OF OECANTIIUS AND TELEAS. 



253 



those of the cross commissures and arise from the ganglionic cells of the lateral cords. In 

 longitudinal sections of the cord (pi. 22, fig. 1) one finds the longitudinal commissures 

 extending; as unbroken bundles of fibres from the brain to the last abdominal segment. 

 The portion in the brain is of two or three times the size of that in the hind abdominal 

 region. In transverse sections (pi. 23, figs. 1 and 2) it forms a varying proportion of the 

 cord, dependent upon the part of the ganglion through which the section passes as well as 

 upon the region of the body from which it is taken. During its passage through each 

 ganglion it suffers a slight enlargement, which is augmented by the decussation of fibres 

 from the adjacent surfaces of the two long commissures through the cross commissures. 

 The fibres of the longitudinal and cross commissures remain distinct from one another 

 until cpiite late in embryonic life, when the peripheral fibres of the long commissures 

 become Avoven among some of the fibres of the cross commissures. (PI. 22, fig. fi.) 



The brain (pi. 19, figs. 6, 10 ; pi. 20, figs. 22 and 23) is developed as two separate kid- 

 ney-shaped lobes from the internal cell mass of the ectoderm of the head folds, and hence 

 corresponds in its origin to the lateral cords of the thoracic and abdominal regions, but the 

 invaginated median element is lacking here, and the union of the two lobes is on this account 

 accomplished only at a very late date. The union of the brain with the ventral nerve 

 cord is accomplished shortly before revolution. The posterior portion of each lobe is pro- 

 longed backward for a short distance and unites with the anterior ends of the lateral cords 

 which are prolonged as far as the upper border of the oesophageal opening. The union of 

 the two lobes of the brain is accomplished by the coalescence of outgrowths from their 

 median surfaces near the posterior end of either lobe. In cross sections of the brain 

 (pi. 22, fig. 1 ; pi. 23, figs. 1, 3, 6, 15) the fibres are shown to be limited to the central 

 portion of the mass of ganglionic cells, while in the ventral cord they lie dorsad to 

 the centre. The most of the fibres in the brain appear to form concentric layers, the 

 remainder curving about in all directions, giving the fibrous mass the appearance of a felt 

 work in which a few of the fibres are larger and more sharply defined than the others. In 



the ventral ganglia the fibres are arranged principally in 

 the direction of either the long or the cross commissures. 

 The fibres of each cross commissure are collected into two 1 

 bundles more or less closely united, depending upon the 

 development of the embryo, while those of the long com- 

 missures remain permanently distinct from one another. 

 The nerves of the adult insect are, in their basal portions 

 at least, simple outgrowths of the fibres of the cross 

 commissures, each with a sheath of ganglionic cells. At the 

 time of the closure of the body wall, they are finger-like pro- 

 cesses and they, like the outgrowing ocellar and antennal 

 nerves, project in pairs from either side of the ganglia. 

 (PI. 20, figs. 22 and 23.) The ganglionic cells give rise to 

 the fibrous portion of the nervous system, probably by the 

 prolongation of their cell walls into filaments. PI. 22, fig. 

 2, shows the nucleus of a single ganglionic cell, from the 

 periphery of which fine filaments radiate and pass into the 

 1 Compare Bobretz];y (G). 



Fig. 28. 



Ventral nerve cord and other structures as 

 seen from abeve. The ganglia are connected 

 by a mass of cells below the fibres, /.c.of the 

 figure. The 1. com. probably contains most of 

 the commissural fibres. The ganglia are 

 entirely covered by a reticulum within the 

 meshes of which are seen fat bodies. Along 

 the median dorsal line there is seen a del- 

 icate tube containing a non-corpuscular 

 fluid. Six tracheal branches extend over the 

 surface of the ganglion. On cither side of 

 which are seen the lateral blood sinuses. From 

 a living embryo. X 400. 



