CEREBRAL COMMISSURES IX THE VERTEBEATA. 



463 



{at least in the region of the foramen of Monro) never forms part of the cerebral lieiui- 

 spbere, but is merely an appendage of the roof of the primitive forebrain. The mesial 

 portion of the epithelial plate from which the choroid jilexuses (lateral) appear to spring 

 may be distinguished as the lamina chorioidoa. Tlie exact situation of this lamina is well 

 shown in figure 2 (/3). From that figure it is seen that the lamina chorioidea is separated 

 from the commissura aherrans h\ the paraphysis, and this explains the presence of the 

 paraphysis in the coronal section which has just been imder consideration. It consists 

 of a very irregular median diverticulum of the lamina chorioidea, which extends upwards 

 between the two cerebral hemispheres. 



A comparison of the last three figures will siiow that the paraterminal Iwdy diminishes 

 in bulk as it is followed in the caudal direction. At tlie level of the coinmisura aherrans 

 (fig. 10) it has assumed a triangular shape, and has estal)lished relations which are 



Portion of a coroaal section through the brain o{ Splnuod'ni ( Deiidy's stage It): passing through the 



coiumisSKra aherrans. 



extremely peculiar and perplexing. Thus tlie mesial l)orders of the bodies in question 

 have extended mesially, and fused the one with tlie other upon the roof of the third 

 ventricle. In addition, the ventral surface of the paraterminal body is fused to the dorsal 

 surface of the optic thalamus, producing on the whole an appearance which (especially 

 to the student of the Mammalian brain) is in the highest degree bizarre and astonishing. 



The ventrolateral angle of the paraterminal body is prolonged downward, and tapers to 

 become continuous with a plain epithelial lamina («), which completes the gap in the 

 mesial wall of the hemisphere which would otherwise exist in the interval between the 

 corpus striatum and the paraterminal body. 



The commissura aherrans consists of a strand of nerce-fihres which crosses ahove the roof 

 of the thii-d ventricle in the hridije of grey matter formed by the fusion of the caudal 

 extremities of the tioo corpora paraterminalia : after passing through the paraterminal 



SECOND SERIES.— ZOOLOGl', VOL. VIII. 68 



