THE BRAIN OF THE LOCUST. 
227 
A. The large ganglion cells (PI. XI, Figs. 3, 3 a, 3 b, 3 e, 3 d } 3 e) are 
oval, and send off usually a single nerve fiber ; they have a thin fibrous 
cell wall, and the contents are finely granular. The nucleus is very large, 
often one-half the diameter of the entire cell, and is composed of large 
round refractive granules, usually concealing the nucleolus (the granules 
are much larger and fewer in number and the nucleolus is less distinct 
than in the brain of Limulus, the king crab). These large ganglion cells 
are most abundant and largest on each side of the upper furrow, and in 
front of the "ceTitral body," also at the bottom of the lower furrow, 
and along the exterior of the optic and antennal lobes, and along the 
commissural lobes. 
B. The small ganglion cells apparently differ chiefly in size from the 
large ones, and are most numerous in the front swelling of each hemi- 
sphere ; they surround and fill the calices of the mushroom bodies, and 
they extend along each optic nerve and form a large proportion of each 
optic ganglion, especially the layer next to the retina of the eye, though 
they are replaced by large ganglion cells at the junction of the fibrous 
part of the optic nerve with the dilated granular portion. The brain 
is surrounded more or less completely by the connective tissue cells 
belonging to the mesoderm or middle germ layer, and which are some- 
times liable to be confounded with the ganglion cells, as they stain the 
same tint with carmine. It should be borne in mind that the nervous 
system, ganglia and nerves, originate from the tegumental or exodermal 
layer. 
II. The medullary or inner part of the brain consists of matter which 
remains white or unstained after the preparation has remained thor- 
oughly exposed to the action of the carmine. It consists of minute 
granules and interlacing fibers. The latter often forms a fine irregular 
net- work inclosing masses of finely granulated nerve matter. 
In the antennal and commissural lobes is a third kind of matter, in 
addition to the granular and fibrous substances, which forms irregularly 
rounded masses, cream -colored in picro-carmine preparations, and which 
stain dark with osmic acid. This is called by Dietl u marlcsubstanz^ 
and is described by Newton as " a peculiar arrangement of nervous mat- 
ter, which appears sometimes as fine fibrillar, with an axial arrangement, 
sometimes as a very fine net-work of different thicknesses, and some- 
times as thin lamella?, or altogether homogeneous. Under all these 
forms this third group of textures is characterized by turning very dark 
under the influence of osmic acid, whilst the other elements are only 
turned brown." 
It is to be noticed that this central unstained portion contains few, if 
any, ganglion cells, and it is most probable that the fibers of which it is 
composed originate from the cortical ganglion cells. At one or two 
points at Fig. 3, PI. XI, I have seen the fibers passing in from ganglion 
cells towards the middle of the brain. In the horseshoe crab (Limulus), 
owing to the simple structure of the brain, it is evident that the optic 
