Herkick, Brain of Certain Reptiles. 97 



If our interpretation is correct, the front of the mesenceph- 

 alon is limited dorsad by the post-commissure, while it lies close 

 behind the tuber, ventrad. The substance of the basal region, 

 which is displaced by the " saddle cleft, " is pushed cephalad 

 and dorsad into the third ventricle and optic ventricle. 



1 . A section near the dorson of the mesencephalon con- 

 sists of two ovoid segments apposed along the median line and 

 enveloped on all sides by fibres from the optic tracts. The in- 

 terior exhibits the cellular structure characteristic of the tectum. 



2. At a lower level the two important tracts of the tectum 

 appear, at first, in one median region, where, however, the ob- 

 lique fibres which make up the brachial tracts are easily distin- 

 guished from the radial bundles lying between them, which lat- 

 ter can be traced to the lemniscus. Fibres cross from one side 

 to the other along the median line and constitute the Sylvian 

 commissure. The lemniscus fibres gradually accumulate near 

 the ventricular aspect while the brachial proceed cephalad. 



3. In a section where the aqueduct is cut, the post-com- 

 missure is reached. It is underlaid by a remarkable thickening 

 of the epithelium that can be traced far ventrad. The optic 

 tracts are now collecting on either side of the habenae and the 

 tract of the supra-commissure is separated only by a narrow 

 fissure from the former. The cells of the habena nidulus are 

 large and stain deeply. The cells of the mesencephalic nidulus 

 of the trigeminus are numerous along the dorsal walls of the 

 aqueduct, especially caudad. 



4. Somewhat farther ventrad the tuberosities of the cor- 

 pora posteria appear upon the caudal aspect of the optic tec- 

 tum. This is a mass of densely and irregularly clustered cells 

 with interspersed fibres. Cephalad, the less dense and lighter 

 colored Meynert's nidulus appears and lies laterad and ventrad 

 of the nidulus of the habena. The lemniscus fibres collect in a very 

 distinct zone entad of the brachial bundles. The post-commis- 

 sural tract continues obliquely caudo-ventrad and is in about the 

 zone occupied by the lemniscus. 



5. Still farther ventrad the fibres of Meynert's bundle can 

 be seen to emerge from the caudal aspect of Meynert's nidulus 



