DR, H. WATNEY OK THE MINUTE ANATOMY OE THE THYMUS. 1097 
pressing further into it (see Plate 86, fig. 10), and sending out lateral processes which 
join on to the other invading processes, they isolate portions of the cortex, or even 
large parts of the follicle (see Plate 86, fig. 10). The processes have generally a 
narrow neck, and widen out considerably at the end. In these projections we find at 
first some retiform tissue, stretching from the vessel to the perivascular sheath. This 
soon gives place to cells of two kinds: plasma and granular cells. The plasma cells 
are the forerunners of fat cells. The granular cells are here often found on the walls 
of large veins (see Plate 88, figs. 33 and 34), they form fibrous-tissue and blood vessels. 
Thus we have penetrating into the follicle bands of fibrous-tissue containing fat cells; 
and by this means fat invades the follicle ; but there is never any fatty metamorphosis 
of the lymphoid corpuscles. 
Changes in the vessels. 
In the invading processes of connective-tissue there is a considerable formation of 
new vessels : they are very peculiar. They are enclosed first of all between the mem¬ 
branes formed by the capsule of the follicle ; they often have, in addition, a perivascular 
membrane, in some cases a membrane inside this again, like a double perivascular 
sheath; but the most remarkable feature is their adventitia, which often looks like a 
second layer of endothelium outside the vessel (see Plate 94, fig. 95). In some cases, 
there are two or even three layers of cells, forming such an adventitia (see Plate 94, 
fig. 96). These vessels are often seen to be filled with blood, and the lumen does not 
appear to differ from that of other vessels ; they do not become concentric corpuscles, 
as Afanassiew (51) has stated ; for they are not found in numbers until the thymus 
is undergoing involution, nor found to any extent except in the invading processes of 
connective-tissue. - *' These vessels are much enlarged during involution, and it is 
possible that this adventitia is the means by which the vessel increases in size. 
To sum up the changes in involution : we find a large formation of connective-tissue 
and fat between the follicles; this tissue invades and divides the follicles. At the 
same time there is a change taking place in both the cortex and medulla—produced 
in the cortex by the connective-tissue-corpuscles, in the medulla by the granular cells, 
giant cells, and connective-tissue-corpuscles, the cortex disappearing much the more 
rapidly of the two. 
The gland, diminished in size, though with enlarged blood vessels, is transformed 
into connective-tissue, and in Mammals is finally buried in fat. (“In adipe circumfuso 
sepelitur ,” Haller (35)). 
The Thymus of Birds, Reptiles, Amphibia, and Pish. 
During the growth of the organ the microscopical characters of the thymus of all 
animals are very similar. When the point of full development has arrived, then, and 
* Except, in tlie Tortoise, where all the vessels in involution partake of this nature. 
7 A 2 
