CHAPTER IV. 



THE SUBDIVISIONS OF THE BRAIN. 



I. THE PROSENCEPHALON, OR FOREBRAIN. 



The fore brain of arthropods is that part of the neuron that usually lies in 

 front of the stomodaeum. In the embryos it is the anterior expansion of the 

 medullary plate called the procephalic lobes. As nearly all traces of mesoderm 

 and appendages have disappeared from this region, there is but little evidence 

 accessible to indicate the presence there of metameres. In many arthropods 

 the lobes are divided into three main divisions, with no recognizable separation, 

 at any time, between them and the postoral sections of the nerve cords; hence we 



Kt: 



FIG. 43. Sagittal section of a young scorpion. 



may, for the present, regard the main divisions as greatly reduced metameres, 

 and the central portions as neuromeres. 



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Acilius. The structure of the procephalic lobes is best seen in the embryos 

 of those insects which lead an active larval existence, as for example in Acilius. 

 (Fig. 14.) Here they are divided transversely into three similar parts, which 

 probably represent all there is left of three procephalic metameres. Each meta- 

 mere is also divided into three parts: a. a median one, representing a forebrain 

 neuromere, corresponding to the postoral neuromeres; b. a middle part, repre- 

 senting a segment of the optic ganglion; and c. a lateral one, forming a segment of 

 the optic plate, each plate containing two ocelli. Between each segment of the 

 optic ganglion and the optic plate is a deep infolding, w. 1 " 3 , which later closes, 

 covering up the optic ganglia, but leaving the ocelli and neuromeres in their 

 original position. 



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