104 PERCIVAL BAILEY 



in this latter region tiiat the lateral choroid plexus makes its 

 appearance in urodele Amphibia as we have just remarked, 

 and here also it makes its first appearance in the human em- 

 bryo. The hippocampal area is not apparent in this embryo but 

 later developments show that its anlage must lie in some such 

 position as indicated in figure 15. The part of the telencephalon 

 which is evaginated to form the cerebral hemisphere is the part 

 anterior to the line d-a-b. The division between telencephalon 

 and diencephalon is represented by the line b-a-c. 



Keeping these relations in mind, we may easily understand 

 His' embryo CR, 13.6 mm. in length. Figure 16 shows a me- 

 dial view of the forebrain. An invagination of the telencephalic 

 roof has taken place between the taenia fornicis and the mid- 

 line, forming the fissura chorioidea in the region which was 

 marked area chorioidea anterior in figure 15. The hemisphere 

 vesicle has enlarged mainly by enlargement and evagination 

 of the neo-pallial area (fig. 15, 7i-p.a.), so that the hippocampal 

 area which in figure 15 lay below the roof plate and in front of 

 the area chorioidea posterior, now lies above the roof plate and 

 behind the area chorioidea posterior, and its ependymal face 

 has now come to look lateralward. So also with the area chorioi- 

 dea posterior, which lies now in the evaginated cerebral hemi- 

 sphere behind the di-telencephalic groove, and its ependymal 

 face is now its lateral face. When the hemisphere evaginated, 

 the wall bent for the most part along the line of the taenia 

 fornicis in figure 15, the di-telencephalic groove as far as the 

 point a, and then along the line a-d. 



If now the hemisphere in embryo CR be excised, along a 

 line homologous with the line b-a-d in figure 15, the relations 

 of the area chorioidea posterior and area chorioidea anterior 

 are more clearly seen. The medial surface of such a hypothetical 

 hemisphere is represented in figure 17. The area chorioidea 

 anterior has invaginated into the lateral ventricle. The result- 

 ing plexus has been cut off in figure 17, leaving the fissura chori- 

 oidea. Since the invagination takes place between the taenia 

 fornicis and the mid-line of the telencephalic roof, the taenia 

 fornicis lies along the lateral and upper margin of the fissura 



