The gland that promotes the development of certain lymphocytes is the thymus.

Here's why:

* T lymphocytes (T cells): The thymus is the primary site of T cell development. It provides a unique environment where immature T cells mature and learn to distinguish between self and non-self antigens. This process is crucial for the immune system to function correctly and avoid attacking the body's own tissues.

* B lymphocytes (B cells): While the thymus focuses on T cells, B cells primarily develop in the bone marrow.

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