104 Histology and Embryology. By C. S. Minot. 
carmine, while the outside layer is hardly stained at all. This 
affords another excellent illustration of the ease with which valuable 
discoveries may he made, when well-known histological methods 
are apphed 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 labour than almost any other 
department of zoological investigation. 
It would be easy to add illustration after illustration to those 
already given, hut it is not our purpose to review the progress of 
histology, but merely to give incentives to w r ork 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 methods hitherto 
employed for these purposes are few in number and limited in 
application, hut 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 suitably with 
certain silver salts the intercellular lines are coloured 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 remove 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 completely, taking great pains to stretch it out by 
pulling it at various points so that it shall dry fully extended. 
.Before it is quite dry put on a drop of glycerine and a thin glass 
cover in the usual way. If the impregnation has been successful, 
the lines will appear very sharply. If the impregnation was not 
sufficient the lines do not appear, but that is also the case if it has 
been too prolonged, for then the cells fall off altogether. The 
membrane may be coloured with hfematoxiline or carmine, if so 
desired, after impregnation, and then the stained nuclei appear 
within the dark outlines, making exceedingly pretty preparations. 
Maceration gives the means of isolating layers of cells. If the 
skin of an amphibian, a toad, for example, be pinned out on a bit of 
