Final answer:
The cells with a round, densely stained nucleus and a small amount of cytoplasm are known as lymphocytes, which are a key component of the immune system and can be recognized by their large, darkly stained nuclei when observed under a microscope.
Step-by-step explanation:
The class of cells described as having a round, densely stained nucleus with a small amount of cytoplasm are lymphocytes. Lymphocytes are a type of white blood cell and are integral to the immune system. They can be found in different sizes, with both large variants like natural killer cells (NK cells) and smaller ones like B and T cells. Under a microscope, when stained with a dye such as Wright stain, lymphocytes' nuclei absorb the stain actively, making them appear very dark. These cells generally have a single large nucleus that occupies most of the volume inside the cell, often leaving a very thin rim of cytoplasm visible. This distinct appearance aids in the identification of these cells during microscopic examination of blood or lymphoid tissue samples.