Mustard ovules. Light micrograph of a section through two ovules from a mustard plant (Brassica nigra). The outer wall of the ovule (blue) contains the embryo sac. Below this is another layer, the nucellus (red-black). In the centre of the ovules is an inner fluid formed from the endosperm. The developing embryos within the ovules are the dark blue areas. When fully developed, the ovules will form the fruit and seeds of this mustard plant. Magnification: x103 when printed at 10 centimetres across.