DR, H. WATNEY ON THE MINUTE ANATOMY OF THE THYMUS. 
1099 
Thymus of the Reptile. 
The thymus of the Tortoise will be described first, as specimens of that animal were 
procured of various ages. The thymus of one young American Alligator, three months 
old, and of a fully-grown Python, were also examined. 
The thymus of the Tortoise, when the animal is quite small, resembles that of the 
Mammal; and then the follicle is composed of cortex and medulla; but as the animal 
grows older the follicle is composed of three parts : an outer cortical part, which is 
formed of flattened cells, with a delicate reticulum stretching between them (see 
Plate 95, fig. 105); a medullary portion, containing most of the concentric corpuscles ; 
and between these two a zone of tissue, looking as if it were compressed, containing 
a ring or rings of vessels, and many lymphoid cells. This peculiarity of the thymus 
of the Tortoise depends on the fact that the cortex does not disappear more rapidly 
than the medulla, as in the Mammal or Bird ; but the lymphoid cells disappear, and 
the network of connective-tissue-corpuscles increases, and takes their place. A change, 
something like- this, is shown in the Mammal in Plate 93, fig. 80, but it does not there 
proceed to anything like the same extent, as in the Tortoise, where the cortex finally 
contains hardly any lymphoid cells. Many pigmented cells can be seen in the connective- 
tissue at the edge of the follicle, and surrounding the invading blood vessels; these 
appear to correspond to the plasma cells of the thymus of Mammals. 
Further, there are follicles (as has been noticed by Afanassiew (33)) which are 
entirely formed of flattened, spindle-shaped cells. These follicles will be more com¬ 
pletely noticed in the description of the thymus of the Alligator. 
The concentric corpuscles of the thymus of the Tortoise present intermediate forms 
between the concentric corpuscles of the Bird and the ciliated cysts of the Dog. There 
are at times found in the thymus a few granular cells (see Plate 95, figs. 104 and 107). 
The protoplasm of the largest of these cells is marked by concentric rings (see Plate 95, 
fig. 104). The concentric corpuscles are formed in great measure by epithelioid cells ; 
they contain vacuoles of various sizes, in which sometimes a granular cell is present 
(see Plate 95, fig. 106). In Plate 95, fig. 107, two granular cells are seen to be enclosed 
in a large vacuole. 
The fully-formed concentric corpuscles contain either a mass of cells balled together 
in the interior of the vacuole, evidently undergoing degeneration, or masses and granules 
of haemoglobin (see Plate 95, fig. 108). 
The blood vessels of the medulla are very large ; most of the vessels in the thymus 
of old animals have perivascular sheaths and thickened adventitia, and are very similar 
to those which we have described in the thymus of the Calf. A few giant cells are 
present. As involution proceeds the ring of vessels between the cortex and the medulla 
enlarges. 
The thymus of the young Alligator is composed of cortex and medulla, the cortex 
