THE MIDDLE CORD OF INSECTS. 



325 



the posterior margin of the neuromere, A 5 , the middle cord is still a solid tri- 

 angular block of cells, with its deep lateral angles spreading outward underneath 

 the longitudinal connectives. Still farther back, A Q , the middle cord is again 

 a deep groove, open at the surface, and with thin lateral walls. The sections show 

 that the floor of the intra-ganglionic middle cord is raised over the cross com- 

 missures, and that the cells at the deep angles of the middle cord are spreading 

 forward and backward around the cross commissures and longitudinal connec- 

 tives. These cells form the so-called inner neurilemma, or neuroglia. There is 

 nothing to indicate that the cross commissures arise from the intraganglionic cells 

 of the middle cord, as is claimed by many authors. They appear to be out- 

 growths from the ganglion cells of the lateral nerve cords. 



m.eH. 



FIG. 222. Nerve cord of an embryo of Acilius. A , '- fi , serial sections of an abdominal neuromere showing a later 

 stage in the development of the neuroglia and middle cord (median nerve) ; B l , inter- ganglionic infolding of middle 

 cord of the thorax in a newly hatched larva of Acilius, showing modifications of middle cord for the attachment 

 of muscles; B-, section in same stage of an inter-ganglionic space of the abdomen, showing the middle cord as the 

 ganglion of the median nerve. 



In one of the next stages, a series of sections beginning at the anterior margin 

 of the second pedal neuromere, shows that the middle cord has now lost its con- 

 nection with the surface ectoderm. (Fig. 222.). In the center of the neuromere, 

 A 4 , its cells have multiplied and form a thick neuroglia investment around the 

 cross and longitudinal connectives. At the posterior end of the neuromere, A 6 , 

 the middle cord assumes its characteristic cell structure, and gradually merges 

 into the interganglionic cord. 



In the later stages, at the time of hatching, the interganglionic segments of 

 the thoracic middle cord are still in connection with the ectoderm, as they are 

 through life, and wing-like bundles of muscles are attached to their sides, B l . 

 The interganglionic segments in the abdomen become completely separated from 

 the ectoderm, forming a cylinder of nerve cells like those of the lateral cords, B 2 . 

 At each end the cord merges into the central tissues of the neuromeres. The 

 chain of abdominal interganglionic segments of the middle cord may now be 

 recognized as the median nerve of the adult. 



Thus the thoracic furcae for the attachment of muscles, the median nerve, 

 and the neuroglia are but different stages, or modifications, of a single structure. 



