Cabbage white butterfly eggs. Coloured scanning electron micrograph (SEM) of an egg of the cabbage white butterfly (Pieris brassicae). The sculpted egg shell is made of a protein called chorionin. There is an open micropyle at the top of unfertilized eggs to allow fertilization. When fertilized this opening closes (as here). The other small openings are aeropyles (breathing pores) that allow air to flow to the developing embryo. These eggs are laid in batches of up to 100 to 150 on the underside of brassica plant leaves, hatching after about 10 days. Once hatched, the caterpillars feed on the leaves. Magnification: x490 when printed at 10 centimetres across.