Here's why:

* Bone marrow is the primary site of hematopoiesis, the process of blood cell formation. This includes the production of all blood cell types, including lymphocytes.

* Lymphocytes are a type of white blood cell that are crucial for the immune system.

* Undifferentiated lymphocytes are immature cells that haven't yet developed into specific types of lymphocytes like T cells or B cells.

While lymphocytes mature and differentiate in other organs like the thymus (T cells) and bone marrow (B cells), they all originate from the undifferentiated lymphocytes produced in the bone marrow.