THE MESENCEPHALON. 65 



cranial flexure, and marks the anterior termination of the notochord. The 

 chamber lying between and above these neuromeres (the third ventricle) was 

 for a certain period an extension of the stomodaeum, or an antechamber to that por- 

 tion that actually perforates the brain floor. The final closing of the entrance of 

 this antechamber took place probably at a point just back of the cerebellum 

 (rhomboidal fissure). (Figs. 3, 46, 58.) 



III. THE MESENCEPHALON. 



In typical arachnids there are six thoracic neuromeres. The first one (two or 

 three) (cheliceral neuromere) has already been described as the tween-brain, the 

 remaining ones constitute the midbrain. They differ from all other neuromeres 

 in their great breadth, and in the enormous size of their ganglia and gustatory 

 nerves. 



C 



FIG. 53. Longitudinal, horizontal sections of the head of an embryo scorpion, showing origin of the lateral 

 stomodaeal ganglion. A, At the level of the suprastomodaeal commissure; B, at a lower level, showing the inner 

 end of the lateral stomodaeal ganglion; C, still deeper level, showing its origin from the sides of the stomodaeum. 

 Camera outlines. 



In the scorpion, they rapidly increase in size with the growth of the append- 

 ages, forming a compact group in which the original segmentation is clearly in- 

 dicated by the arrangement of the nerves and cross commissures. They are 

 never so completely fused in the adult as to lose their identity, differing in this 

 respect from those in the forebrain in front of them, and in the vagus region 

 behind. 



In the adult scorpion, the cross commissures are very short, and the thick 

 cords, or crura, are but slightly separated, leaving a very small opening for the 

 passage of the oesophagus. (Figs. 40-43.) 



In Limulus, this opening is larger, the anterior ends of the crura are widely 

 separated, and the elongated anterior commissures are bent into wide loops by the 

 backward movements of the cesophagus. (Figs. 38-39.) These conditions, and 

 the absence of a tween-brain flexure, give the brain of Limulus a different out- 

 ward appearance from that of the scorpion, although structurally they are very 

 much alike. 



The great size of the neuromeres and the divergence of the crura make this 

 the broadest and most voluminous part of the brain, giving it a rhomboidal out- 

 line, when seen from above. 

 5 



