522 



PERCIVAL BAILEY 



Figure 15 shows ;i parasagittal section still farther laterally 

 through the triangle. The velum transversum is obvious, the 

 plexus invaginating the roof plate medial to the taenia fornicis 

 and tilting the portion of the roof between itself and the taenia. 

 Above and lateral to the taenia fornicis the brain wall is his- 

 tologically differentiated; medial it is ependymal. This differ- 

 ence is much more apparent in later stages (fig. 17). Poste- 

 riorly the plexus is less well developed and as we approach the 

 lateral angles of the triangle, plexus, velum, and taenia fornicis 

 tend to merge into one groove which becomes continuous with 



Fig. 13 Sagittal section no. 135 from an 8.8 mm. embryo of Chrysemys mar- 

 ginata. Harvard Embryological Collection, no. 1433. X 33j. 

 Fig. 14 Parasagittal section no. 130 from the same embryo. 

 Fig. 15 Parasagittal section no. 124 from the same embryo. 



the di-telencephalic groove. Lateral and anterior to the point 

 where the taenia thalami meets the di-telencephalic groove and 

 velum transversum the hemisphere wall is massive and unin- 

 vaginated. It would seem, then, that in this embryo, the 

 lateral telencephahc plexus lies entirely in the roof plate of the 

 telencephalon. The condition is diagrammatically represented 

 in figure 11. 



Figures 23 and 24 depict the region around the foramen of 

 Monro in an embryo with a carapace 8.6 mm. long, about 14 mm. 

 greatest length. Figure 23 looks at the foramen of the right 

 half of the brain from the medial side. The cephalic end of the 



