(2) Gelatine 10 grms. Distilled water 50 c.c. Allow it to stand in the cold water until the water is absorbed and the gelatine has become soft. Warm (1) almost to boiling point over a Bunsen burner, and add the gelatine slowly. Stir thoroughly and add a ten per cent. solution of acetic acid until the solution becomes slightly acid. This will be shewn by the mass assuming a darker and duller colour. A little salicylic acid may be added to preserve it. *Blue injection mass.*--To the gelatine mass (2) prepared as above, and liquefied by heat, add instead of carmine Soluble Prussian blue 5 grms. Distilled water 60 c.c. Every trace of alkali must be kept away from the mass during and after the preparation. Sections of injected organs should be mounted in Farrant’s solution slightly acidulated with formic or acetic acid. With every care, however, the blue colour is apt to fade in the course of time. *Green injection mass.* Robin’s formula (modified). (1) Arseniate of soda (sat. sol.) 80 c.c. Glycerine 50 " (2) Sulphate of copper (sat. sol.) 40 " Glycerine 50 " Mix and add one part to three parts of the gelatine mass made as for the red and blue injections. *Method of injection.*--In injecting the vessels of tissues it is necessary that the organ or the entire animal, as the case may be, shall be kept during injection at a temperature well above that at which the gelatine mass will melt, otherwise the gelatine will “set” in the arteries and will never reach the capillaries. This warming is effected by immersing the animal in a water bath. The liquefied gelatine is forced into the artery by a syringe or by air pressure. It is essential that the pressure be uniform and steady. This is so much more easily managed with air pressure that this method is strongly recommended to the beginner. But, whatever method be adopted, perfect results can only be obtained with certainty after long practice. Sometimes too high pressure will be employed and the vessels give way, at others the injection may not reach the capillaries at all. The most scrupulous attention to details is essential. By far the most effective apparatus for injecting is the modification of Ludwig’s constant pressure apparatus devised by Fearnley.[2] Although the apparatus appears complicated, the various parts are easily obtained and it would be easy to improvise a substitute for the water bath. [2] “Practical Histology,” (Macmillan & Co.).