The pharyngeal lymphoid tissue of lampreys. A morpho-functional equivalent of the vertebrate thymus?

PE, pharyngeal epithelium; S, blood sinus; T, connective tissue trabecula; End, endothelial cell; Er, erythrocyte; L, lymphocyte; Lb, lymphoblast; Lm, lymphoblast in mitosis; Ma, macrophage; Ret, reticular cell; col, collagen fibres; db, dense body; g, Golgi apparatus; gr, granules; n, nucleus; nu, nucleolus; m, mitochondria; rer, rough endoplasmic reticulum.