DR. H. WATNEY ON THE MINUTE ANATOMY OF THE THYMUS. 
1085 
tissue-corpuscles, or (2) of epithelioid cells, or (3) of protoplasmic masses not differ¬ 
entiated into cells : and also contains a reticulum, which only occurs at rare intervals. 
Besides these, there are many granular cells, giant cells, and concentric corpuscles, 
found in the medulla, each of which will now be considered. 
Granular cells. 
During the latter part of foetal life, the medulla is studded in places with granular 
cells (see Plate 90, fig. 49), and during the remaining period of growth and involution 
these cells are present (see Plate 88, fig. 31). However, there are many more during 
the first period of involution than at any other time. They are more numerous in the 
Calf than in other Mammals, but they are found in all classes of Vertebrates, and in 
much greater number in the Mammalia and in the Frog, than in the Bird, Beptile, or 
Fish. They are easily recognised, in sections made from tissue hardened in bichromate, 
and stained in hsematoxylin, as yellow masses with somewhat coarse granules; they 
are often found lying close together, arranged in bands, or in masses lying over one 
another in great numbers. They are not readily stained by the ordinary hsematoxylin 
solutions, or by carmine, if the tissue has been hardened by chromic acid and alcohol. 
They are, however, stained very readily if the tissue has been hardened in osmic acid 
(see Plate 90, fig. 49), and are deeply stained by red hsematoxylin solutions (see 
Plate 85, fig. 8). 
These granular cells vary considerably in size (compare Plate 93, fig. 82 ; and Plate 88, 
fig. 36). In fresh preparations, their protoplasm closely resembles in appearance that 
of the giant cells of medulla of bone. In sections, some cells having very coarse 
granules are met with, close to others with much finer granules (see Plate 88, fig. 33, 
and compare figs. 33 and 34). 
The granular cells may be divided, for the sake of convenience, into four varieties, 
although it should be clearly understood that there are intermediate forms. 
They are found (1) as polygonal or rounded epithelioid cells, often lying in close 
approximation; the central part of the cell only is granular (see Plate 88, figs. 31 
and 33); (2) as vacuolated cells, in this case the mass lying in the vacuole is granular 
(see Plate 88, fig. 32 ; and Plate 89, figs. 45 B, va, and 45 C, ra ); or (3) as spheroidal 
masses lying in cavities between the branching processes of the connective-tissue- 
corpuscles, as in Plate 88, fig. 30, and Plate 89, fig. 46 E ; or (4) as club-shaped masses, 
attached to blood vessels and to newly-formed connective-tissue, as in Plate 88, 
figs. 35, 37, and 36, 38. 
We will first consider the epithelioid cells which contain granular masses. They are 
well seen in Plate 88, figs. 33 and 31. The granular mass in the cells varies in size 
from quite small spheres (as in Plate 89, figs. 44 A, 45 A, g, and 41) to masses which 
nearly fill the cell (see Plate 89, fig. 44 F). When the contained granular mass is 
quite small the nucleus of the cell is nearly central (see Plate 89, figs. 44 A and 41), 
but as it increases in size the nucleus is forced to one side (see Plate 88, fig. 31), and 
