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The ultrastructure of the dog pineal gland from the first postnatal day to the seventh month is described. In the first postnatal stages, pineal parenchyma only shows immature proliferative cells with abundant cytoplasmic glycogen. Nerve fibers are seen in the pineal connective tissue spaces. The differentiation of the dog pineal cell types begins in the first postnatal week. Both pinealocytes and pigmented cells are first seen on the fourth postnatal day. The pineal astrocytes are observed on the tenth day. Immature cells are still found in the pineal gland of 1 mo-old dogs. The differentiation of the dog pineal cell types is completed by the second postnatal month.

Immunoperoxidase methods for the demonstration of three glial antigens, vimentin, glial fibrillary acidic protein, and S-100 protein, were applied to routine-fixed paraffin sections of rat pineal gland. A pre-embedding electron microscope immunoperoxidase method was also used to study the ultrastructural localization of S-100 protein in pineal cells. Light and electron microscopic results showed the presence of these antigenic glial markers in the second pineal cell type. The term glial cell is proposed for the second of parenchymatous cell in rat pineal gland.

A study of the embryonic development of the pineal gland of the chicken was performed with the optical microscope. The time of apparition of the first outline, as a derivative of the roof of the third ventricle, was fixed at 3 days. The major portion of the pineal parenchyma is derived from the frontal wall of the outline. All the vesicles and follicles were formed by solid mammilliform projections which subsequently presented a central lumen. In no case was a communication of the follicular cavities with the pineal recess observed. Three categories of vesicles or follicles can be distinguished according to their origin.