Final answer:
In fetal development, the spleen participates in the process of extramedullary hemopoiesis, producing erythrocytes or red blood cells, as part of the overall production of blood's formed elements.
Step-by-step explanation:
The spleen is involved in a special type of hematopoiesis in a fetus called extramedullary hemopoiesis, during which erythrocytes, or red blood cells, are produced. This process occurs outside of the medullary cavity of adult bones and is taken up by organs like the spleen when conditions such as bone cancer cause failure of hemopoiesis in the bone marrow. In addition to erythrocytes, other formed elements of blood, including leukocytes (white blood cells) and platelets, are produced through hemopoiesis which initially begins in the red bone marrow from hematopoietic stem cells that differentiate into myeloid and lymphoid lineages. During fetal development, the spleen, along with the liver and lymph nodes, temporarily take over the role of red bone marrow in producing these cells.