66 



THE SUBDIVISIONS OF THE BRAIN. 



S.A3. 



Mesencoele. The mesencoele is formed in part by a marginal, epithelial 

 overgrowth, and in part by a deep median depression or infolding, between the 

 two cords. The epithelial overgrowth is formed along the whole margin, as a 

 thin overarching shelf that projects about a third way over each cord. (Figs. 

 136, 227 and 231.) Later, the cords settle bodily below the surface, and as there 

 is no delamination of a superficial epithelium, a wide opening is left that is 

 gradually closed in by the union of the thin edges of the two overgrowths. 



As the two cords in this brain region remain horizontal, the potential mesen- 

 ccele is broad and shallow, except in the middle line where it extends to the bottom 

 of the deep median fissure. 



In the more posterior parts of the brain, in the vagus and branchial regions, 



similar overgrowths occur. But the cords are 

 much narrower here and their margins are brought 

 together, like the closed covers of a book, by the 

 deep median infolding, so that the chamber is con- 

 verted into a deep, narrow fissure. The bottom 

 of this fissure is converted into a "canalis cen- 

 tralis," when the neural commissures grow across 

 the fissure, just above the floor. (Figs. 55, 69.) 

 * * * * * * 



Comparison with Vertebrates. The mid- 

 brain neuromeres of arachnids are represented in 

 vertebrates by the group of conspicuous neuromeres 

 forming the floor of the brain, from the infundi- 

 bulum to the vagus region. The region is charac- 

 terized by: a. its great width; b. its enormous 

 FIG. 54.Vagus and branchial cran i a i ganglia, widely separated from the crura 



neuromeres of an embryo scorpion & & J 



about ready to hatch. Camera outline, and associated with the oral and hyoid arches ; c. by 



the segmentally arranged gustatory organs; and d. 



by the unusual distinctness of the neuromeres in the early embryonic stages, in 

 marked contrast with the regions just in front and behind. 



In vertebrates confusion has arisen from the failure to distinguish between 

 the true neuromeres on the floor of the brain and the various structures that have 

 been forced out of their original positions onto the roof of the brain. The cerebel- 

 lum and the optic lobes, as we have indicated above, are of very unequal value, 

 and in no wise comparable with neuromeres. Their position in vertebrates is a 

 purely secondary one, a long way caudad to their original position and connec- 

 tions. The optic lobes, that in vertebrates form the roof to the midbrain, clearly 

 belong to the procephalic neuromeres, while the primitive cerebellar "neuromere" 

 is a special commissure primarily associated solely with the diencephalon. The 

 area covered by these structures therefore, varies greatly, and has no constant 

 relation to the underlying neuromeres. 



If we remove the cerebellum and optic lobes from the brain of a vertebrate 



