The dark round structure within the nucleus that produces ribosomes is called the nucleolus.

Here's a breakdown:

* Nucleus: The control center of the cell, containing the genetic material (DNA).

* Nucleolus: A dense, spherical structure within the nucleus that is responsible for the synthesis of ribosomal RNA (rRNA). rRNA is a key component of ribosomes, which are essential for protein synthesis.

* Ribosomes: Tiny organelles found in the cytoplasm of cells that translate the genetic code from mRNA into proteins.

So, the nucleolus is the factory for ribosomes, which in turn are the factories for proteins.