The structure and physico-chemical composition of the egg of Limnaea stagnalis and the changes occurring during the uncleaved stage of the egg were studied by a variety of methods. A. Composition of egg: I. The cytoplasm of the egg consists of 2 parts: ectoplasm and endoplasm,which differ in their staining reactions. Immediately after oviposition, the ectoplasm occupies a sector at the vegetative pole of the egg, the endoplasm the rest of the egg. 2. Three sorts of granules can be distinguished: Î±-granules (probably mitochondria), Î²-granules and Î³-granules. Moreover, fat droplets and Golgi bodies are present in the egg. 3. The Î±-granules, which are small, are accumulated especially in the endoplasm. Probably, glycogen, phenolases and peroxydases are bound in some way or other to these granules. 4. The Î²-granules, of medium size, form a major part of the ectoplasm. They consist of albumen and contain pentosenucleic acids. 5. The Î³-granules are coarse, probably albuminous in nature; they lie mostly in the endoplasm. 6. Both fat droplets and Golgi bodies are distributed rather evenly in the cytoplasm, leaving free only the spindle and asters. "Praesubstances" and "Golgi systems" can be distinguished. 7. The hyaloplasm contains pentosenucleic acids in small quantity and, probably, bound sulfhydril components, especially in its central part. 8. The freshly laid egg contains the first maturation spindle in metaphase. The spindle area is free of fat droplets and Golgi bodies, but is characterized by the presence of free glutathion in reduced form. The chromosomes contain thymonucleic acid. B. Changes during uncleaved stage: 9. The course of the maturation divisions and the formation of the polar bodies are described. The egg shows amoeboid movements shortly after the extrusion of either of the polar bodies. This is accompanied with a distinct drop of the tension at the surface. 10. The Î±- and Î³-granules of the endoplasm are attracted by the maturation spindle and asters, forming a halo surrounding the amphiaster. 11. The ectoplasm spreads beneath the egg cortex to the animal side. At first, a gap remains at the animal pole; after the completion of the maturation divisions, the ectoplasm surrounds the whole egg. 12. A fine chorion is formed, which lies inside the first, but outside the second polar body. 13. The spermastcr makes its appearance shortly before the extrusion of the first polar body; during the maturation divisions, it grows slowly. This is accompanied with a gradual rise of the viscosity of the protoplasm. No division of the spermaster with formation of an amphiaster takes place. 14. The sperm-head remains in a subcortical position till shortly after the extrusion of the second polar body; then, it migrates to the spermaster and develops into the male pronucleus. 15. The chromosomes left in the egg after the maturation divisions swell into karyomeres which fuse to the female pro-nucleus. 16. The copulation of the pronuclei takes place immediately beneath the egg cortex at the animal pole. 17. About one hour before cleavage, the animal pole plasm is formed by a local accumulation of substances attracted, probably, by the egg cortex at the animal pole. The centripetal flow of protoplasm in the dilating maturation aster may aid in this localisation process. Many Î±-granules are transported by it to the animal pole plasm. 18. From the time of oviposition till first cleavage the egg swells considerably, probably by the intake of water. This is accompanied with a decrease in density. At the same time, the Î³-granules of the egg give rise to the formation of vacuoles, by an attraction of water from the neighbourhood. Eventually, the egg protoplasm has a vacuolated appearance throughout, with the exception of the animal pole plasm. 19. The viscosity, which is high one hour before cleavage, has a minimum 30 minutes later, then it rises again. The tension at the surface reaches a minimum immediately before the beginning of cleavage.