18381.] The Brain of the Locust. 289 
B. The small ganglion cells apparently differ chiefly in size from 
the large ones, and are most numerous in the front swelling of 
each hemisphere; they surround and fill the calices of the mush- 
room 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 gan- 
glion.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 sometimes 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 ex- 
odermal layer. 
1. The medullary or inner part of the brain consists of matter 
which remains whiteor unstainedafter the preparation has remained 
thoroughly 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 mat- 
ter, 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 “ sarksubstanz,’ and is described by Newton as 
_ “a peculiar arrangement of nervous matter, which appgars some- 
times as fine fibrillae, with an axial arrangement, sometimes as a 
very fine net-work of different thicknesses, and sometimes as thin 
lamellz, or altogether homogeneous.” 
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, Pl. 1, I have seen the fibers pass- 
ing 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 and ocellar nerves and pos- 
terior commissures originate from the large ganglion cells which 
in this animal are situated in or near the center of the brain. In 
the last abdominal ganglion also the fibers arising from the pe- 
ripheral ganglion cells can very plainly be seen passing in towards 
