The thyroid glands of the dog, cat, rabbit, rat, mouse, and guinea pig were stained by a modification of the silver impregnation method of DAVENPORT (1930). The parafollicular cells were selectively demonstrated with their cytoplasm filled with fine, silver blackened granules. As fixative Bouin, Carnoy or 10% formalin could be used for this method. Because of its simple and quick procedure and better fixation, this impregnation method has advantages over that of Cajal which has been used for the demonstration of this type of cell.
Puppies were given three injections (2.5hr intervals) of 50mg/kg CaCl₂ and 100, 000 IU Vitamine D₂. The parafollicular cells after this treatment showed a marked decrease in their argyrophil granules and a strong shift of the granules to the basal side of cell. Some cells even completely lost their granules and could be identified as parafollicular cells by bleaching the impregnated section and restaining it with hematoxylin-eosin which revealed the nuclei characteristic of this type of cell.
The relative population of the parafollicular cells to follicular cells was calculated and it became obvious that they were by far more numerous than ever estimated (e. g., 30-90 parafollicular cells per 100 follicular cells in the dog and 30 parafollicular cells in the rat).
As to the problem of the origin of the parafollicular cells, the hypothesis that these cells were derived from the ultimobranchial body was considered. The ciliated epithelium of the ultimobranchial cyst in the thyroid of postnatal animals, however, contained few argyrophil cells.