THE PROSENCEPHALON. 



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The cerebral hemispheres arise as mushroom-like expansions of the 

 second neuromere. In Limulus, they are very conspicuous in the early stages, and 

 ultimately grow to an enormous size. They consist of dense masses of minute cells, 

 with deeply stained nuclei, unlike any others in the nervous system. (Figs. 37 and 38.) 



As these cells multiply, the hemispheres project above the surface of the 

 brain and then mushroom, forming large, overhanging lobes. We may dis- 

 tinguish anterior, lateral, and posterior lobes, the latter being much the largest. 

 In addition, there is a large lobe on the median face of each hemisphere. (Figs. 

 47, B, 48 and 49, g.c.) The hemispheres, throughout life, are connected with the 

 neural surface of the second neuromere by a thick, vertical stalk, or peduncle, 

 composed of nerve fibers. 



As the hemispheres increase in volume, the 

 posterior lobe completely overlaps the third neu- 

 romere, and the lateral and anterior lobes partly 

 en\-elop the haemal surface. In the adult, the Rm 

 hemispheres are irregularly convoluted, and 



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their median faces are flattened against each 

 other so that they form a large spherical mass 

 that has a striking resemblance, in external form, 

 to the hemispheres of vertebrates. (Figs. 38-48.) 



In the scorpion, the hemispheres are much 

 smaller than in Limulus and are crowded farther 

 forward by the optic ganglia, which have almost 

 united in the median line behind them. Later 

 the whole prosencephalon is bent toward the 

 haemal surface, through an angle of something 

 more than 90°. (Fig. 47, A.) When the fore- 

 brain flexure is completed, about the time of 

 hatching, the hemispheres lie on the anterior 

 haemal surface of the procephalon. (Figs. 42 

 and 43, c.h. or h.) This flexure is very marked 

 in all arachnids, so far as known, except in 

 Limulus. 



The third neuromere undergoes very little change. It may be recognized for 

 a considerable period as a separate neuromere whose neural surface is covered 

 with tufts of large ganglion cells. It is gradually incorporated into the thick 

 mass of tissue that constitutes the body of the forebrain commissures, and upon 

 which the hemispheres rest (basal ganglia). (Fig. 46.) 



The history of the procephalic sense organs and their nerves and ganglia 

 will be considered under their appropriate heads. 



II. The Diencephalon. 

 The diencephalon in arthropods consists of a variable number of neuromeres 

 surrounding the mouth. The first neuromere following the procephalic lobes 

 (antennal neuromere of insects, cheliceral neuromere of arachnids) may be re- 



FlG. 45. — Diagram of the arachnid 

 brain, showing the number and grouping 

 of the neuromeres, the ventricles, the vagus 

 lobes, and the longitudinal gustatory 

 tracts and their relation to the stomodaeal 

 ganglia. 



