425 
no fibrous connection with it but passes to a specific terminus in the 
nidulus corticalis, Frirscn, on the lateral aspects of the thala- 
mus near the intrance of the anterior optic tract into the tectum. 
None of these fibres cross in the postcommissura. There is a 
direct connection between the thalamus and the nucleus rotundus by 
means of a non-medullated tract containing cells. The homologies of 
this organ are not difficult to seek. First, considering the structure, 
the only body in which such intimate association of cells and blood- 
vessels has been observed in higher vertebrates is the nidulus 
ruber, in which Mrynert describes the perforation of bloodvessels 
by the long processes of a peculiar variety of cell; second, the position 
also warrants this reference, lying dorsad of the pes pedunculi and 
caudad of the thalamus and extending as far caudad as the exit of the 
third nerves; third, and particulary, the fibrous connections agree with 
those of the nidulus ruber. In mammals the anterior peduncle of the 
cerebellum perforates or joins this body. In fishes the tract from the 
cerebellum to the thalamus unites withe the horizontal commissure 
tract at its exit. Part of the dorsal peduncle from the cerebrum per- 
forates and surrounds it (tegmental radiation). 
If there are other bodies indistinguishably fused in it nevertheless 
the ,,corpus rotundum“ must be regarded as a functional homologue 
of the nidulus ruber. 
6. The hypoaria. It is remarkable that, in spite of the 
comparative simple structure of these bodies, they have so long baffled 
interpretation. They have generally been regarded as appendages of 
the tuber cinereum and more or less perfect homologues of the mam- 
millaria. That this is impossible is shown, first, by the fact that they 
lie morphologically caudad of the tuber and adjoin caudad the pons 
region. The third nerve often is forced backward by them and always 
emerges immediately caudad. Second, the specific homologues of the 
mammillaria are present in all fishes examined in exactly the proper 
place (fig. 8) and have a small tract leading toward the cerebrum 
and are also associated with fibres to the dorsal region of the thala- 
mus. Third the fact that the tectum opticum has been thrust back- 
ward over the testes implies a certain amount of folding of the base 
of the mesencephalon, which has resulted in a protuberance of the 
pes pedunculi. These considerations might all be over-ruled but the 
crucial test lies in the course of the fibres. It is assumed that in 
mammals there is a strong intercalation of cells in the pes pedunculi 
which connect on one hand with the cerebellum and on the other 
with sensory niduli. It must be confessed that we were quite unpre- 
