48 EDGAR J. ALLEN. 
in the other similar cases in the abdominal ganglia, that they 
appear as one. 
The element S (figs. 2 and 3) is similar to the element S 
already described in the thorax (Th. x1, fig. 1). The cell, 
which is very small, lies near the centre of the ganglion and 
at the ventral surface. The fibre runs first upwards, back- 
wards, and outwards, and then curves forwards and inwards. 
After running for some distance in this direction it decussates 
with its fellow of the opposite side, the two fibres lying for a 
short distance close together. The fibre then turns forwards 
and outwards, gives off an arborescent branch to the neuropile, 
and then proceeds forwards in a straight line to the ganglion 
in front, where it passes out by the middle root of the three 
which spring from that ganglion. 
From the foregoing description of the motor elements found 
in the abdomen, it will be noticed that they, in nearly every 
case, supply fibres to the posterior nerve-roots of the ganglia, 
whilst the greater number of those described for the thorax 
in this and the previous paper send their fibres to the anterior 
nerve-roots, a few only supplying the posterior roots. The 
probable reason for this difference is not difficult to find. 
The anterior roots chiefly supply the appendages, which are 
well developed in the thorax of the embryo, whilst in the 
abdomen they are wanting. The posterior roots, on the other 
hand, supply the muscles of the body itself, which are well 
developed in the abdomen, but less so in the thorax owing to 
the considerable space occupied by the still unabsorbed yolk. 
III. ELements Arisinc FRoM CELLS OUTSIDE THE 
CENTRAL Nervous SystEmM. 
In Part I a number of elements were described, arising from 
cells which lay in the ectoderm of the dorsal surface of the 
abdomen. These cells give off fibres which enter the abdo- 
minal ganglia and there bifurcate in the Y-shaped figure, 
which is characteristic of sensory nerve-fibres in all divisions 
of the animal kingdom, sending one branch forwards and the 
