This is a cell that secretes large molecules that are unable to pass through the lipid bilayer of the plasma membrane. They leave the cell instead by a process known as exocytosis. After production in a system of membranes called the endoplasmic reticulum the molecules are packaged in a small sac of membrane called a vesicle. The vesicle moves to a stack of membranes called the Golgi apparatus. Their membranes merge and the vesicle releases its contents for modification. As the molecules leave the Golgi apparatus they are again packaged in a vesicle which moves to the plasma membrane. As membranes merge again, the contents leave the cell without actually crossing the plasma membrane. This process can also occur in reverse, allowing large molecules to enter a cell. This is termed endocytosis.