822 THE NERVOUS SYSTEM 



of the arachnoid layer is a cavity, the subdural cavity. The external boundary 

 of the subdural cavity is formed by the dura mater. The latter is a tough con- 

 nective tissue membrane of mesenchymal origin (fig. 355H). 



2. Development of the Brain 

 a. The Development of Specialized Areas and Outgrowths of the Brain 



1) The Formation of the Five-part Brain. The primitive vertebrate brain 

 from its earliest stages of development begins to show certain enlargements, 

 sacculations and outpushings. Furthermore, it possesses two main areas which 

 are non-nervous and membranous in character, namely, the thin roof plate 

 of the rhombencephalon and the thin roof plate of the posterior portion 

 (diencephalon) of the prosencephalon (figs. 354E; 356A). These thin roof 

 plates ultimately form a part of the tela chorioidea. Vascular tufts, the chorioid 

 plexi, also project from these roof plates into the third and fourth ventricles. 



The anterior region of the primitive brain known as the prosencephalon 

 or forebrain soon divides into the anterior telencephalon and a more posterior 

 diencephalon (fig. 354C-E). The telencephalon gives origin to two lateral 

 outgrowths or pouches, the telencephalic vesicles (figs. 354E; 357E). The 

 telencephalic vesicles represent the rudiments of the cerebral lobes. From 

 the diencephalon, four or five evaginations occur, namely, a mid-dorsal evagi- 

 nation, the epiphysis or rudiment of the pineal body (fig. 356A), and in front 

 of the epiphysis a second mid-dorsal evagination occurs normally in most 

 vertebrates, namely, the paraphysis (see Chapter 21); two ventro-lateral 

 outgrowths, the optic vesicles (fig. 354B-D) from which later arise the optic 

 nerves, retina, etc., and a mid-ventral evagination, the infundibulum. The 

 infundibulum unites with Rathke's pouch (figs. 354E; 356A), a structure 

 which arises from the stomodaeum. Rathke's pouch ultimately differentiates 

 into the anterior lobe of the pituitary body (see Chapter 21 ). 



The mesencephalon, unlike the fore- and hind-brain regions, does not di- 

 vide. However, from the mesencephalic roof or tectum dorsal swellings occur 

 which appear to be associated with visual and auditory reflexes. In fishes and 

 amphibia, two swellings occur, the so-called optic lobes or corpora bigemina. 

 In reptiles, birds and mammals four swellings arise in the tectum, the corpora 

 quadrigemina. (fig. 357H-0). 



The rhombencephalon divides into an anterior metencephalon and posterior 

 medulla or myelencephalon (fig. 354E and G). Two cerebellar outpushings 

 arise from the roof of the metencephalon. 



The primitive five-part brain forms the basic embryonic condition for later 

 brain development in all vertebrates. 



2) The Cavities of the Primitive Five-part Brain and Spinal Cord. As pre- 

 viously observed, the brain and spinal cord are hollow structures, and its 

 generalized cavity is called the neural cavity or neurocoel (fig. 357A). From 



