ABSTRACT
For developmental study of thymus in different stages of fetal period, from thymus glands of 75 healthy fetuses, histological sections were prepared and stained by H and E, PAS, Verhoeff, Toluidine blue and Van Geisson's methods. This study revealed that, the infiltration and accumulation of lymphocytes in thymus takes place at second month. Medullae were expanded and cortices were seen as accumulation of lymphocytes around them at first half of third month. There were progression in the thymus lobulation and the thicknesses of lobules was increased at second half of the third month. At the fourth month of development, the sizes of thymic lobules were reached to their maximum diameter, but the trabeculae became very thin. In overall, it seems that the most critical period of thymus development in sheep fetus was second half of second month till the end of third month