DEVELOPMENT OF THE THYMUS 477 



are formed in that organ. The free eosinophile granules are never 

 very numerous but the thymus of late developmental stages in 

 which the erythrocytes are comparatively numerous contains 

 more than the thymus of early stages. Erythrocytes and normo- 

 blasts with granular (degenerating) cytoplasm are also present. 

 The red blood-cells in the thymus usually have an irregular 

 outline. This is not a sign of degeneration but is brought about 

 by purely mechanical factors as stated above. In late develop- 

 mental stages phagocytes ingested with erythrocytes and other 

 degenerated products can occasionally be found in the lobules of 

 the thymus. These, however, appear first and are more numer- 

 ous in the interlobular septa. In some groups of red cells in 

 the superficial thymus of the 270 mm. fetus some of the erythro- 

 cytes are apparently fused, forming as seen in section, irregularly 

 and deeply red stained and quite homogeneous patches which 

 contain only a few lymphocytes. Whether or not the fused 

 erythrocytes undergo degeneration could not be determined with 

 the material at hand. The thymus of post-natal pigs needs 

 to be investigated to determine the fate of the numerous free 

 erythrocytes present in that organ. It is evident, however, 

 that not all, if any, enter the circulation. 



In all the developmental stages examined eosinophile cells^ 

 are, in general, more numerous in the connective tissue of the 

 interlobular septa than in the lobules of the thymus. A con- 

 sideration first of their origin in the former place will, therefore, 

 aid in determining their origin in the lobules. Eosinophile 

 cells are already present in the mesenchyme of quite young 

 stages (20 to 25 mm.). In all these and in somewhat older 

 stages (55 mm.) they are not found in localized areas but may be 

 found in almost any part of the connective tissue. In the 55 

 mm. embryo they are somewhat more numerous than in the 

 younger stages but can be found only after considerable search- 

 ing. From embryos more than 55 mm. in length only the 



^ The value of the distinction of eosinophile myelocytes (myeloid eosinophiles) 

 and eosinophile leucocytes is not considered and cells containing eosinophile 

 granules are therefore simply designated as eosinophile cells regardless of the 

 shape of their nucleus. 



