Thymocytes of human embryos at the 7–8-week stage have irregularly shaped nuclei with one to three distinct nucleoli, characterized by absence of compact chromatin or heterochromatin. The electron-dense cytoplasm of these cells contains polysomes and a few mitochondria. Receptors for sheep's red blood cells and T-antigen are absent on the surface of the cells. In 11–12-week human embryos, the T-lymphocytes are reduced in size, clumps of heterochromatin appear in their nuclei and receptors for sheep's red blood cells (79%) and T-antigen (60%) appear on the surface of the cells. Later, the quantity of compact chromatin in the nuclei of the thermocytes increases and the cells acquire their definitive properties and structure.