374 J. B. JOHNSTON 
Fig. 3 Petromyzon dorsatus, ammocoetes. Medial view of a model of the 
left half of the forebrain. 
Fig. 4 Lateral view of the model shown in figure 38. The hemisphere is divided 
by a vertical sulcus into a rostral portion containing the bulbar formation and a 
caudal portion containing the secondary olfactory centers. About opposite the 
caudal pole of the hemisphere appears a vertical sulcus in the wall of the brain 
stem. This corresponds in position to the di-telencephalic boundary line as deter- 
mined by other data and is to be regarded as a di-telencephalic sulcus or fissure. 
