The adult organisation of Paragordius varius. 407 
are nerve cells found on the dorsal side of this tract, while they are 
most abundant at its ventro-lateral angles. 
2) On each side of the preceding is a lateral tract, which shows 
three not very sharply demarcated subdivisions: a) a dorsal tract 
(D. T Figs. 22, 24, Pl. 38; Fig. 26, Pl. 39) composed mainly of chromo- 
phobic fibres, which are here more densely grouped than elsewhere in 
the cord; this is also the smallest tract. b) A latero-ventral tract 
(L. V.T Figs. 22, 24, 26, Pl. 39) which is bounded peripherally by 
the lateral layer of chromophobic nerve cells; this is composed mainly 
of longitudinally coursing chromophilic fibres. c) A medio-ventral 
tract (M. V. T Figs. 22, 24, 26), larger than the preceding and placed 
between it and the median tract; it contains chromophilic fibres cour- 
sing in all directions but mainly longitudinally, and chromophobic nerve 
fibres ; the nerve fibres are generally less abundant in this tract than 
in any of the others. 
Horizontal longitudinal sections (Fig. 32, Pl. 39) of the nerve 
cord show: the median tract (JZ. 7’) with longitudinally directed fibres ; 
on each side of this a tract with finer chromophilic fibres and with 
more numerous chromophobic fibres, the medio-ventral tract (M. V. T), 
which appears the least deeply stained of all the tracts on account 
of its having the fewest fibres; on the outer side of each of the latter 
tracts the latero-ventral tracts (LZ. V.7’), with large chromophilic fibres 
running longitudinally; and outermost on the sides of the nerve cord 
the nerve cell layer (chromophobic nerve cells, Chb. C). 
The nerve cells being on ‘the periphery of the cord send their 
fibres radially inwards. The “dotted substance” (Punktsubstanz, LEYDIG) 
of the central nervous system is thus not a network of fibres nor a 
nuclear reticulum, but is composed of the fibres of two kinds of nerve 
cells. The chromophobic nerve cells seem to lie in clear areas since 
the chromophilic fibres are least numerous around them. The nerve 
cord has no peripheral envelope (no neurilemma) of its own, but is 
immediately invested by the small-celled parenchym; external to the 
peripheral layer of nerve cells is a sheet of chromophilic fibres. 
Transverse commissures. At intervals along the nerve 
cord are found transverse commissures of fibres (both chromophobic 
and chromophilic), extending from the dorso-lateral angle of one side 
to that of the other (N.C. Com Fig. 26, Pl. 39); the cells from which 
most of these fibres arise are placed at the corresponding angles of 
the cord. The distances between these commissures as determined 
on a series of sections of the anterior trunk region were found to be 
Zool, Jahrb. XVIII. Abth. f. Morph. 27 
