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THE MIDDLE CORD, THE LEMMATOCHORD AND THE NOTOCHORD. 



each neuromere, and close to the lateral branches of the median nerve, as a thin, 

 dark, nucleated band, extending across the haemal surface of the median nerve. 



In the young pupae the band expands very rapidly, forming in front of each 

 neuromere a transverse muscular sheet, broad over the median line and tapering 

 to a point at either side. They ultimately form a continuous sheet beneath the 

 cord. In the adult, the ends of many of the muscle fibers spread out in fanshaped 

 masses of fibers that penetrate the substance of the cord in all directions. (Fig. 

 224, B, m.) 

 II. THE MIDDLE CORD OF THE SCORPION. (Figs. 15, 16, 43, 71, 225 to 230.) 



The middle cord and associated parts are in some respects imperfectly de- 

 veloped in the scorpion, so that it is difficult to follow their local modifications; 

 but the conditions appear to be essentially the same as those we have described in 

 embryos of Acilius, and in the larvae of Cecropia. 



In the scorpion, the median nerve itself is hardly recognizable; its neurilemma 

 forms in part the walls of a blood sinus. The neurilemmas of the median and 

 lateral cords form the bothroidal cord of the abdomen and the merochord of the 

 posterior thoracic neuromeres. Chiten-lined neural apodemes are absent. 



A. Neural Sinus, Merochord and Bothroidal Cord of the Adult. In the 

 adult scorpion a large blood vessel extends along the haemal side of the nerve cord, 

 exclusive of the brain. In the abdomen and tail it opens into vertical channels 



ml. 



FIG. 225. Cross-sections of the nerve cord of an adult scorpion. No. i, Section through the posterior portion 

 of the first free abdominal neuromere; No. 2, section between the vagus and abdominal neuromeres, showing the 

 very thick walls and small lumen to the neural sinus; No. 3, section just in front of the second free abdominal 

 neuromere, showing the conspicuous hamal tracts; No. 4, section through the anterior half of the last caudal 

 neuromere, showing the solid cord of cells derived from the middle cord and continuous with the neural artery; 

 No. 5, section just in front of the anterior vagus neuromeres showing the merochord. 



which, behind each neuromere, pass between the connectives to the skin on the 

 neural surface of the body. There are also in each segment two lateral branches 

 which follow pretty closely the course of the spinal nerves. Between the succes- 

 sive neuromeres the vessel is much enlarged, and is either round or triangular in 

 section. (Fig. 22 5'.) Beneath each neuromere it is much flattened and in some 

 cases hardly visible. (Fig. 225 2 .) The w r alls of the sinus consist of an inner epithe- 

 lial layer of clear cells, sharply contrasted with the dark coagulum in the sinus. 



