAIN. 



141 



mid-brain and hind-brain, the constriction between mid- and hind- 

 brains being the isthmus. In the sides, as in the cord, two zones 

 may be recognized, dorsal and ventral, separated internally by a 

 groove, the sulcus of Monro, which lies at about the middle of the 

 tube. At the extreme anterior end a small region, the optic recess, 

 is' wedged in between the two zones on either side, the end of the tube 



FIG. 146. Diagrams of (i) primitive brain. (2) an intermediate stage, ancT(3) with 

 the definitive parts. (Compare 3 with fig. 147). AQ, aqueduct; AC, anterior commissure; 

 C, cerebral region; CB, cerebellum; CS, corpus striaturn; HC, habenular commissure; 

 I, infundibulum; LT, lamina terminalis; MO, medulla oblongata; O, olfactory region; 

 P, epiphysial region; PC, posterior commissure; RO, optic recess; ST, subthalamica; T, 

 tegmentum; TH, thalamus. Dorsal zone plain, ventral zone dotted. 



just above the recesses being the lamina terminalis. The most 

 marked modifications in converting the primitive into the adult brain 

 take place in the dorsal zone. 



In the fore-brain the anterior part of the dorsal zone on either side 

 forms an outgrowth which rapidly increases hi size, the two eventually 

 forming a pair of hollow vesicles, the cerebral hemispheres (telen- 

 cephalon, prosencephalon) which extend far beyond the lamina 

 terminalis. In the wall of each hemisphere may be recognized a basal 

 ganglionic portion, the corpus striatum, while the rest of the wall 

 is the pallium or mantle. An olfactory lobe (rhinencephalon) 

 grows out from the lower anterior part of each hemisphere to meet 



