450 J. A. BADERTSCHER 



movement they take on various forms. Some are round with a 

 layer of cytoplasm of uniform thickness around the nucleus. 

 Some are irregularly oblong with the bulk of the cytoplasm massed 

 at one or both poles of the nucleus. Others are very irregular 

 in outline, sending out one or more pseudopods. The size of 

 the large lymphocytes varies somewhat but all possess a large 

 amount of cytoplasm. In the smaller of the large lymphocytes 

 the nucleus is proportionally smaller than in the larger ones. 



The nuclei of the lymphocytes are large, but on the whole 

 a little smaller than the epithelial nuclei, i.e., the nuclei of the 

 large lymphocytes are smaller than the large epithelial nuclei 

 while the nuclei of the smaller lymphocytes are smaller than 

 the small epithelial nuclei. A distinct nuclear membrane is 

 present. The form of the nuclei may vary considerably. Some 

 are spherical, some oval in outline, while others have a very 

 irregular shape. The varied forms of nuclei are undoubtedly 

 brought about through the motile activity of the lymphocytes. 

 The chromatin is generally distributed in the form of larger or 

 smaller irregular granules some of which are attached to the 

 nuclear membrane. These are often united with each other 

 by fine irregular chromatin threads. The nucleoli are large, 

 and usually very irregular in outline, having an extremely jagged 

 surface. On the whole the nucleus of a large lymphocyte con- 

 tains a more generous supply of chromatin than does an epithelial 

 nucleus. The amount of chromatin present and the manner of 

 its distribution in the two kinds of nuclei is, however, not the 

 main feature by which one can readily distinguish a large Ijrm- 

 phocyte from an epithelial nucleus. The basophilic character 

 of the cytoplasm of the lymphocytes is the main distinguishing 

 feature between them. At no stage in the histogenesis of the 

 thymus is the value of a differential stain more appreciable 

 than at the stage marking the appearance of the first Ij^mpho- 

 cytes, for, by the use of it, a lymphocyte can be distinguished 

 unmistakably from an epithelial cell or nucleus. 



The most critical developmental stages in which the source of 

 the lymphocytes in the thymus is to be determined, are those 

 in which only a few lymphocytes are found. The evidence indi- 



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