DEVELOPMENT OF THE THYMUS 479 



in others they are thinly and quite evenly scattered throughout 

 the basophilic cytoplasm, while still others are completely 

 gorged with them. 



A striking peculiarity is that the large majority are mono- 

 nuclear. Only very seldom can one of the polymorphonuclear 

 type be found. The nuclei are round, slightly indented, or 

 crescentic in outline and are usually eccentrically located in the 

 cell. In those cells that are gorged with granules the nuclei 

 are crowded to one edge of the cell and stand out conspicuously 

 among the eosinophile granules. The structure of the nuclei 

 is identical with that of the nuclei in the large lymphocytes which 

 have been described. 



The thymus of a 125 mm. embryo was chosen to consider 

 the origin of the eosinophile cells. In this stage the inter- 

 lobular septa, loose in structure, contain numerous lympho- 

 cytes, red blood-cells, and many eosinophile cells lying both 

 singly and in groups. Also in a single group can be found 

 eosinophile cells containing varying numbers of granules, as 

 stated above. An interesting and instructive feature often 

 to be observed is the presence of large lymphocytes containing 

 only from one to three or four eosinophile granules which are of 

 the same size and shape as those found in cells completely gorged 

 with them. Often in very hmited areas — covered by very 

 slightly moving the slide under high magnification — can be 

 found large lymphocytes and* a series of eosinophile cells with 

 gradually increasing numbers of granules (fig. 7, l.m.n.). Only 

 one interpretation can be given to microscopic pictures of this 

 kind, namely, that the eosinophile cells are derived from lympho- 

 cytes. This conclusion also accounts for the large numbers of 

 eosinophile cells along the course of blood vessels in late develop- 

 mental stages, for it is along the blood vessels — in the loose por- 

 tions of the septa — that the lymphocytes are most numerous. 



I believe that the groups of eosinophile cells in the septa are 

 identical with the granular cells of Watney which he found in 

 the interlobular septa of the thymus in various classes of ani- 

 mals, although none of the cells were attached to the tunica 

 externa of the vessels, as was observed by him. The ultimate 



