SUMMARY. 



335 



The condition in Limulus may be compared with the early larval stages in 

 the development of the lemmatochord in Cecropia, where the lemmatochord tissue 

 completely surrounds the cord. 



IV. SUMMARY AND COMPARISON. 



1. The middle cord of arthropods is of the same fundamental importance 

 in the morphology of segmented animals as the lateral nerve cord. 



2. In the arthropods we may recognize two parts in the middle cord, viz. 

 the intra- and interganglionic segments. The intraganglionic segments are 

 located in the central portion of the neuromeres, on the neural side of the haemal 

 cross commissures. They may give rise in each neuromere to both ganglion 

 cells and neuroglia cells, and they may persist a longer or shorter period as solid 

 cords, or as epithelial canals representing the walls of the original median groove. 



A 





i.^.^ f .g$$!& 



' .' y '.'. t,'.:.'. 1 .-'* ' "." ! 



bc^-JKr-~~--Tnck. 



D 



FIG. 231. Diagrams to illustrate the mode of growth of the axial organs in the arachnids; such as the lateral 

 nerve cords, their infolding, the overgrowth of the ectoderm, and the formation of the cross commissures; the local 

 modifications of the middle chord to form the neuroglia, the lining of the central canal, blood cells, and lemmato- 

 chord; and the origin of nerve cords, ectoderm, and germ cells from the primitive streak. A, Sagittal section of 

 the axial oigans; B, C, and D, cross-sections in front of the apex of the caudal lobe, showing the separation of the 

 nerve cords and germ cells from the tissue of the primitive streak; E, section farther forward, through the middle 

 of a neuromere; F, between two neuromeres; G, still farther forward, through the middle of a neuromere; H. same, 

 between two neuromeres. 



The interganglionic segments of the middle cord gave rise primarily to 

 the median nerve and its neurilemma. They undergo various local modifications. 

 In the insects (Acilius), the oral segments disappear; those in the 'eg segments may 

 produce the chiten-lined supports, or furcae, which are permanently connected 

 with the ectoderm, and which serve for the attachment of muscles; in the abdom- 

 inal region, the same kind of infoldings are formed, but they separate from the 

 ectoderm at an early period and are converted bodily into the median nerve. 

 In lepidoptera, during the metamorphosis, the neurilemmas of the median and 

 of the lateral nerve cords become enormously enlarged and form a semi-cylin- 



