Â­Mesoamericans began writing during the mid-Pre-classic period. The Mayans were the first to keep any sort of historical record, and the beginnings of the calendar were born. The Mayans used stelae, or stone monuments, to carve their civil events, calendars and astronomy knowledge. They also inscribed their religious beliefs and mythology on pottery.
The nucleus is a membrane bound structure that contains the cell's hereditary information and controls the cell's growth and reproduction. It is commonly the most prominent organelle in the cell. The nucleus is surrounded by a structure called the nuclear envelope.
The nucleus is perhaps the most important structure inside animal and plant cells. It is the main control center for the cell and acts kind of like the cell's brain. Only eukaryotic cells have a nucleus. In fact, the definition of a eukaryotic cell is that it contains a nucleus while a prokaryotic cell is defined as not having a nucleus.