FOREBRAIN MORPHOLOGY 393 
in distinctness. This groove undoubtedly represents the remains 
of a previous lateral ventricle and represents the sulcus limitans 
externus, whose most characteristic feature is its leading out into 
the bulbar ventricle. 
In Acipenser and other cartilaginous ganoids the eversion 
of the telencephalon is very much less pronounced than in bony 
ganoids and teleosts, and the bulbar ventricle is much greater. 
This fact seems to point in the direction that the inversion is a 
primary character of the forebrain of fishes. The same conclusion 
must be drawn from the fact that the forebrain vesicle of a young 
embryo of Lepidosteus is very clearly inverted. 
After these introductory notes, we will turn attention to the 
pallial structures. What is, then, the pallium in the morphologi- 
ealsense? It is not easy to define this idea in a manner to include 
therein the pallium of all vertebrates. It seems not to be possible 
to accept the definition of Edinger and others, whose opinion is 
that a true pallium (Episphaertum=Neencephalon) does not 
receive olfactory fibers of lower order than the third. I cannot 
accept this view, since it supposes that a brain nucleus would 
not be able to change its connections without changing its mor- 
phological value. It is a well-known matter of fact that the con- 
nections of a nerve-nucleus can change in different closely related 
species (for instance, the nucleus rotundus in bony fishes). But 
none will for that reason declare that this nucleus is not a homolo- 
gous one. While the hind part of the roof of telencephalon in 
selachians is said not to receive olfactory fibers of the second 
order,! but only such of the third, the conclusion is not at all 
justified that this hind part has a quite different morphological 
value than the anterior part of similar construction, but with 
secondary olfactory tracts ending therein. In such a case the 
conclusion indicated by the facts is that a process of differentiation 
has taken place in such a manner that the brain roof (the pallium) 
has become divided into two parts, of which the one is connected 
with secondary olfactory fibers, the other with tertiary. The 
morphological unity of the roof remains nevertheless as before. 
1 My studies have shown that this is a false statement, as there are really olfac- 
tory fibers of the second order ending in the posterior part of the pallium. 
