426 NILS HOLMGREN 
The above description is based upon a transverse series of sec- 
tions through an adult Polypterus forebrain. But I have also 
studied the forebrain of younger stages. Through the kindness of 
Professor Jiigerskidld (Gothenbourg), I had the opportunity of 
investigating two stages, one (with retained outer gills) of 4.5 
em. total length and the other of 12.5-em. These stages exhibit in 
principle the same morphological arrangement in the forebrain as 
the adult. In the 4.5-cm. stage the lateral olfactory nucleus is 
under formation from the pars superior of the precommissural | 
body, as suggested in the adult. The eversion of the pallium is in 
principle the same as in the adult, but the bending of the upper 
portion of the pallium is in the youngest stage by far not so 
advanced as in the 12.5-em. stage and in this it is much less 
advanced than in the adult. Of very great phylogenetic interest 
is the fact that the descending part of the pallium is much greater 
in the adult than in the 12.5-cm. stage and that in the 4.5-cm. 
stage one can scarcely speak of a descending portion, the pallium 
being composed of a high ascending part and a bent-out portion 
of smaller size. In the larger stages the brain-case is very spa- 
cious dorsoventrally and there is a rather wide free space between 
the roof of the brain case and the pallium. In the 4.5-cm. stage, 
however, the pallium lies in the closest contact with the roof of the - 
skull and the relations between skull and forebrain are such as to 
make it evident that the bending out of the pallium is caused by 
lack of space for the outgrowing of the pallium in the dorsal 
direction. This stage of 4.5-cm. makes it very probable that it 
has been preceded by a stage where the pallium was uneverted, 
directed straight dorsad or perhaps bent in a little. The latter pre- 
sumption is based on the fact that the pallium at the line of attach- 
ment of the tela makes a little bending in opposite direction tothe 
general bending of thepallium. Thus, I findit very probable that in 
younger Polypterus stages there is even a somewhat inverted 
forebrain present. This being the case, the forebrain of Polyp- 
terus may serve very well for connecting the everted forebrain 
type with the inverted as present in dipnoi or holocephalians. 
The chief difference is the lack of forward evagination in Polyp- 
terus, but in Chimaera this evagination is very small, as pointed 
out before. 
