BLOOD. 3 



examine. The tannic acid acts on the serum-albumen of the plasma and forms a finely granu- 

 lar precipitate which somewhat obscures the field, but it also acts (like some other acids) on 

 the corpuscles, causing them to become globular and the colouring matter to separate from the 

 stroma. The separated colouring matter tends to pass out of the corpuscle, but as it does 

 so it becomes coagulated by the acid, and remains attached to the corpuscle in the form 

 of one or more granular-looking buds (PL I., Fig. 5). 



None of the preparations of the blood of the frog or newt hitherto described can be 

 permanently preserved ; the two following preparations may, however, be kept as permanent 

 slides. 



(A) Picric Acid and Picrocarmine. Place a drop of blood on a slide, and add a drop of a 

 saturated solution of picric acid, put the slide aside and allow it to remain for five minutes, and 

 at the end of that time, when the acid has ' fixed ' the corpuscles (that is, has coagulated their 

 contents), the excess of acid should be removed by means of a narrow slip of blotting-paper. 

 A drop of a solution of picrocarmine (p. xliii) should now be added, and a trace of glycerine 

 to prevent evaporation, and the preparation set aside for an hour. At the end of that time, 

 remove the picrocarmine solution by means of a slip of blotting-paper, and add a drop of 

 Farrant's solution or glycerine (p. xlviii) and apply a cover. The preparation may then be ex- 

 amined, when the peri-nuclear part of some of the corpuscles will be seen to be highly granular 

 and of a deep yellow colour ; while the nucleus is stained red. In some of the corpuscles 

 there may also be seen delicate yellow-coloured threads, extending from the nucleus to the 

 envelopes. In others the peri-nuclear part remains uniformly homogeneous. 



(?) Osmic Acid and Picrocarmine. Mix a drop of blood with a drop of a one per cent, 

 aqueous solution of osmic acid (p. xlvi), and allow the slide to stand. This ' fixes ' the corpuscles 

 without altering their shape. At the end of five minutes remove the excess of osmic acid with 

 blotting-paper, add a drop of solution of picrocarmine and a trace of glycerine to prevent evapo- 

 ration, and set aside for three or four hours (or even longer, as no 6ver-staining takes place). 

 At the end of this time, the preparation is treated as (/i) and examined. The nucleus will 

 now be found to be stained red, and the peri-nuclear part homogeneous and yellow. If 

 a drop of blood (taken from a frog which has been kept through the winter) be prepared by 

 this method, some of the corpuscles may show ' vacuoles ' in the peri-nuclear part. (PL I., 

 Fig- 7)- 



(j) Osmic Acid and Logwood. A similar preparation may be made by using a solution of 

 logwood (p. xlii) as the staining agent instead of picrocarmine, with the advantage that the 

 staining of the nucleus takes place much more rapidly in about five minutes. 



(/) Ammonium Chromate and Picrocarmine. Mix a few drops of blood with a small quantity 

 of a five per cent, solution of ammonium chromate in a small glass vessel, and leave the mix- 

 ture for twenty-four hours, taking care to prevent evaporation. Then pour off the chro- 

 mate solution and substitute picrocarmine solution for another twenty-four hours. Mount a 

 drop of the deposit in glycerine and examine it. The effects of this substance are most 

 remarkable. Some corpuscles retain their shape though the nucleus enlarges, and shows an 

 intra-nuclear plexus of fibrils. In others the haemoglobin is arranged in groups around the 

 enlarged nucleus ; while others have their wall partially dissolved oh one side, with part of 

 the cell-contents extruded through it. 



The preparations (z) and (j} show colourless corpuscles with their nuclei stained. 



2. Observe the Colourless Corpuscles (H). Prepare a drop of blood as for the examination 

 of the coloured corpuscles. On careful examination there may be -seen, scattered amongst 



