The thymus is the lymphoid organ that consists of lymphoid and epithelial cells The thymus is composed of two identical lobes and is located anatomically in the anterior superior mediastinum. In spite of remarkable progresses made in the field of the immuno histochemical characterisation of the thymus parenchyma, the diagnosis of thymoma largely depends on the interpretation of conventional morphologic aspects. In the present study thirty eight foetuses ranging from 9^th to 24^th gestational weeks were studied to find out the histological changes that occurred at different stages of the foetal growth period. After proper fixation, alternate sections of the thymus tissues stained with Haematoxylin and Eosin, Masson’s Trichrome were examined histologically under light microscope. Lobulation and differentiation of cortex and medulla of the thymus were observed at 9th week. Lobulation was completed by 12^th week. The differentiation of cortex and medulla was completed between 12^thand 14^thweeks. The normal structure and its variants are extremely helpful to differentiate normal from pathologic aspects.

Keywords: Thymus,cortex,medulla, gestational age, lobulation, thymoma. 

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