132 Journal of Comparative Neurology. 



nidulus fibres arise from the cells and collect into Meynert's 

 bundle with the usual course. 



At a section farther ventrad {Fig. j,) the dorsal part of the 

 tectum appears. The optic tract enters laterad. The structure 

 of the tectum is exceedingly simple, consisting of ganglion cells 

 arranged about the v^entricle with long bifurcating processes of 

 great length and delicacy. There is no stratification into zones 

 as in the -higher forms but, nevertheless, the essential elements 

 are present. The optic nerves are small and hollow (i. e., the 

 old cavity of the primary optic ventricle is not wholly obliter- 

 ated) and the small number of fibres in the optic tract calls out 

 relatively few neurons from the second layer to the first. These 

 few cells are scattered irregularly somewhat above the level of 

 the general gray matter. A second layer of fibres approxim- 

 ately parallel to the ventricle is the brachial system. These 

 fibres are smaller than those of the optic tract. They arise 

 from cells in the region where in other vertebrates the genicu- 

 lata occur and pass caudad to constitute the inner horizontal 

 fibre zone. In the caudal aspect of the tectum bundles enter 

 from obliquely ventro-laterad and caudad and penetrate the tec- 

 tum between the meshes of and nearly at right angles to the 

 preceeding. This tract can be none other than the lemniscus. 

 Its course is obliquely latero-caudad to near the region of exit 

 of the trigeminus. In the middle region of the tectum occur a 

 few giant ganglion cells — the mesencephalic nidulus of the tri- 

 geminus. Each of these cells gives off a large dark fibre which 

 passes entad of the lemniscus to the exit of the trigeminus, 

 There are several bundles of such fibres in each side. The 

 fibres are larger than those of the 4th nerve. 



T/ie optic tectum in the frog is exactly similar to that of the 

 fishes in all essential particulars. It, therefore, differs from that 

 in Urodela much as the cortex of reptiles differ from that in the 

 embryo. It is very curious that none of the writers on the am- 

 phibian diencephalon seem to have systematically sought for the 

 optic brachia or Sylvian commissure. The latter is a very evi 

 dent band crossing from one tectum to the other and lying be- 

 neath the ectal ganglion layer. The cephalic brachia extend 



