Studies of the Egg of Bacillus

3. The microscopic structure of the egg envelopes is described. The egg is covered by three main envelopes: a hard, brittle exochorion built of seven distinct layers, two of which consist almost solely of calcium salts; a double-layered, membranous endochorion; a thin vitelline membrane. Layers of the chorion may be summarized as follows: (i) exochorion—tubercular layer, dark layer, lamellar membrane, calcium oxalate layer, intermediate lamella, calcium carbonate layer, shell membrane, (ii) endochorion—outer and inner membrane. 4. In developing eggs the calcium carbonate layer of the exochorion becomes reduced owing to loss of material from its inner surface. Consequently the shell membrane, initially attached to this layer, becomes almost completely detached. The calcium carbonate layer in shells of eggs which do not develop remains unchanged.