ostrich egg

An ostrich egg is the egg laid by an ostrich, and it is famous in biology because its yolk is one single cell that can be seen easily with the naked eye.

• The yolk is the actual egg cell (the female reproductive cell). In an ostrich, this cell is unusually large—about 13–17 cm across.
• The egg white (albumen) is not a cell. It mainly provides water and proteins and also cushions the developing embryo.
• The hard shell is also not a cell. It acts like strong packaging: it protects the egg and has tiny pores that allow gas exchange (oxygen in, carbon dioxide out).
• This is a neat reminder that cells can be very different in size: many cells are microscopic, but some (like an ostrich egg cell) are giant because they store a lot of food for early development.
• Even though it starts as one cell, that cell can divide again and again to form a multicellular organism—showing how a whole body can begin from a single starting cell.