Presumably, there's only a sponge inside one of those aforementioned egg toys; but, what is inside a plant seed? The workings of a plant seed are far more complex. Reproduction and germination take place within the plant seed, and nutrition is provided by a starchy source called the endosperm. The first thing that materializes from the seed is the initial root, called the radicle. Soon afterward, seed leaves called cotyledons emerge. Different plants have different numbers of seed leaves; plants with one seed leaf are called monocots, and plants with two seed leaves are dicots.
One fun experiment to try is to take a bunch of unknown seeds (have someone pick out about three or more different seeds of different plants, so you can be surprised!), place them on damp paper towels, put them in a plastic bin, and watch the seed leaves emerge. (Be sure to keep the paper towels damp.) Did you see the radicle emerge? Is the plant a monocot (one seed leaf) or a dicot (two seed leaves)? Did the leaves come out green, yellow, or a different color? Carefully plant the seedlings in a pot containing potting soil, or outside in the garden (make sure the seed leaf/leaves poke above the soil surface!) and see what kind of plant you get.
For more detail about the inner workings of plant seeds, check out The Plant Seed: The Secrets Within.
Photo: A germinating pea seed. S. Trueman.