1. The growth of the trout eye is negatively allometric in relation to body-length.
2. The relative thicknesses of the retinal layers, and their cell densities, change as the eye grows.
3. The addition of new cells takes place from a peripheral growth zone.
4. Measurements from a wide size-range of trout retinae show that the dimensions of the cone pattern increase as the retina grows.
5. The relative importance of these two processes changes as the eye grows; in early growth the addition of new cells is the principal factor, but the enlargement of the cone pattern, which occurs at an almost constant rate throughout growth, becomes more significant later.