THE NERVOUS SYSTEM. 



547 



At the beginning of the sixth week the foramen of Monro has changed some- 

 what in shape. The pallio-thalamic part of its boundary passes forward and 

 forms the above-mentioned (p. 540) acute angle (angulus praethalamicus) with 

 that part of the wall uniting the two hemispheres (lamina terminalis). The 

 latter wall descends to the region of the optic recess. The inferior part of the 

 foramen is partly closed by the medial part of the corpus striatum as already 

 described. (Comp. Figs. 479, 464 and 466.) In the ependymal mesial wall 

 of the hemispheres just below the taenia, described above, there arises a folding 

 inward, which begins anteriorly near the angulus praethalamicus and proceeds 

 caudally along the upper (pallio-thalamic) border of the foramen of Monro. 

 This infolding is the chorioid fissure. In the ependymal mesial wall there are 



Pallium 



Foramen of Monro 

 Corpus striatum 



Eye 



III ventricle 

 Chorioid fissure 



Mesodermal tissue, 

 forming later the 

 chorioid plexus. 



FIG. 479. Transverse section through fore-brain of a i6mm. embryo (six to seven weeks). His* 



now the following : limbus chorioideus (the infolded part) and a small strip of the 

 ependyma wall below the fold, the lamina infrachorioidea (Fig. 480). This 

 imagination soon becomes very deep, resulting in the formation of a double- 

 layered ependymal fold (the chorioid fold, plica chorioidea) lying in the lateral 

 ventricle over the corpus striatum (Figs. 479, 464 and 482). Later, vascular 

 mesodermal tissue passes in from the falx between the lips of this fold and 

 thereby forms the chorioid plexus of the lateral ventricles. The chorioid fissure 

 is at first quite short, but becomes elongated (Fig. 481) with the above-described 

 posterior elongation of the hemisphere of which it is a part, and thus extends 

 into the inferior horn of the temporal lobe. (Figs. 481 and 482.) 



Toward the end of the second month, according to some authorities (His), 

 but not until considerably later, according to others (Hochstetter, Goldstein), 

 another furrow appears in the limbus corticalis above and parallel to the chori- 



