The nucleolus was first recognized by

The nucleolus is a dense, spherical structure found within the nucleus of eukaryotic cells.

It was first observed and described by Fontana in 1781, making him the earliest known scientist to recognize this sub-nuclear body.

Robert Hooke is known for discovering cells using a microscope and coining the term "cell".

Stelly and Feldherr are associated with later studies in cell biology but not with the discovery of the nucleolus.

Hence, Fontana is credited with the first recognition of the nucleolus.