106 PERCIVAL BAILEY 



Here again, as in the brain of Necturus maculatus, the half 

 grown frog tadpole, and His' embryo CR, the taenia fornicis 

 can be followed along the lateral margin of the sulcus tegmenti 

 past the velum trans versum to the taenia thalami. But when 

 the sulcus tegmenti invaginates to form the fissura chorioidea, 

 the posterior end of the taenia fornicis and a small part of the 

 medial hemisphere wall are drawn in also. This latter process 

 is carried much farther in the human embryo. 



In the evagination of the hemisphere in the human embryo, 

 the point marked x in figure 15, at the junction of the hippo- 

 campal area, striatal area, and area chorioidea posterior, re- 

 mains always at the posterior pole of the hemisphere. The 

 result on the corpus striatum, as the hemisphere extends back- 

 ward, is to draw out the tail of the caudate nucleus. The result 

 upon the area chorioidea posterior is to draw it out in a thin 

 lamina marked in figure 17 by small circles. 



The area chorioidea posterior now buckles into the lateral 

 ventricle, and we have the condition found in the 19 mm. em- 

 bryo, H 173. The area chorioidea posterior is clearly shown 

 by figure 19, being all of the plexus back of the point marked 

 z, and has buckled inward only slightly, leaving a very wide 

 fissura chorioidea. The point z lies opposite the velum trans- 

 versum. 



It thus appears that the lateral choroid plexus is composed 

 of two parts, a pars anterior plexus chorioidei ventriculi lateralis 

 which is formed by the invagination of the area chorioidea 

 anterior, between the paraphysal arch and the taenia fornicis, 

 and a pars posterior plexus chorioidei ventriculi lateralis formed 

 by the infolding of the area chorioidea posterior in the medial 

 wall of the hemisphere. 



If the taenia fornicis be now followed in the 19 mm. embryo, 

 it will not be found to become continuous with the taenia thalami 

 because the area chorioidea posterior, to which it was attached 

 toward its posterior end, has now buckled into the ventricle. 

 If, therefore, we follow the attachment of the lateral choroid 

 plexus to the medial hemisphere wall, we follow the taenia forni- 



