The respiratory epithelium of the nasal cavity started its development as non-ciliated pseudostratified columnar in foetuses of 50 mm. CVRL. The tall cells had swollen apical portions and their free surfaces were either smooth or carrying scattered short microvilli. At 60-80 mm. CVRL some cells lost gradually their swollen portions, and long, narrow microvilli appeared on their free surfaces. During this stage many basal corpuscles appeared at the supranuclear regions, then arranged themselves under the free cell border and finally young cilia appeared among the latter microvilli and increased in length and density with advancing age. At 140 mm. CVRL, the number of the ciliated cells became nearly aqual to the non-ciliated ones, then the former type of cells exceeded the latter during the following foetal ages.