DEVELOPMENT OF THE THYMUS 483 



large lymphocytes that have mgested eosinophile granules (debris 

 of degenerated erythrocytes) which as was stated above can be 

 found lying free in the meshes of the reticulum among the lym- 

 phocytes. The free eosinophile granules do not seem to be 

 numerous enough, as in the case of the septa, to account for all 

 granules found in eosinophile cells. However, an apparent 

 general relationship exists between the latter type of cells and 

 the erythrocytes which indicates that at least some of the gran- 

 ules of eosinophile leucocytes are derived from degenerated 

 erythrocytes. The features indicating this relationship may be 

 expressed as follows: (1) As in the thymic septa, the red blood- 

 cells are present in advance of the eosinophile cells; (2) The 

 eosinophile cells increase in numbers in successively advanced 

 developmental stages as do also the red blood-cells; (3) They 

 are most numerous in the thymus of a full term fetus in which 

 developmental stage the red blood cells are also most numerous; 

 (4) In the thymus of a 270 mm. embryo the eosinophile cells 

 are more numerous in the superficial thymus than in the thymus 

 head, the difference in the numbers corresponding favorably 

 to the difference in the numbers of red blood-cells which are 

 much more numerous in the former than in the latter segment. 

 Here also it must be said that if all the granules of the eosino- 

 phile leucocytes in the lobules are derived from degenerated 

 erythrocytes it must also be assumed that their degenerated 

 products are taken up in soluble form by the lymphocytes in 

 which it is transformed into granules. 



Phagocytes (fig. 7, o. and p.) are found in the interlobular 

 septa of the thymus in a wide range of developmental stages. 

 They are most numerous in those stages in which the septa 

 have a loose structure and contain many lymphocytes. They 

 possess a large amount of cytoplasm which in some cells is 

 vacuolar. Some are gorged with ingested material which con- 

 sists mainly of lymphocytes (apparently) in various stages 

 of degeneration. Occasionally one can be found in which an 

 entire erythrocyte or a part of one makes up a part of the in- 

 gested material. They arise directly from connective tissue 

 cells some of which contain ingested particles even before their 



