INANITION OF THYROID IN RATS 321 



est stage for at any rate that tlei^arting least from the normal 

 type) consists in a mere thickening of the nuclear membrane 

 ("nuclear wall hyperchromatosis") or an increase in the number 

 and size of the nuclear chromatic granules, so that as a whole 

 the nucleus appears darker and more deeply-staining than usual. 

 This form appears normally in the more flattened cells of the 

 larger peripheral follicles, and in the maintenance rats occurs 

 more or less freciuently throughout the gland, associated with 

 the shrinkage and flattening of the cells (especially where the 

 nucleus is also flattened). 



Associated with or following the nuclear hyperchromatosis just 

 described (in some cases perhaps independently thereof) is found 

 a deepening coloration of the homogeneous nuclear background, 

 which gives the appearance shown in figure 8. This appearance 

 of incipient pycnosis is very characteristic and represents tlie 

 predominant type of nuclear change oliserved. Apparently the 

 process involves a progressive dissolution of the basic chromatin 

 masses, which are probably dissolved in the nuclear saji. \'ari- 

 ous stages occur. The earliest show merely a slight darkening 

 of the homogeneous background, which gi-aduall^' increases un- 

 til the nuclear net and karyosomes are obscured and finally dis- 

 appear (figs. 8 and 9j. In size, the nucleus at first shows no 

 marked change (beyond perhaps a slight preliminary shrinkage), 

 but as the process continues the nucleus usually decreases greatl.v 

 in size. In the typical rounded, homogeneous, dense pycnotic 

 nuclei, the diameter rarely exceeds 4 niicra, which repi'esents a 

 tremendous shrinkage. When this stage is reached, the nuclei 

 may apparently persist for a long time without further change, 

 even though the cytoplasm has been reduced to a ^'estige or 

 entirely removed (as, for example, in the mass of nuclei on the 

 right in figure 9). In many cases of advanced cellular degen- 

 eration, however, the pycnotic nuclei become irregular in form 

 and may later undergo karyorrhexis, l:>reaking up into fragments 

 which eventually through karyolysis are dissolved antl disappear. 

 Some of these extreme stages of karyorrhexis are shown in fig- 

 ure 9. 



