105 
Histology and Embryology. By C. S. Minot. 
cork and then placed in a disk of water containing three or four 
drops of strong carbolic acid to prevent the development of germs, 
and then left for a day or two, the superficial layer of cells may be 
peeled off with a pair of pincers, and so on, successive layers from 
day to day until the whole skin has been removed. The bits thus 
peeled off usually contain but a single layer of cells, and if coloured 
with carmine they make very beautiful preparations. 
But besides investigating cells in their relation to one another, 
the histologist endeavours to determine the form of single cells, and 
employs therefor means of isolation or dissociation. These may be 
either mechanical, such as shaking up a tissue in a fluid or teasing 
it out with fine needles, &c., or chemical. Usually a combination 
of the two is the most effectual. 
In most tissues the cells are united by intercellular matter, just 
as above described in the epithelium of the mesentery. This sub- 
stance acts as a cement binding the cells together. In some cases 
it reaches an extraordinary development, so that the cells come to 
be quite far apart, as in cartilage, for instance. But usually it is 
very thin, and may be dissolved, in some cases, without altering the 
appearance of the neighbouring cells. The cells that line the in- 
testine and stomach are particularly adapted to illustrate this action 
of certain chemicals. Thus if a small bit of the wall of the digestive 
canal be left in alcohol of 30 per cent, for twenty-four hours, the 
lining cells all become loosened so that they are easily scraped off 
with a needle or scalpel, and if mounted in glycerine mixed with a 
little picrocarmine, they become stained in a week or so, and show 
the details of structure of the single cells very admirably. 
Chromic acid has a similar action, and solutions of two parts in 
ten thousand of distilled water have a great value from their so 
affecting the brain that the ganglion-cells may be quite easily 
isolated. To effect this a very small piece of the brain — calf’s 
brain is perhaps the best — is placed in fifty or sixty times its 
volume of the solution for twenty-four hours, and then carefully 
teased out under a good dissecting microscope. 
Both weak chromic acid and alcohol may be used for isolating 
muscular fibres. Flies and beetles are perhaps the best for this 
purpose. The muscles of the wings (not those of the legs) should 
be torn out with fine forceps, and little bits, the smaller the better, 
placed in 30 per cent, spirit for twenty-four hours, and then dis- 
sociated or pulled apart on a glass slide, wdth fine needles. With 
sufficient care it is possible to separate the single fibrillse of each 
fibre, and when stained with liaematoxiline the alternating lines, 
dark and light, appear very sharply. These lines are those that 
make the muscles transversely striated. The cause of this striated 
appearance is not yet fully determined, but it is apparently con- 
nected with greater perfection of the muscular fibre than is found 
