238 REPORT UNITED STATES ENTOMOLOGICAL COMMISSION. 
As tbe fibers of the adult brain are evidently secondary products, it 
would appear that they must be transformed granules or nuclei, and 
not in all cases, at least, the fibers thrown off from the ganglion cells, 
although at this time the ganglion cells have no fibers, the fibers of 
those seen in the adult brain being also secondary growths. It may be 
that the white inner granulo -fibrous matter of the adult brain is (1) 
made up of modified granules, which in some cases remain such, and in 
others form libers, and (2) of fibers sent in from the cortical ganglion 
cells. 
Comparison of the brain at this stage with the first thoracic ganglion. — 
If we compare at this stage of development of the nervous system the 
brain with one of the ganglia of the trunk, we shall obtain a fair idea 
of the primitive difference between the brain and one of the ordinary 
ganglia (PI. XIII, Fig. 5). By a glance at the figures of the two it will 
be seen that the organization of the thoracic ganglion is essentially 
simple. It is divided into two portions or regions. The central granular 
region is enveloped by a thick stratum of cortical ganglion cells. The 
whole ganglion in section is rudely hour-glass-shaped and much smaller 
than the brain. There is no differentiation into distinct lobes as in the 
brain. The formation of the brain, as is well known by embryologists, 
is one of the earliest steps in the development of the nervous system, 
the entire system being at an early date in the life of the embryo set 
apart from the epidermis or integument, the latter with the nervous 
system originating from the ectoderm or outer germ-layer. 
Second embryonic stage, B (PI. XII, Figs. 2-9). — In embryos more ad- 
vanced, and just ready to hatch, the eyes being now dark red, the cen- 
tral body is formed, but our sections do not show any traces of a mush- 
room body. The sections are frontal, and we will describe them in 
order. The fifth section is through the head and front part of the eyes, 
but does not graze the brain itself. Fig. 2 shows the structure of the 
interior of the head, being filled with connective tissue cells not dis- 
tinguishable from the ganglion cells. 
Section G (Fig. 3) passes through the outer portion of the optic and 
antennal lobes, now clearly differentiated. 
In section 7 the cerebral lobes are seen, and in section 8 are larger, as 
are the optic lobes, while the antennal lobes are somewhat reduced in 
size. Section 10 passes through the cerebral lobes and also grazes the 
optic lobes, passing through the optic ganglion. 
Section 11 (Fig. 8) shows the central body, separated from the upper 
cerebral lobes by a thin layer of loose ganglionic cells. The relation of 
the central body to the upper and lower cerebral lobes is well shown in 
this section. 
Plate XIII, Fig. 4, shows the relation of these and their structure 
greatly enlarged. Through the granular substance of the lobes are 
sparsely scattered ganglionic cells. 
Section 12 passes through the lower cerebral kobes and the upper left 
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