1. It is generally believed by cytologists that fats blacken quickly in osmic acid, whereas lipoids take a much longer time. This view seems to be erroneous, as the time required by osmic acid to blacken a particular sample of fat or lipoid depends entirely on its degree of unsaturation and its previous state of oxidation.
2. Films of a sample of completely hydrogenized fat, with iodine value nil, failed to blacken in osmic acid, even in seven days at 40° C. Samples of fat, therefore, must exist which actually take longer time to blacken in osmic acid than some highly unsaturated lipoids.
3. It follows, therefore, that a granule in the cell, which goes black in osmic acid in a short period, cannot be identified as true fat unless it also stains intensely in Sudan III and Scharlach R.
4. These fat tests have been employed on eggs of animals representing many groups.
5. In Pheretima, Culex, Dysdercus, Crossopriza, Plexippus, and the rabbit, the Golgi material can be blackened in short periods of osmication, but it cannot be stained even slightly with Sudan III and Scharlach E during any stage in oogenesis. It must, therefore, be interpreted not as fat but as very highly unsaturated or very little oxidized lipoid.
6. In earlier stages of the oogenesis of Ophiocephalus Eana, Emyda, Gallus, Lueiola, Periplaneta, Palaemon, and Paratelphusa, the Golgi material does not stain at all with Sudan III and Scharlach E, even though it can be blackened in short periods of osmication. It must again be interpreted, not as fat, but as highly unsaturated or very little oxidized lipoid. At a certain stage in oogenesis these lipoids are converted into fats when they stain brilliantly with Scharlach E and Sudan III.
7. In Rita the Golgi material of early oocytes consists of very nearly fully saturated lipoid, as it does not go black in short periods of osmication, and takes thirty-two days to impregnate in Mann-Kopsch. It does not stain with Sudan III and Scharlach R, but by the time the egg measures 1 mm. it is converted into fat and begins to stain intensely with these dyes.
8. A chart is published recording the reactions of fats, lipoids, and vacuoles to various microchemical tests.