53 



ANATOMY OF THE CENTEAL NEETOUS SYSTEM. 



life for nearly its whole length as a simple epithelial layer. At the point 

 of juncture with the Prosencephalon the epithelial plate is displaced down- 

 ward by the encroachment of a highly-yascular growth from the cranial 

 cavity: the velum chorioideum. From the relation of the hemispheres to the 

 Thalamencephalon, their inner border must be continuous with the velum. 

 In the frontal section through the cerebrum, shown in Fig. 22, this is made 

 apparent. The figure also shows that the cerebral cavity is divided into 



Fig. 22. — Frontal section through the head of a, human embryo of two and 

 a half months, showing the invagination of the cerebral cavity and the fundament 

 of the corpus striatum. 



one median and two lateral ventricles. The velum chorioideum sends ex- 

 pansions into the lateral ventricles: the Plexus chorioidei laterales. The 

 place where the walls of the hemispheres pass into a simple epithelial layer 

 (Fig. 22, a) is called the margin of the hemispheres. This margin is later 

 marked by a fasciculus of white fibers throughout its whole length: 

 Fornix. 



When the most important parts of the cerebrum have once become 



