- 13 - Such follicles often contained darker staining colloid. such follicles have essentially no uptake of 131 I. Autoradiogrfphs showed that The cells compribing the wall of such a follicle are thus functionally different from others nearby Such giant follicles may be found containing a cluster of cells aris in the wall rather than the normal single layer of cells lining the t are functioning from one local est of the follicl Some of the masses of cells may be forming papillary tufts prot into the follicle These cells have a discernably higher incidence of nuclei bearing *H-TdR label thar cells in other parts of the wall of that follicle or in other folli es nearby. follicle appears to enlarge to accomodate the expanding papillary 1 ion within. Ul- The timately the cells ofthe parent follicle are replaced by commective tissue forming a capsule as the total mass cames to occupy more of a lobe. Although there may be- quite a mmber of the enlarged non-functioning follicles in a single irra jiated thyroid and « a few of the lesions described above appear in a single thyroid, buq they are in diffe: ent stages of development in a given animal. This is the evolution of papillary neo- Plasms. The follicular neoplasms appearto evolve much as the papill follicles develop among the papillary structures. lesions. Tiny The parent folli le contimes to | expand until the structure appears to be an encapsulated follicular adenoma. Multiple lesions are again seen in a single thyroid, where as solid cellular lesions are most often single but may be accampanied by papillary or mixed papillary and follicular lesions in the same thyroid. The application of autoradiography us ing 131) and 4y-TaR separately prepared from adjacent sections of tissue, with such obs prvations on very large mmbers of rats, have made the described sequences of develophent rather obvious The material acquired for these observations was based on four expe riments composed of several Immdred animals each and lasting 16 to 22 months.