Final answer:
The spleen developmentally forms from the mesoderm, which is one of three primary germ layers in the embryo that contributes to various body structures.
Step-by-step explanation:
Developmentally, the spleen forms from the mesoderm. The mesoderm is one of the three primary germ layers in the early embryo that later differentiates to give rise to various structures, including connective tissues, muscle, and the circulatory and lymphatic systems. The spleen, being an important part of the lymphatic system and involved in filtering blood as well as an immune response site for blood-borne pathogens, originates from this middle layer. Other germ layers, the ectoderm, and endoderm give rise to different body systems. The ectoderm forms structures such as the skin and nervous system, while the endoderm forms internal structures like the digestive and respiratory tracts.