BRAIN. I4I 
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 
Fic. 146.—Diagrams of (1) primitive brain. (2) an intermediate stage, and (3) with 
the definitive parts. (Compare 3 with fig. 147). AQ, aqueduct; AC, anterior commissure; 
C, cerebral region; CB, cerebellum; CS, corpus striatum; 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. 
Tn the fore-brain the anterior part of the dorsal zone on either side 
forms an outgrowth which rapidly increases in 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 
