90 MINUTE STRUCTURE OF THE BRAIN AND COED OF ARACHNIDS. 



there a special condensation of commissural bundles, that have been crowded 

 together from before backward, in order to leave a large opening for the end of 

 the oesophagus. (Figs. 57 and 58.) In the vertebrates, there is a similar back- 

 ward dislocation of commissural bundles, forming a special group just back of the 

 choroid plexus of the fourth ventricle, "commissurainfirma." Johnston says (p. 287) 

 that "Behind the choroid plexus the c. infirma contains the visceral, sensory 

 elements proper to the segments of the VII, IX and X nerves. It is probable that the 

 course of the root fibers of these nerves within the brain has been influenced by the 

 crowding backward of their decussation and median nucleus by the choroid 

 plexus." The choroid plexus could hardly have the power to dislocate the 

 cerebral framework. The dislocation was probably brought about, as in Limulus, 

 by the backward migration of the outer end of the old stomodasum, and when 

 that closed, the choroid plexus grew over it, leaving the permanently distorted 

 commissures to testify to the event. 



h. In both classes, there is a remarkable ganglionated commissure extending 

 over the neural surface of the brain, the stomodseal commissure of arthropods 

 and the cerebellum of vertebrates. Both structures represent very primitive 

 commissural tracts, the only ones which develop, primarily, from the roof of the 

 brain chamber. Both commissures may be ganglionated, and they are the only 

 ganglionated transverse commissures in the primitive brain. Both commissures 

 develop in connection with the fourth neuromere. Both have special relations 

 with the gustatory centers on the posterior median face of the stomodaeal opening 

 (infundibulum) . 



In arachnids, this commissural arch lies behind the cerebral lobes and the 

 optic ganglia. In vertebrates, it is crowded backward by the enlarged optic 

 lobes so that part of its fibers are directed downward and forward, toward the 

 posterior wall of the diencephalon. (Figs. 3, 46, 57, 58.) 



IV. Commissures. 



Summary. — The facts, bearing on the cross commissures of arachnids, 

 that have been brought out in the preceding pages, may be summarized as follows : 



I. The right and left cords of the primitive neuron were united by a series 

 of transverse commissures, two for each neuromere. The anterior commissure 

 is primarily related to the anterior segment of the neuromere and to its peripheral 

 nerve, the other to the posterior segment, and to its nerve. The commissures 

 arise, during the early embryonic periods, as fibrous, non-cellular bands, extending 

 across the floor of the middle groove. They are separated from one another by 

 deeper infoldings of the groove. In the adult, they are still separate, and each 

 contains several distinct fiber bundles, differing in origin and in histological 

 characters. 



These commissures always retain their primitive position on the floor of the 

 neuron, hence they are called the haemal commissures. They are the primary 



