There is recent evidence that the pineal gland in some species may have lymphatic qualities during early post natal life. In this study the development and structure of the pineal complex in the CDF1 mouse is examined in fetal, neonatal, juvenile, and adult stages. The gland appears as an out growth of the dorsal wall of the third ventricle of the brain and at ten days gestation attains a mean size of 90 microns in the transverse and 35 microns in the median saggital plane. General and specific staining shows fetal and early post partum tissue to be compact and mitotic with scant vascularity and poorly developed connective tissue structure. Mitotic activity ceases around three weeks post partum marking the beginning of the juvenile period. Neuroglia become evident as cell volume increases throughout the juvenile stage. Parenchyma cells reach a size of 15 microns in the late juvenile and adult glands. In the adult the gland has grown to a mean diameter of 630 microns in the transverse by 390 microns in the median saggital plane. At this stage the gland is supported by an extremely fine connective tissue framework which is continuous with the capsule. There is no evidence of lymphatic aggregations or germinal centers as described in other species, although occasional lymphocytes are seen in the capsule of the gland and adjacent meningeal tissues of juvenile specimens
