Fig. 4. Loss of MASK inhibits photoreceptor differentiation. (A,B) Third instar larval eye imaginal discs stained with -Elav to mark photoreceptor nuclei. (A) Optical section of a wild-type disc; each ommatidial cluster contains eight photoreceptor nuclei. (B) Optical section of a mask^6.3/mask^7.29 disc. Yellow arrows indicate ommatidial clusters containing fewer than eight photoreceptor nuclei. (C-E) A third instar larval eye imaginal disc containing a mask^10.22 clone. Arrows mark incomplete ommatidia. (C) Photoreceptor nuclei are stained with -Elav. (D) The clone is identified as those cells not expressing GFP. (E) Color merge: Elav is in red and GFP is in green. (F,G) Tangential sections of adult eyes. (F) Small mask^10.22 clone induced by the expression of hs-flp. White arrows indicate ommatidia missing photoreceptors. (G) A portion of a large mask^10.22 clone induced by the expression of ey-flp. Almost all ommatidia are missing at least one photoreceptor.