FOREBRAIN MORPHOLOGY A39 
embryo of Petromyzon, selachians, Dipnoi, ganoids, teleosts, and 
amphibians is formed at the lateral surface of the forebrain 
vesicle and from this position brought to its frontal position. 
The forebrain of the common ancestors of the selacho-holocephalians 
and the crossopterygians (hypothetical form y) 
Comparing the schemes of the brain types of selacho-holocepha- 
lians and primitive crossopterygians, we find them to cover each 
other almost perfectly, the former being, however, more primitive 
in the supposed lateral position of the bulbi olfactorii. 
From this agreement the conclusion may perhaps be permitted 
that the common ancestors of the selachians and primitive 
crossopterygians had a forebrain built up more conforming to the 
selacho-holocephalian ancestral type than to the crossopterygian. 
The forebrain of the common ancestors of gnathostomian fishes and 
cyclostomes (hypothetical form x) 
The cyclostomes represent a very old vertebrate group. The 
position of the Devonian genera Palaeospondyius and Hetero- 
spondylus being very uncertain, no unquestionable extinct 
species are hitherto known. The morphology of the living genera, 
however, indicates that these are in many respects reduced forms. 
The absence of paired limbs seems to connect thecyclostomes with 
those Silurian fish-types included in the class of anaspids (Lasanius, 
Birkenia, etc.). Perhaps the living cyclostomes could be eel- 
‘shaped semiparasitic descendants from such fish-types. 
The evagination of the forebrain in Petromyzon probably 
is reduced together with the reduction of the lateral ventricles, 
as has been explained before, but there are, on the other hand, no 
signs present indicating that the frontal evagination has ever been 
more than a bulbar one. The septum, if regarded as present, is 
built up only by cells belonging to the granular layer of the bulbus 
olfactorius. The pallium thus is excluded from the ‘septum.’ 
In the pallium there are no signs indicating a subdivision. 
Also in the subpallial parts no special nuclei are indicated. 
The presence of a sulcus limitans externus undoubtedly is a 
primitive character. 
