Final answer:
Agranular leukocytes with a round nucleus and minimal cytoplasm are known as lymphocytes, important immune cells originating from the lymphoid stem cell line.
Step-by-step explanation:
Small, agranular leukocytes with a round nucleus and very little cytoplasm are called lymphocytes. Agranular leukocytes, which include lymphocytes and monocytes, have smaller, less-visible granules in their cytoplasm compared to granular leukocytes. Lymphocytes are one of the two major types of agranulocytes that arise from the lymphoid stem cell line and are crucial for the body's immune response.