vii PRACTICAL DIRECTIONS 121 



4. Three or four dissecting needles, made by sticking a fine 

 sewing needle into the end of a wooden penholder, allowing 

 the point to project about half an inch. 



5. A few reagent-bottles for holding the various fluids used 

 for applying what are called micro-chemical tests to the 

 tissues. Special bottles can be bought for the purpose, but 

 sufficiently convenient ones can be made from ordinary 

 one-ounce phials, fitted with sound corks. Bore a hole 

 lengthwise through the cork, and insert into the hole a piece 

 of narrow glass rod, pointed at the end, just long enough to 

 reach nearly to the bottom of the bottle when the cork is 

 inserted. This arrangement allows of the ready application 

 of a single drop of fluid to the object under examination. 



6. The following micro-chemical reagents : 



a. Salt solution. Dissolve 0-75 gramme of sodium chloride 

 in 100 c.c. of distilled water, so as to make a f per cent, 

 solution. 



b. Acetic acid, i per cent. One c.c. of strong acetic acid 

 to 99 c.c. of distilled water. 



c. Distilled water. 



d. Solutions of one or two aniline dyes. For fresh tissues, 

 dissolve enough mefhyl-green in distilled water to make a 

 deep bluish-green solution, and add a trace of acetic acid. 

 For preserved (or fresh) tissues, make a saturated solution 

 of magenta or safranin in strong alcohol, and dilute with an 

 equal bulk of water. 



e. Glycerine, 50 per cent. Equal parts of pure glycerine 

 and distilled water. 



Microscopical Examination of the Simple Tissues. 



For the following work, a freshly-killed frog must be used. 



(If you wish to measure each kind of tissue-element, you 

 must learn to use a micrometer, which consists of a circular 

 piece of glass, marked at regular intervals with lines or 

 squares, the distance between which can be calculated by 

 comparing them with a scale engraved on a slide known as 

 a stage-micrometer.) 



i. The Blood. Have ready a clean dry slide and cover- 

 glass. In a freshly-killed frog open a vein or make an 

 incision in the heart, and with a clean glass rod remove a 

 drop of blood to the middle of a slide. Take hold of the 

 edge of the cover-glass with small forceps, and supporting it 

 with a mounted needle, gently lower it on to the drop of 

 blood until the latter is spread out into an even, transparent, 

 yellowish film. This operation of covering the drop of blood 

 requires a little practice : if not done quickly (unless a drop 

 of salt solution is added), there is danger of the blood 



