IXANITION OF THYROID IN RATS 315 



maximum sometimes reaching 150 micra, the largest folHcles be- 

 ing frequently found at the periphery of the gland. While cubi- 

 cal cells still occur to a variable extent, the average cell is some- 

 what more flattened than at 10 weeks, especially in the larger 

 peripheral follicles. The colloid is variable in appearance. Oc- 

 casionally the follicular content may be clear and unstainable. 

 The stroma remains as previously described. 



The most striking difference in the thyroid of the older rats is 

 found in increasing preA'alence of the degenerative process al- 

 ready described as appearing at 10 weeks. In the older control 

 rats it was found not only more frequently, but more pronounced 

 in character. In extreme cases, the follicles in some regions (see 

 fig. 10) are entirely obliterated, being replaced by masses of epi- 

 thelial cells with irregularly disintegrated cytoplasm and nuclei 

 in various stages of pycnosis or karyorrhexis. Among 20 glands 

 from apparently normal older rats, only 3 thyroids appeared en- 

 tirely normal; 6 showed slight degeneration, 5 were moderately 

 involved, while in 6 the degeneration was extensive, involving 

 the larger portion of the gland. The significance of this appear- 

 ance of degeneration in normal animals will l)e discussed later. 



h. Structure of thyroid gland in young rats held at maintenance 



The appearance of the thyroid gland in young rats held at 

 constant body-weight from 3 to 10 weeks of age is shown under 

 low magnification in figures 4, 5 and 6, and more highly magni- 

 fied in hgures 7, 8 and 9. The follicles in general appear in a\'- 

 erage size smaller than the normal at 10 weeks, although it is 

 somewhat difficult to make exact comparisons, on account of the 

 irregularity in their size. The maximum diameter found in 9 

 normal glands was al^out 100 micra, whereas in 14 maintenance 

 rats it was 85-90 micra. The average in both cases was of course 

 much lower, as follicles are found of all sizes down to those with 

 only a minute cavity. Although in the maintenance rats at 10 

 weeks a few follicles are larger than found in the normal rat at 

 the age of 3 weeks (where the maximum w^as 70 micra), it is 

 doubtful whether the average follicle is any larger. It is proba- 

 bly slightly smaller. 



