Tissues for histological observations were fixed in FAA (95 % ethyl alcohol + glacial acetic acid + formaldehyde + water, 10:1:2:7), dehydrated in a tertiarybutyl-alcohol series, embedded in paraffin wax, sectioned at 10 µm thickness and stained with 0.5 % safranin-O and 0.1 % fast green (Jensen 1962).