1. A convenient form of nomenclature for the various forms of yolk is suggested.
2. In the centrifuged oöcytes of Helix three layers are produced; an upper layer of G-yolk (Golgi-yolk) (10 per cent.) formed by a direct metamorphosis of some of the Golgi rods, a middle layer of ground cytoplasm free from yolk and containing the nucleus, and a lower layer of swollen mitochondria (50 per cent.). The unaltered Golgi rods remain scattered throughout the cell.
3. In the centrifuged oöcytes of Patella three layers are also produced; an upper layer of G-yolk (75 per cent.) formed by, not from, the Golgi rods, and containing the nucleus together with a number of mitochondria caught between the G-yolk spheres, a middle layer of ground cytoplasm free from yolk, and a lower layer of swollen mitochondria (10 per cent.).
4. The bearing of these results on early development is discussed, especially in the light of Conklin's work.
5. A generalised account of yolk-formation in the Mollusca is given and graphically represented.