Development of the secondary palate in man.

A study of secondary palate formation in man employing histochemical techniques indicates that (1) tongue muscles are sufficiently differentiated at the time of palatal shelf elevation to produce active tongue movements; (2) an embryo with asymmetrically arranged shelves and tongue suggests that the tongue is indeed active at this time; (3) few blood vessels are present beneath the epithelium at fusion sites both before and after fusion has occurred suggesting that this epithelium is not metabolically active; (4) only slight lysosomal enzyme activity is seen in the disintegrating epithelium at the junction of palatal shelves and nasal septum, and (5) macrophages are infrequent near to the disintegrating epithelium and their scarcity is consistent with the prolonged survival of epithelial rests in the midline of the human secondary palate.