In which specimen were cells first identified

The specimen where cells were first identified was cork. Robert Hooke was examining a piece of cork using a microscope in 1665.

cork (thin slices of cork)

Explanation:

Robert Hooke used the term "cell" in 1665. He made the first compound microscope, and he used it to see the cork cells. He took thin slices of cork and examined it under the compound microscope. He found smaller compartments in it. These compartments were arranged like a honeycomb structure. The cork was a dead plant part, hence inside of each compartment was empty.  He called the outer wall is the cell wall.