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The Development of the Thymus 
unreasonable to suppose that its primitive function was the formation 
of a colloid secretion such as occurs in the thyroid, and that the corpuscles 
are abortive expressions of this primitive function.’ 
GIANT CELLS. 
Polykaryoeytes may often be seen in the medulla. These bodies de- 
velop from the syncytium of the medulla. They are first noticeable as 
groups of small spherical nuclei in a solid area of the syneytium. These 
nuclei stain with medium intensity and are all very similar in size and 
color. ‘The area containing this group of nuclei becomes a well-defined 
node of the reticulum and persists as such. A polykaryocyte is, there- 
fore, a large node of the reticulum containing a number of small nuclei 
very similar in appearance. These cells often occur in groups. They 
are entirely distinct from the corpuscles. They are evidently similar 
to the polykaryocytes found in bone marrow and other lymphoid tissues. 
SUMMARY. 
The following is a resume of the development of the thymus in the 
pig: 
The thymus of the pig is probably developed entirely from the endo- 
derm of the third gill pouch. 
By a gradual process of vacuolization and liquefaction of the cyto- 
plasm, the epithelial syncytium of the thymic anlage is converted into a 
cellular reticulum. 
From the first appearance of vacuolization, three types of nuclei are 
present: large pale nuclei; small dark nuclei (lymphoblasts), and large 
dark intermediate forms. 
The lymphoblasts gradually break loose from the cellular reticulum, 
moving into its spaces and forming lymphocytes. Mitoses are most nu- 
merous at the period of the most rapid formation of lymphocytes. The 
medulla continues to form lymphocytes at least as late as birth. 
Lymphocytes appear in the connective tissue around the thymus 
shortly after they are formed; and lymphoblasts, which are distinguish- 
able from lymphocytes only by being embedded in the syncytium, are 
present in the thymus a long period before lymphocytes are found any- 
where in the neighborhood of the thymus. 
The cellular reticulum of the earlier stages persists in a modified form 
as the reticulum of both cortex and medulla. It retains more cytoplasm 
7Ver Eecke (28) believes that the corpuscles in amphibians are of a 
glandular nature. 
