A mature seed has seed coat, an embryo and reserve food material in the endosperm in monocots and in cotyledons in the dicots. Figure shows structure of dicots and monocot seeds. The embryo as shown is already well-developed and is differentiated into embryonic root (radicle) and shoot (plumule). You know that the process of seed formation begins with the fusion of a male gamete with an egg cell. The 'zygote' thus formed undergoes divisions resulting in the development of an embryo. At this stage the development is stopped and there is cessation of metabolic activity which resumes only after the seeds get dispersed from the parent plant and start germination.
Figure: Seed structure and seedlings of 3 different angiosperms. The food is stored in cotyledons (bean, dicot), endosperm (maize, a monocot, castor bean, a dicot)