[The effect of ethidium bromide on ommatidium development inEphestia kühniella zeller]

1. The effect of ethidium bromide (EB) on ommatidium development inEphestia was investigated. Doses ranged from 10^-3 to 10 μg EB/pupa. 2. Even the lowest dose applied blocked imaginal ecdysis. The degree of development toward the imago was progressively lower with increasing doses of EB. 3. The development of pigment in the retinula cells, which proceeds from the dorsocaudal portion of the eyeanlage in a ventrorostral direction, is usually completed in pupae which have received injections of up to 0.80 μg EB. Five μg prevents all further pigment development. 4. A peculiar deficiency phenomenon (Pigmentaussparung, PA) appears in animals treated with 0.90-3.00 ug EB. A band-shaped unpigmented region appears in an otherwise pigmented region. The area is perpendicular to the direction of successive development of ommatidia. The region appears to be undifferentiated histologically. 5. The location and width of the affected regions are dependent on dose and upon the age of the pupa when injected. They are coincident with the distribution of the 2 successive series of cell divisions which precede differentiation. 6. The results support the conclusion that the waves of cell division are a prerequisite for the appearance of all cell types in the ommatidium.