50 The Development of the Thymus 
A. The concentric corpuscles include those that from their earliest 
appearance are concentric in structure. Adopting Ecker’s classification, 
I distinguish simple concentric corpuscles and compound concentric 
corpuscles. 
(a) Three types of simple concentric corpuscles are to be considered. 
(1) The ordinary type is far more numerous than any other. The earl- 
lest recognizable stage is shown in Plate II, Fig. 11. A nucleus (n) 
of the syneytium of the medulla has enlarged to perhaps twice its ordinary 
volume and has lost the ability to stain in the characteristic way with 
hematoxylin. Its sap is clear and a few reddish stained granules repre- 
sent its chromatin. Around it in the cytoplasm is an indistinct uneven 
layer of colloid (cf). The colloid is not yet solid and is being formed 
in concentric fibers or sheets. A shghtly later stage is shown in Plate IT, 
Fig. 14 and Fig. 15 (left side of figure). Some of the colloid (¢ s) 
next to the nucleus is now solid. The next stage is shown in Plate II, 
Fig. 15 (right side of figure). These corpuscles show a thick layer of 
colloid (¢ s d) that stains intensely with Congo red. Just outside the 
deeply staining colloid, colloid in formation may be seen. The nuclei 
are clear, and have become smaller and irregular in outline. The colloid 
seems to be pressing upon them and obliterating them. The colloid 
transformation gradually involves the adjacent cytoplasm of the syn- 
eytium until other nuclei are involved. The corpuscle has now reached 
the condition shown in Plate II, Fig. 12. The central area (0 c) is 
solid, the nucleus having disappeared entirely. Another (n’) is nearly ob- 
literated by the colloid. Part of the central area (0 c) no longer stains 
intensely, and it is breaking loose by the formation of a concentric space. 
Several nuclei are surrounded by colloid in formation. Their long axes 
are nearly in a tangential direction. ‘These nuclei are clear but only 
moderately swollen. 
In the further development of the corpuscle (Plate III, Fig. 17 and 
Plate II, Fig. 7), the central area (c s d) increases in size. The nuclei 
involved in this area become obliterated probably by the pressure of the 
colloid and are no longer distinguishable. This central area usually splits 
off and may break up into many smaller masses. The peripheral part of 
the corpuscle increases by extension of the colloid formation into the 
adjacent part of the syneytium. This extension takes place in the early 
stages by direct progressive involvement of the immediately adjacent cyto- 
plasm; in later stages (Fig. 7), by the formation of concentric lamelle 
which cut off unchanged areas of cytoplasm. The lamellxe increase in 
size and number, the cytoplasm included between them is changed into 
colloid. They finally become closely packed, giving the characteristic and 
