226 REPORT UNITED STATES ENTOMOLOGICAL COMMISSION. 
the larva and pupal stages before attaining the fully developed structure 
of tbc adult locust. 
THE BRAIN OF THE ADULT LOCUST. 
The method of examination employed by us has been to cut the brain 
into a number of thin sections by means of the microtome, previously 
hardening the tissues in alcohol. The labor of cutting these sections 
has been performed by Mr. Norman X. Mason, of Providence, R. I., who 
has brought to his work an unusual degree of .skill and care in prepar- 
ing such delicate sections. In all these sections the brain was not pre- 
viously removed from the head, but the entire head was cut through, 
having previously been hardened in absolute alcohol for twenty-four 
hours or more, and then soaked in gum arabic for one or two or more 
days. The objects were then embedded in a preparation of paraffine and 
sweet-oil and wax, or, in some cases, in soap and oil. After the sections 
were cut they were stained with picrocarmine, or partially stained with 
osmic acid, and then treated with picrocarmine. Finally the slices were 
mounted in glycerine jelly for study under the microscope. 
The sections were in most cases frontal ones, namely, cut transversely 
from the front of the head or brain backwards, while a few were longi- . 
tudinal or vertical ones, viz, cut parallel to the median line of the body. 
They were either or of an inch in thickness. 
It should be observed that the brain is divided by a furrow into two 
halves or hemispheres ; these are deepest above and below, and the 
upper and lower portions may be called, respectively, the frontal and 
posterior furrow. 
The brain is mostly surrounded by a thin delicate membrane, the neu- 
rilemma, also called by Krieger the perineurium ; it is formed of very 
dense fibrous connective tissue. 
Histological elements of the brain. — The brain is histologically or struct- 
urally divided into two kinds of tissue or cellular elements. 
I. An outer, slightly darker, usually pale grayish white portion is 
made up of " cortical cells," or " ganglion cells." (PI. XI, Fig. 3, a, ft, c, d.) 
This portion is stained red by carmine, the ganglion cells composing it 
readily taking the stain ; when thus stained by carmine, the nucleus of 
the cells is rendered quite distinct, but the cell wall is also distinct ; 
when stained by haeniatoxylin the large nuclei are remarkably distinct, 
but the cell walls are not well brought out ; when stained by osmic acid 
these cells are not so clearly shown as by a picrocarmine or carmine 
stain, and the nucleus is less distinct than when treated by the two other 
stains mentioned. 
This outer loose cellular envelope of the brain consists of large and 
small ganglion cells. Where the tissue consists of small ganglion cells, 
it is naturally from the denser arrangement of the smaller cells, which 
are packed closer together, rather darker than in those regions where the 
tissue consists of the more loosely disposed, large ganglion cells. 
