56 



THE SUBDIVISIONS OF THE BRAIN. 



garded as the initial neuromere of this subdivision of the brain. Its large size 

 and its special relations, on one side with the hemispheres, and on the other with 

 the stomodaeum and the gustatory organs, lend to this neuromere and those im- 



FlG. 46. — Semi-diagrammatic sagittal sections, showing the relations of the piincipal nerve centers in the 

 brains of arachnids and vertebrates. A , Embryo scorpion (stage B, Fig. 15) ; B, embryo scorpion (stage G, Fig. 18) ; 

 C hypothetical intermediate condition, based on the conditions in both scoipion and Limulus. The embryonic 

 palium and the anterior neuropore have been carried over into the adult, and the lateral eye ganglia have been pro- 

 jected onto the neural surface, otherwise the typical arachnid conditions remain essentially unmodified. D shows 

 the probable position and relation of these parts in a vertebrate. The ventricles are indicated in Roman numerals, 

 the neuromeres in Arabic numerals. 



mediately associated with it in the circumoral region, a special distinction that 

 justifies their elevation to the rank of a distinct brain region. 



*^t^ ^Am mS* ^ ^ k^ «1* «i« 



In the scorpion (Figs. 15, 16, 43), by the time the embryo hatches, the fore- 

 brain is bent through something more than 90°, onto the anterior hsmal surface 



