FOREBRAIN MORPHOLOGY 2399 
SELACHIANS 
In selachians the inversion of the forebrain is much more ad- 
vanced than in Petromyzon, the inverted parts reaching each 
other in the medial plane and there being bent down and grown 
together to form the short dorsal septum hanging down between 
the lateral ventricles in the region of the foramen monroi. The 
evagination also has a different character than in Petromyzon. 
The lateral ventricle is evaginated into the olfactory bulb, 
which has a lateral situation, but to this there is added an evag- 
ination in the rostral direction which causes a ventricular 
pouch medial to the olfactory crus. In consequence of the latter 
evagination, brain parts, occupying in other vertebrates a medial 
position in the septum, should be brought forward and lateralward 
to cover the topographically foremost part of the brain. Mor- 
phologically, the frontal parts of the brain hemispheres thus 
should belong to the medial side—a matter of fact that should be 
of some importance in comparing the selachians with other verte- 
brates. The reason why I have here not followed this scheme is 
this: 
This scheme is only applicable under the presumption that the 
terminal situation of the olfactory crus is primary. But early 
ontogenetic stages not only in selachians, but also in cyclostomes, 
ganoids, teleosts, dipnoans, and amphibians, show that the pri- 
mary situation of the bulbus is a lateral one and that in these ani- 
mals except in selachians, the bulbus rudiment is dislocated during 
ontogeny to a terminal position. As the bulbus rudiment is a 
subpallial structure, this wandering of the bulbus rudiment is 
generally effected in the subpallial part without changing 
the pallial conditions. In early ontogenetic stages the rudiment 
of the pallial cortex is found medial and in its foremost part rostral 
to the bulbus rudiment, in the same way as in selachians, as shown 
below. Thus I consider the selachian condition to be primary; 
those of the other vertebrates, with the hippocampal formation 
extending directly to the olfactory crus, as secondary. 
The pallial structures in selachians seem to have been very little 
understood and as to the cortical layers they have been nearly 
