CEREBRAL COMMISSURES IN THE VERTEBEATA. 



459 



extends across the mesial plane as a transverse band of an ejjithelial or nenroglial natnre 

 which is devoid of nerve-cells. In most cases, however, the commissures which grow 

 across the mesial plane in contact with the lamina terminalis cany with them some grey 

 substance (derived from the corpora paraterminalia) which forms the "commissure-bed" — 

 a secondary thickeninsr of tlie lamina terminalis. 



Fig. 4. 



A scheme to indicate the fundamental relationsliips of ecrtain portions of the cerebral hemispheres to the lamina 

 terminalis and optic thalanii ; being the plan of an ideal horizontal section. 



If a coronal section be made througli the cerebral hemisphere of Sphenocloa, imme- 

 diately in front of the lamina terminalis, the relationship of the corpus ixiraterminale to 

 the suri'ounding areas will be more readily appreciated. 



A coronal section through the two cerebral hemispheres of ISphfuodon (Dendy's foetal stage R) immediately in front 



of the lamina terminalis. 



Each paraterminal body will then appear as the thick plump mesial Avail of the 

 hemisphere, which is continuous Aentrally around the ventricle with the corpus striatum 

 on the lateral aspect of the ventricle. 



The upper edge of the thick paratermiual body is fused with a relatively thin portion 

 of the wall of the ventricle, in Avhich the nerve-cells are collected into an extremely 



