The observations of many individuals are correlated to indicate that the myeloid and lymphoid derivatives of mesenchyme carry on important complementary roles in tissue nutrition. Whereas the myeloid tissues specialize to the internal transport of oxygen by producing erythrocyte hemoglobin, it is suggested that the lymphoid tissues and, to some extent, the reticulum become specialized to the transport of water and food by producing plasma containing a variety of low and high molecular weight proteins, and by producing lymphocytes which may carry nucleoproteins to the tissues. Fundamentally, the lymphoid tissues (and the reticulum elsewhere) appear to subserve this nutritive function by utilizing molecular substrate from multiple sources—including the arteries, the branchial pouch entoderm (in the embryo), the gastrointestinal mucosa (in the adult), and the peripheral lymphatics—to produce a variety of mononuclear cells which "give of themselves" by undergoing dissolution, by shedding soluble cytoplasmic droplets, or by migrating to other tissues and dissolving there. It is stressed that dissolution of the reticulum, the mononuclear cells, and the cytoplasmic droplets (essentially gels) will yield sols (plasmas) whose water makes possible the intravascular migration and flow of proteins and other nutritive elements, and whose proteins (colloids) osmotically enable the intravascular retention of water. While it is assumed that the parenchymal cells of the liver are the source of most of the plasma proteins, circumstantial evidence is presented to suggest that this assumption is incorrect, and that derivatives of mesenchyme are the direct source of the blood, its cells, its plasma, its water, and its diverse extracellular as well as intracellular proteins.