I. The development of Limnaea stagnalis from the first cleavage till the trochophore stage has been studied with various cytochemical methods. 2. The uncleaved egg is surrounded by a vitelline membrane. At cleavage this membrane is carried inward with the cleavage furrows; it forms a partition wall separating the blastomeres. The cleavage cavity arises by a splitting of this wall. Apparently, the vitelline membrane of Limnaea is a layer of living protoplasm forming the outer layer of the egg cortex. 3. The cleavage cavity, which is formed by the coalescence of several lenticular spaces between the blastomeres, widens by the secretion of fluid into this cavity by special secretion cones on the adjacent sides of the blastomeres. The fluid is expulsed to the exterior at regular intervals. In this way, the cleavage cavity plays an important part in the water regulation of the egg. 4. Prior to the 3d cleavage, the subcortical plasm of the egg, which formed a subcortical layer of uniform thickness at earlier stages, concentrates at the animal side and fuses with the animal pole plasm and the perinuclear protoplasm to a common mass of dense protoplasm; the rest of the egg consists of vacuolar protoplasm. Both substances are distributed unequally over the cells at further cleavage; the dense protoplasm, forming the ectoplasm of all cells, is most abundant in the animal blastomeres and decreases in a vegetative direction, whereas the vacuolar protoplasm, which forms the endoplasm, is most abundant at the vegetative side. The ectoplasm is rich in Î±-granules (mitochondria) and Î²-granules; the endoplasm in Î³-granules and fat droplets. 5. During cleavage and gastrulation, the nuclei have a constant position at the boundary between ectoplasm and endoplasm. The nucleoli show an intense activity; intranucleolar vacuoles are formed, which are extruded, apparently, into the nucleoplasm. An extrusion of entire nucleoli into the cytoplasm is indicated. The nucleoli are rich in ribonucleic acids, sulfhydril compounds, iron and benzidine peroxidase. The cytoplasmic ribonucleic acids and glutathione are, in many cells, most concentrated in the neighbourhood of the nuclear membrane. These facts point to an active role of the nucleus, and especially the nucleolus, in cell metabolism. 6. During later cleavage stages, a transformation of the proteid yolk occurs; the Î²-granules are dissolved in the cytoplasm, the y-granules increase in number. At the same time, albumen is ingested by the cells from the surrounding egg capsule fluid, and laid down in numerous albumen vacuoles in the ectoplasm. Probably, the nucleus plays a part in these transformation processes ; the same holds true of the Golgi bodies. During further development, the Î³-granules decrease in size; ultimately, they disappear altogether. The ingestion of albumen is restricted more and more to the albumen cells of the gut. Both extracellular and intracellular digestion of albumen can be observed. 7. Probably, no direct continuity exists between the Golgi bodies of the uncleaved egg and those found in the cells of the embryo. 8. In the course of development, there is a marked increase of the thymonucleic acid contents of the nuclei. 9. Particular granules, rich in ribonucleic acid, accumulate at the central end of the macromeres, where they fuse into single dark bodies; these bodies are transmitted into the cells of the 4th quartette at the next division; at later stages, they disappear. 10. Glycogen is accumulated in the "central plasm" at the 24-cell stage; at later stages, especially the cells 4a-4c and their descendants are rich in glycogen. 11. Four equatorial groups of thickened ringlets are formed on cells of the 2d quartette during cleavage. 12. With basic vital dyes, in early stages a weak granular staining of the yolk occurs. At later stages, the basic dyes are accumulated in the albumen vacuoles of the cells which may be considered to represent a "vacuome" in the sense of PARAT. Nile blue hydrochloride stains, furthermore, the yolk granules and mitochondria; brilliant cresyl violet gives a diffuse staining of the cytoplasm. Methylene blue and Janusgreen do not penetrate into intact cells. 13. Probably, there exists a relation between the composition of the cells and their further development; especially, the proportion between ecto- and endoplasm seems to be important for the fate of the cells. 14. At the late gastrula stage, the velum and head vesicle show an elective indophenol oxidase reaction; at still later stages, the reaction is especially localized in the shell anlage and mantle fold and in special subepidermal cells of the mesenchyme. 15. At the late gastrula stage, a strong elective benzidine peroxidase reaction of 2 pairs of cells at the lateral angles of the embryo, representing, probably, the descendants of the anterior trochoblasts, occurs; the velum and head vesicle react more weakly. At the late trochophore stage, the gut shows also a rather strong reaction; still later, it is localized in the mantle fold, the shell, and particular mesenchyme cells in the interior of the embryo. 16. The histological differentiation is accompanied with the appearence of considerable differences in chemical composition between the cells. This leads in each of the 3 germ layers to the formation of 2 types of cells: I. large cells, without cell division, with albumen vacuoles, many Golgi bodies, mitochondria and fat droplets, rich in glycogen and iron, but poor in thymo- and ribonucleic acid; they form the larval organs (ciliary cells, albumen cells, protonephrideum); 2. small, dividing cells, without albumen vacuoles, poor in Golgi substance, mitochondria, fat, glycogen and iron, but rich in thymo- and ribonucleic acids; they have to build up the body of the adult snail. These two cell types can be regarded as representatives of 2 different directions of cell life.