In this study development of the semilunar plica was examined histologycally by making sections through the eyes of eleven foetuses at different stages of gestation, two newborns and an old man. We found that in the early stages of its development the semilunar fold covered a bigger part of the orbit and later did not keep up with the growth of the eyeball and the lids. In its development three different kinds of germinal glands could be seen in the semilunar plica. Beside poorly differentiated buds of the surface epithelium which can be classified as rudiments of the nictitating or Harderian gland, serous glands were detected which could be beginnings of Krause's glands. Additionally, a new kind of plica gland was identified in which ductular structures, with an onion skin appearance, could be discriminated from mucous acini. The surface of the plica developed slowly by an increase in the layers of its epithelium as well as by the maturation of the epithelial cells from a two-layered cuboidal to a multi-layered cylindrical epithelium. In general the palpebral side of the plica consisted of a higher number of epithelial layers and more goblet cells than its bulbar side. Moreover, the surface of the palpebral side appeared more irregular and enlarged by numerous pleatings of its epithelium. The initially loose mesenchymal connective tissue was soon condensed by firstly an increase of the number of cells and later by an increase in the fibre density. In the tight collagenous connective tissue no elastic or reticular fibres or cartilaginous structures could be found. Very dense vascularization of the plica semilunaris was seen early in development. The blood vessels rising from the root of the plica divided themselves into a central and a subepithelial vascular net. Later on, some of them showed an enlarged lumen and were covered by a thin layer of muscle cells. A few unmyelinated neurofibres were found next to blood vessels and glands. Muscle cells could not be detected. A very dense concentration of leukocytes in the plica even in early intrauterine development was very striking and could be observed in this kind for the first time. These were mostly lymphoplasmocytic elements but granulocytes and macrophages could also be seen around the blood vessels and in the subepithelial area where they were arranged as follicles. Some of these cells even passed through the epithelium and could be found in the conjunctival fissure. The discovery of dense infiltration with both specific and non-specific immune cells, abundant vascularization and secretory structures (goblet cells and surface enlargement) in the semilunar plica suggest that it plays an important role as a specialized organ in human eye protection. Its anatomical position at the medial border of the eye supports this theory. Its origin from the third eyelid of the mammals could be recognized to a certain extent.