STRUCTURE AND DEVELOPMENT OF LEPIDOSTEUS. 
377 
with the brain of the larva (vide p. 378) shows that this is not the case, but that 
the body in question is represented in the larva by a special outgrowth of the roof ol the 
thalamencephalon. The vesicle of the roof of the thalamencephalon is therefore to be 
regarded-as a peculiar development of the tela choroidea of the third ventricle. 
How far this vesicle has a homologue in the brains of other Ganoids is not certain, 
since negative evidence on this subject is all but valueless. It is possible that a 
vesicular sac covering over the third ventricle of the Sturgeon described by Stannius, 7 ' 
and stated by him to be wholly formed of the membranes of the brain, is really the 
homologue of our vesicle. 
WiEDERSHEiMt has recently described in Protopterus a body which is undoubtedly 
homologous with our vesicle, which he describes in the following way :— 
“ Dorsalwarts ist das Zwischenhirn durch ein tiefes, von Hirnschlitz eingenommenes 
Thai von Vorderhirn abgesetzt; dasselbe ist jedoch durch eine hautige, mit der Pia 
mater zusammenhangende Kuppel oder Kapsel iiberbruckt.” 
This “ Kuppel” has precisely the same relations and a very similar appearance to 
our vesicle. The true pineal gland is placed behind it. It appears to us possible 
that the body found by Huxley;}; in Ceratodus, which he holds to be the pineal gland, 
is in reality this vesicle. It is moreover possible that what has usually been regarded 
as the pineal gland in Petromyzon may in reality be the homologue of the vesicle we 
have found in Lepidosteus. 
We have no observations on the pineal gland of the adult, but must refer the 
reader for the structure and relations of this body to the embryological section. 
The infundibulum (Plate 25, fig. 47 A, in.) is very elongated. Immediately in front 
of it is placed the optic chiasma (Plate 25, figs. 47 A and C, op.ch.) from which the 
optic fibres can be traced passing along the sides of the optic thalami and to the optic 
lobes, very much as in Mullee’s figure of the brain of Polypterus. 
On the sides of the infundibulum are placed two prominent bodies, the lobi 
inferiores ( l.in .), each of which contains a cavity continuous with the prolongation 
of the third ventricle into the infundibulum. The apex of the infundibulum is 
enlarged, and to it is attached a pituitary body ( pt .). 
The mid-brain is of considerable size, and consists of a basal portion connecting the 
optic thalami with the medulla, and a pair of large optic lobes ( op.I .). The iter a 
tertio ad quartum ventriculum, which forms the ventricle of this part of the brain, is 
prolonged into each optic lobe, and the floor of each prolongation is taken up by a 
dome-shaped projection, the homologue of the torus semicircularis of Teleostei. 
* “ Ub. d. Gehirns des Stors,” Muller’s Archiv., 1843. and Lehrbuch d, vergl. Anat. d. Wirbelthiere. 
Cattie (Archives de Biologie, vol. iii., 1882, has recently described in Aoipenser sturio a vesicle on the 
roof of the thalamencephalon, whose cavity is continuous with the third ventricle. This vesicle is clearly 
homologous with that in Lepidosteus. (June 28, 1882.) 
f R. Wiedersheim, ‘ Morpliol. Studien,’ 1880, p. 71. 
f “ On Ceratodus Forsterei &c., Proc. Zool. Soc., 1876. 
3 c 2 
