Parafollicular cells of thyroids of female fowls are studied. The materials used are the same as the author's previous papers. The results obtained are as follows. 1) From the successive observations of the materials (Fig. 1 to 8), especially from the state of colloids contained, the parafollicular cells may be classified to many transitional types. 2) Concerned to the fact above described, the supposition is able to think that the clusters of the parafollicular cells are the precursors of the nowly formed follicles.