326 THE MIDDLE CORD, THE LEMMATOCHORD AND THE NOTOCHORD. 



We may conclude from the wide distribution of the middle cord in the arthro- 

 pods, and from the important part it plays in the early embryonic stages: i. 

 That the central nervous system of primitive arthropods consisted of three par- 

 allel, longitudinal bands that were divided into similar segments; 2. that the 

 neuromeres of the middle cord fused with those of the lateral cords, and that the 

 longitudinal connectives remained separate; and 3. that there has been a pro- 

 gressive degeneration, or modification, or specialization, of the intra- and inter- 

 ganglionic segments of the middle cord from the head end backward into 

 non-nervous structures; while the lateral cords have increased in the specializa- 

 tion of nervous tissues from behind forward. 



The Lemmatochord of Lepidoptera (Figs. 223, 224). — The lemmatochord 

 of lepidoptera, or Leydig's cord, is a large, irregular, cyhndrical rod of elastic, 

 semi-gelatinous tissue extending along the hjemal side of the nerve cord from the 

 thoracic neuromeres to the posterior end of the cord. It serves as a support to 

 the nerve cord and for the attachment of lateral sheets of muscle fibers, the ends 

 of which are imbedded in the substance of the cord. (Fig. 224, B.) It resembles 

 the notochord of vertebrates in its position, its consistency and general histological 

 structure; and in its function. Morphologically it represents the interganglionic 

 segments of the abdominal middle cord, enveloped in the thickened neurilemma 

 of the median nerve and that of the adjacent lateral cords. It makes its appear- 

 ance during the metamorphosis of the larvae into the imago. 



The following observations refer to the development of the lemmatochord of 

 Cecropia and Sphinx. 



At the close of the larval period, the lateral and median cords are surrounded 

 by two membranes, an inner, distinctly cellular layer, i.sk., and an outer one, o.sh., 

 that forms a thick hyaline membrane. During the early pupal stages, large 

 polygonal, or oval cells, with clear protoplasm and small nuclei, make their ap- 

 pearance between the inner membrane and the nerve cord. (Fig. 223, A.) In 

 some places they are isolated and imbedded in a darker plasma. In others they 

 are crowded together and appear to have thick, but not sharply defined walls. 



At this time the outer hyaline membrane, o.sh., also increases greatly in thick- 

 ness and becomes distinctly laminated. Here and there small flattened nuclei are 

 seen, and in some places clusters of thick-walled chorda cells, B. These changes 

 take place in the membranes surrounding the ganglia, the longitudinal connectives, 

 and the median nerve. 



During the subsequent period, the distinction between the two investing mem- 

 branes disappears, owing to the conversion of the flattened cells of the laminated 

 membrane into the characteristic, thick-walled chorda cells, and to its invasion 

 by chorda cells formed from the inner membrane. The chorda cells develop 

 most rapidly along the haemal surface of the lateral cords, and around the median 

 nerve; they thus form three large irregular bands of chorda cells, roughly tri- 



