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PERCIVAL BAILEY 



ping down to meet the anterior nucleus of the thalamus, which 

 it does in a few sections and becomes continuous with the taenia 

 thalami. In figure 19, much farther posteriorly, the taenia 

 thalami is present, the plexus being entirely in the medial hemi- 

 sphere wall. The portion of the plexus arising from the medial 

 hemisphere wall is very poorly developed (fig. 19) but its area of 

 invagination is extensive (fig. 24). 



In later stages this posterior part of the plexus develops more 

 rapidly and overshadows the other. Figure 25 shows a lateral 



Fig. 18 Transverse section of the same embryo as figure 16. Slide 13, sec- 

 tion 22. X 33 J. 



Fig. 19 Transverse section of the same embryo; section 5, slide 14. X 33|. 



view of the region around the foramen of Monro in an embryo 

 with a carapace of 10.6 mm. A pen sketch of the entire model 

 is appended (fig. 27) showing the region represen ed in figure 

 25. (Figure 24 is of a homologous region in a younger embryo.), 

 The fissura chorioidea appears merely as a big hole in the medial 

 hemisphere wall; all the landmarks are lost. In still later stages, 

 this hole becomes reduced to a long narrow slit. 



The development of the plexus in size and shape is so well 

 discussed and figured by Warren ('11) that it will not be con- 

 sidered here. 



