1. All the chlorophyll types examined were found to contain the same initial cell structure, minute "proplastids" of the same size and general appearance being present in every type. 2. In normal green plants the proplastid first appears in the cell as a minute granule at the limit of visibility, gradually enlarging and developing chlorophyll until it becomes a mature chloroplast. In plants of the other chlorophyll types studied (Mendelian white, Mendelian virescent, and the maternal inheritance strain) the unusual characters of the plants are due to the failure of the proplastids initially present to develop into plastids with the normal size, or color, or both. 3. The green and colorless plastids found in different plants or in different portions of the same plant do not represent two fundamentally distinct types, but are rather to be regarded as the end members of a continuous series which comprises also all intermediate conditions. No cytological evidence was found favoring the view that the primordia...