THE COMMISSURES. 91 



communicating paths between the right and left cords, and between the right and 

 left peripheral nerves. 



During the later stages, the cords increase in thickness, the median in- 

 folding forms a deep fissure, and then three new commissures appear in each 

 neuromere, extending across the fissure above the old ones. These secondary, or 

 neural commissures, consist largely of association fibers. 



With the formation of the secondary commissures the bottom of the fissure, 

 at certain points in each neuromere, is converted into a canal, " canalis centralis" 

 (Figs. 55, 56, 68, 69, c.c.), that is lined in the earlier stages with an epithelium 

 derived from a part of the original median infolding. (Figs. 222, 224 and 231.) 

 The floor of the canal is formed in part by the haemal commissures, and the roof 

 is formed in part by the neural commissures. When the neuromeres are widely 

 separated, there are of course wide gaps in the roof and floor of the canal between 

 the commissures in front and those behind. In the vagus region, owing partly 

 to the increased thickness of the crura, the canal is greatly enlarged, marking the 

 beginning of a chamber comparable with the fourth ventricle or metenccele. 

 (Figs. 55 and 56). Here the neuromeres are more closely united than elsewhere, 

 and their neural commissures, together with some of those belonging to the more 

 anterior neuromeres that have been crowded backward into this territory by the 

 oesophagus, form a special group over the posterior part of the region of the 

 fourth ventricle. (Fig. 47, C.) 



In the scorpion also, the immense vagal lobes are united by two special neural 

 commissures. (Fig. 47, A and 69.) 



The combined neural commissures of the vagal neuromeres of the arachnids, 

 and the more posterior thoracic ones, consist largely of somatic sensory associa- 

 tion fibers; they probably represent the beginning of the commissura infima of 

 vertebrates. 



In the arachnids there is a wide gap between the forebrain and midbrain, 

 where there are no primitive commissures. This opening, or infundibulum, is the 

 passageway for the old oesophagus. In front of it, the character of the commis- 

 sures changes greatly, in both vertebrates and arachnids. There are no neural 

 commissures, and the haemal ones form practically a single, but very complex 

 mass of fibers. (Fig. 48.) In it we may recognize the olfactory commissure, 

 representing the commissure of the first cerebral neuromere, and lying, morphologi- 

 cally, at the anterior end of the nervous system. Owing, however, to the in- 

 ward and backward migration of the lobes, it lies, in the adult Limulus and 

 scorpion, on the posterior haemal surface of the forebrain. Fig. 47 A and B. 

 The forebrain commissure also contains the commissures of the second and third 

 neuromeres, and of the lateral eyes; but these commissures, which arise at an 

 extremely early period, are not separated into distinct bundles. (Fig. 48.) 



In Apus, the ganglia of the median ocelli unite with each other in the middle 



