400 The Study of Zodlogy in Germany. [July, 
the inside or right hand one in the cut, being slightly colored by 
carmine, while the outside layer is hardly stained at all. This 
affords another excellent illustration of the ease with which val- 
uable discoveries may be made, when well-known histological 
methods are applied to the study of insects ; indeed, insects offer 
a rich and easily accessible field of research, promising perhaps 
greater rewards in proportion to the necessary labor than almos 
any other department of zodlogical investigation. 
It would be easy to add illustration after illustration to those 
already given, but it is not our purpose to review the progress 
of histology, but merely to give incentives to work in that field. 
We pass on, therefore, to a few additional considerations on the 
“ technique ” of preparing tissues for microscopical examination. 
Experience has shown that it is very difficult to distinguish the 
single cells in sections, in some case almost or quite impossible ; 
or it is even impossible occasionally to make any sections at 
all. On these accounts various means are employed either to 
isolate a few cells or to mark the outlines of them. + The meth- 
ods hitherto employed for these purposes are few in number and 
limited in application, but they have already led to interesting 
observations. 
Many cavities of the body, both of vertebrates and lower ani- 
mals, are lined by a layer of flat cells that are separated by 
lines of intercellular substance; by treating such a surface suita- 
bly with certain silver salts the intercellular lines are colored 
dark brown or black. A solution of one part of nitrate of silver 
in five hundred parts of distilled water (by weight) is very 
convenient. It gives beautiful preparations when applied to 
the mesentery of a rabbit, for example. The mesentery is the 
thin membrane by which the intestine is suspended from the 
back of the abdomen. Cut out a small piece from a freshly 
killed animal, a frog or rabbit or any other vertebrate, and place 
it in a silver solution, where the direct rays of the sun can fall 
upon it, and move it about with a glass rod (metal would be 
corroded) so that all parts may be equally acted upon ; next re- 
move it for a moment into distilled water to wash off the silver, 
and then spread it out on a glass slide and let it dry almost com- 
pletely, taking great pains to stretch it out by pulling it at va- 
rious points so that it shall dry fully extended. Before 1t 18 — 
quite dry put on a drop of glycerine and a thin glass cover M 
the usual way. If the impregnation has been successful, the lines 
will appear very sharply, as in Figure 74, which is from the mes- 
