OLFACTORY CENTERS IN TELEOSTS 245 
but rather a gradual plastic movement of the material, such that, 
while the precommissural body and the medial part of the dorsal 
olfactory nuclei remain in the original position, the intervening 
portions of the lateral wall move toward the lateral part, thus 
bringing the dorsal olfactory nucleus and the precommissural 
body into contact at the sulcus limitans. The palaeostriatum 
moves laterad only a short distance, coming to occupy the middle 
of the basal lobe. Buta portion of the dorsal olfactory nucleus 
and the whole of the lateral nucleus move to the extreme ventro- 
lateral margin, carrying the taenia with them, thus forming at the 
‘rostral end of the basal lobe the tuberculum laterale, and at the 
caudal end the nucleus pyriformis. 
The tuberculum anterius, tuberculum laterale, and nucleus 
dorsalis are parts of the undifferentiated secondary olfactory 
nucleus. The precommissural body and pyriform nucleus are 
more highly differentiated parts of the secondary olfactory nu- 
cleus which have developed under the influence of ascending fibers 
of the medial and lateral forebrain tracts respectively. The 
palaeostriatum has become an efferent correlation center rela-: 
tively free from direct olfactory connections. It is interesting 
to note that the termination of the lateral hypothalamic tract 
caudally in the teleosts has brought about the development of the 
nucleus pyriformis at that point, while in the selachians the more 
rostral ending of this connection (tractus pallii) has induced the 
formation of the nucleus olfactorius lateralis and primordium 
hippocampi in a correspondingly different position. 
The selachians exhibit a considerably more highly differentiated 
condition of all of the forebrain centers than is found in the tele- 
osts (ef. Johnston, ’11). The selachian ascending tract from the 
hypothalamus to the primordium hippocampi (tractus pallii), 
in teleosts is probably represented in the tractus hypothalamo- 
olfactorius lateralis, a condition which resembles that of amphi- 
bians (Herrick, ’10, p. 444). 
The nucleus olfactorius dorsalis or primordium hippocampi 
receives some fibers from the tractus olfactorius medialis, and this 
connection is probably the reason why this portion of the undiffer- 
entiated secondary olfactory nucleus retains its dorso-medial 
