52 The Development of the Thymus 
the outer part of a corpuscle is formed of reticulum cells that have moved 
up and flattened themselves out around it. The description just given 
shows that the corpuscles are never composed of distinct cells, and that 
the increase in size is due to an extension outward of the colloid formation 
and not to a moving in of the adjacent tissue. 
The concentric form of this type of corpuscle is due at first to its being 
formed around a spherical or ellipsoidal nucleus. The swelling of this 
nucleus creates a centrifugal pressure in the adjacent cytoplasm. Before 
or during its transformation into colloid; the cytoplasm also imcreases in 
quantity. That the cytoplasm increases in quantity is shown by the fact 
that the nuclei are fewer in the corpuscle than in any adjacent area of 
the syneytium of equal size. This centrifugal pressure presses the newly 
formed colloid into concentric lamelle. It at first turns the long axes of 
the nuclei tangentially, and later flattens them and makes them concave 
toward the center. 
2. The epithelioid type of corpuscle is characterized by large areas of 
cytoplasm so marked off by colloid lamelle as to give the appearance of a 
mass of large epithelial cells. They may contain only one nucleus em- 
bedded in a well-defined area of cytoplasm (Plate IIT, Figs. 18 and 20). 
These correspond to the monocellular corpuscles that have been described 
for lizards and amphibians. They are rare in the pig. I have not been 
able to trace these very far, as they soon become indistinguishable from 
other forms. The only difference I have noted is that the outer colloid 
lamellae begin to form early, causing the peculiar appearance of a large 
epithelial cell. Again the epithehoid type may present an appearance 
such as shown in Plate I, Fig. 3. These do not seem to be formed around 
any special nucleus. The outer colloid lamelle form before any center 
has been established, marking off large cytoplasmic areas that may look 
like large cells. The centrifugal pressure of expansion caused by the 
great increase of cytoplasm in this area determines the concentric form 
in these corpuscles. Pure epithelioid corpuscles are very rare, but epi- 
thelioid areas in other corpuscles are not uncommon. The occurrence of 
epithelioid areas in corpuscles of the ordinary type shows that it is due 
to variations in a fundamentally similar process. 
3. In the cystic type of corpuscle, the central part, instead of becoming 
transformed into colloid, undergoes early liquefaction, forming vacuoles. 
The central nucleus does not increase in size as in the ordinary type, but 
shrivels up and disappears. The corpuscle begins by the formation of 
outer colloid lamella—the central mass is not changed into colloid. 
In Plate II, Fig. 10, the central area (p m) is undergoing a diffuse 
liquefaction. The nucleus (7) is colorless and shrunken. In Plate IT. 
