Simplify the following text for a Grade 12 native English-speaking person.
Original: When county councils were initially introduced in England in 1889 about 20 % of the area of Middlesex , along with a third of its population , was transferred to the County of London , and the remainder formed a smaller county , in the north-west , under the control of Middlesex County Council .
Simplified: When county councils were initially introduced in England in 1889 around 20 % of the area of Middlesex , and a third of its population , was transferred to the County of London .