MR. A. SANDERS ON THE ANATOMY OF THE 
762 
nervous substance. It is not to be overlooked that Von Baer'" described these 
hemispheres in the embryo of the Teleostei, as being hollow and containing a small 
tubercle occupying the position of the corpora striata. In the Plagiostomata, how¬ 
ever, the walls of the cerebral hemispheres are thickened, and two ventricles are 
present, so that we have here a considerable advance in structure. The theory that 
these represent the lateral ventricles of Mammalia is not universally received. Prof. 
Burt. G. Wilder! looks upon them as implying an extension forward of the third 
ventricle, and the real Foramina Monroi he finds in the openings on each side which 
lead into the ventricles of the olfactory lobes, which latter he considers to be the true 
lateral ventricles. This view seems to me to be scarcely tenable ; not only is the 
structure of these lobes incompatible with this opinion, but the position of the 
so-called Foramina Monroi themselves is against it, the true foramen being a single 
passage, and not one in each lateral wall, which would be the case if this opinion 
should be received. 
With regard to the homologies of the remaining tuberosities of the brain, except 
the optic lobes, there are no great difficulties. 
The hypoaria, lobi infundibuli or inferiores may be considered to belong to the 
thalamencephalon, which may be defined as the space between the anterior com¬ 
missure in front and the posterior commissure behind, and includes the above- 
mentioned lobe together with the infundibulum. Thus the thalamencephalon is 
pushed under the mesencephalon to a great extent in the Plagiostomata. 
To the question, how much if any part of the thalamencephalon is represented 
in these fishes ? I should be inclined to reply, probably no part of it. The only 
consideration which would seem to favour the idea of its presence would be the 
position of the posterior commissure, the homology of which is generally conceded ; 
but this does not necessarily carry with it the homology of the part which it 
connects, and even if it does so, the posterior commissure does not solely nor even 
principally connect the two tlialami optici. But it is probable that the territory 
between the anterior end of the optic lobe and the chiasma nervorum opticorum 
corresponds to the corpus geniculatum externum, or, as Mayser| suggested, the 
corpus geniculatum internum, for the torus semicircularis in Teleostei. 
Bo u on, whose ideas generally seem worthy of respect, has rather gone beyond the 
mark in his theory of the optic lobe ; a combination of the thalamencephalon and 
the mesencephalon in this lobe is an idea that will not bear inspection. In the 
first place there is not the slightest appearance in any sections that I have seen 
of a difference in structure in the optic lobes; on the contrary, the difficult}^ was 
to distinguish any separate layers for description, the characteristic cells extended 
without a break in greater or less numbers from the external to the internal surface. 
* ‘Ueber Entwickelungsgeschichte der Tliiere.’ Zweiter Tlieil, p. 307. 
f ‘ Philadelphia Acad. Nat. Sci. Proc.f 1876. 
f ‘ Zeitschr. Wissenscli. Zool.,’ vol. 36, 1881. 
