Question

How is an egg a single cell? And if it is, where is all that mitochondria and stuff?

Answer 1

Hint: Egg is the most remarkable cell among the animal cells. Once it is activated, it can create a whole new life within some days or months. No other cell of a higher animal has this kind of capacity. Fertilization means the consequence of fertilization. Fertilization refers to the process of fusion of the sperm with an egg.

Complete answer:
Ovum is the starting phase of an egg. An egg is a single cell with all the organelles floating in the cytoplasm.
Cleavage starts after fertilization. It is the division of a cell into two and then four. In the cleavage the cells are connected in a clump.
Even though it appears as a single cell entity during hatching, it will make a multicellular organism inside it, then break the cell and get out.

Additional Information:
However, in some organisms the sperm itself is not mandatorily required and an egg can be fertilized artificially by a variety of nonspecific chemical or physical treatments. In fact, some organisms including a few vertebrates like some lizards, normally reproduce from eggs that become fertilized in the absence of sperm. That is known as Parthenogenesis.
An egg can give rise to every cell type in an adult organism. An egg is a highly specialised cell.

Note:
The cytoplasm of an egg can even be reprogrammed with a somatic cell nucleus. Through this process the nucleus can direct the development of a new individual organism. That is the way through which the famous sheep Dolly was produced.