100 EMBRYOLOGY OF INSECTS AND MYRIAPODS 



intraganglionic sections of the median cord contribute at least in large 

 part, if not all, the median portions of the ganglia, including the transverse 

 commissures. To this opinion, however, Wheeler (1893) took exception. 

 This investigator, although admitting that in Xiphidium the interseg- 

 mental regions of the median cord, the progeny of the median neuro- 

 blasts, are taken up into the central portions of the ganglia to form 

 functional gangHon cells, did not beUeve that the median cord cells in 

 the intrasegmental regions became ganglion cells but maintained that 

 they are used up in the formation of the neurilemma. In the same 

 group to which Xiphidium belongs, the Orthoptera, Heymons (1895a) 

 found that the anterior and central median gangliomeres were actually 

 formed by the median cord, much as in other insects. With this excep- 

 tion, the differences in regard to the development of the median cord 

 center principally about the fate of the intersegmental (interganglionic) 

 sections. Hatschek (1877) stated that these remained in connection 

 with the ectoderm and contributed nothing to the ganglia. 



Graber (1890) found that in Melolontha the median cord was sepa- 

 rated from the epidermis throughout its entire length but that the inter- 

 segmental portions later divide transversely, each half being then drawn 

 cephalad and caudad, respectively, into the ganglia adjoining. In 

 Hydrophilus, Lina, and Stenohothrus, on the other hand, the median cord 

 was not observed to separate from the epidermis. In Lepisma Heymons 

 (1897) found that a continuous median cord was set free from the 

 epidermis and present in the newly hatched insect, extending the entire 

 length of the ventral cord. The findings of Carriere and Burger (1898) 

 with reference to the fate of the median cord essentially confirm those 

 of Heymons (1895a) except that the former do not prove that the 

 roof of the neural groove becomes split up into a dermatogenic and 

 neurogenic layer. According to their observations all its cell material 

 goes to form the median cord, and its covering is produced by the union 

 of the epidermis formed in the region of the primitive swellings. The 

 complete sundering of the median cord from the epidermis takes place 

 about the end of development. Its intraganglionic portions separate 

 from the epidermis at about the same time as the delamination of the 

 lateral cords; its interganglionic portions remain in connection with it, 

 after the nerve fibres have become evident in the lateral cords. In Musca 

 Escherich (1902) found that a continuous cord is separated from the 

 ectoderm. Within the limits of the ganghon, in this insect, the median 

 cord contributes the median portions, as in other insects; in the inter- 

 ganglionic regions it presents swellings of considerable size, one being 

 situated directly caudad of each ganglion. From each swelling, near its 

 posterior end, a pair of lateral processes regarded as nerves are given off 

 which extend to the neighborhood of the stigmata. This indicates that 



