THE NERVOUS SYSTEM. 513 



In front of the cerebellum the tube is narrower and is compressed laterally. 

 This part is the isthmus (Fig. 447). Anterior to this, the roof plate and alar 

 plates expand into the mid-brain roof, the basal and floor plates forming the 

 basal part of the mid-brain. 



Certain gross changes which from now on take place in the medulla riay 

 conveniently be noted here. At about this time (fifth week) the outer borders 

 of the alar plate become folded outward and then downward, being thus turned 

 back on the plate itself (Figs. 452 and 416). This fold is called the prhwry 

 rhombic lip, and is most marked along the caudal border of the cerebellum. 

 The folds of the lip then fuse, forming a rounded eminence composing the border 

 of the alar plate to which the roof plate is attached laterally. Subsequently, 

 the attachment to the roof plate is shifted dorsally in the medulla, caudally in 



D.IV 



Nu. IV. 



FIG. 447. Transverse section through the isthmus of a 10.2 mm. human embryo. D.IV, Decussa- 

 tion of trochlear nerve; M. /., marginal layer; Nu. IV, nucleus trochlear nerve. His. 



the cerebellum. The portion of this lip which thins off into the roof plate is the 

 tcenia of the medulla and the posterior velum and taenia of the cerebellum. The 

 thin roof plate itself becomes tbe epithelial part of the tela chorioidea of the 

 fourth ventricle. At the caudal apex of the fourth ventricle a fusion of the 

 lips of the opposite sides forms the obeoc. 



A further complication is due to the increasing pontine flexure by which the 

 dorsal walls of the tube are brought close together (Fig. 448). The transverse 

 fold of the tela thus produced is the chorioid fold. At about the same time 

 lateral pocketings outward of the dorsal w r alls occur just caudal to the cere- 

 bellum which contain portions of the chorioid fold. These are the lateral 

 recesses. By further growth and vascularization, the mesodermal part of the 

 chorioid fold forms the chorioid plexus of the fourth ventricle (metaplexus). 

 Finally, in the human brain an aperture appears in the caudal portion of 

 the roof of the ventricle the foramen of Magendie (metapore) ; and, according 

 to many authorities, one also occurs in the roof of each of the lateral recesses 



