vii PRACTICAL DIRECTIONS 121 



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 methyl green in distilled water to make a deep bluish-green 

 solution, and add I per cent, of strong 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 sup- 

 porting 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 coagulating before it is covered, in which 



