MORPHOGENESIS OF THE CHOROID PLEXUSES 



521 



The earliest unquestionable appearance of the lateral telen- 

 cephalic plexus is in the Harvard enibrj'o, 8.8 mm. in length. 

 It appears here as a crescent-shaped ridge projecting into the 

 lateral ventricle, with two more strongly developed points 

 showing as small elevations (fig. 22). The anterior extremity 

 of the plexus lies clearly in the roof of the telencephalon lateral 

 to the paraphysis and medial to the taenia fornicia. The taenia 

 fornicis appears in figure 22 as a ridge apparently in the medial 

 hemisphere wall. It bears here a very close superficial resem- 

 blance to the hippocampal ridge in a young human embryo, but 

 its later development and internal structure show plainly that 



Fig. 10 Diagniiii of the region around the paraphysis in tlie roof of the fore- 

 brain of a 5.1 mm. emiiryo of Chrysemys marginata. 



Fig. 11 Diagram of the same region in an embryo of 8.8 mm. 



Fig. 12 Diagram of the same region in an embryo having a carapace of S.6 mm. 



it is the taenia fornicis. Its apparent position in the medial 

 hemisphere wall is an illusion produced by the invagination 

 of the plexus between it and the paraphysis. The triangular 

 area of the telencephalic roof plate is well marked and the lateral 

 angles can be determined. A sagittal section through the tri- 

 angle is practically a straight line, except for the evagination 

 of the paraphysis (figs. 13 and 21) as are also parasagittal sec- 

 tions (fig. 14). Nevertheless the lateral angles are depressed 

 and the sides of the triangle are concave outward. Transverse 

 sections through this region are therefore curved and convex 

 upward (fig. 21). The velum trans versum sharply dehmits the 

 telencephalic roof plate at the base of the triangle (fig. 14). 



