DEVELOPMENT OF THE THYMUS 455 



embryo. Some are represented in figure 1 (L.L.). In slightly 

 later stages they have become more numerous as represented 

 in figures 2 and 5 {L.L.). All the lymphocytes present in the 

 thymus of these early stages are large lymphocytes. No small 

 lymphocytes are present. No transition forms from the normal 

 epithelial nuclei to the large plump lymphocytes with a generous 

 amount of basophilic cytoplasm can be seen. When first present 

 in the thymus they are there in a fully developed condition. 

 It is, therefore, evident that their source must not be sought in 

 the thymus anlage. It was stated above that large lympho- 

 cytes were present here and there in the mesenchyme of a 17 

 mm. embryo. In successively older stages their numbers grad- 

 ually increase until in stages ranging in length from 25 to 30 

 mm. they can be found in all parts of the mesenchyme without 

 much searching. In these later stages, however, they are most 

 nimierous in the neighborhood of the thymus and the large 

 blood vessels in the anterior portion of the thorax. Figure 

 1 represents a portion of a lobule of the thymus head and sur- 

 rounding mesenchyme of a 30 mm. embryo. Three lympho- 

 cytes (L.L.) can be seen in the mesenchyme, two of which have 

 a structure identical to those in the thymus. In one (lower 

 corner to the right) the cytoplasm has a distinctly lighter hue, 

 i.e., less basophilic, than the other two. Only very seldom 

 can this latter type be found in the thymus anlage (fig. 2, L.L.; 

 lower border to the left). These will be considered farther on 

 in the paper. One of the lymphocytes (fig. 1) is in contact 

 with the surface of the thymus. The microscopic picture, which 

 is reproduced in the figure, is suggestive. Since lymphocytes 

 are found in the mesenchyme in -the neighborhood of the thymus 

 before they are found in it, and since there are no transition forms 

 between epithelial cells and lymphocytes nor any blood vessels 

 in the thymus anlage, only one conclusion can be drawn in re- 

 gard to the source of the lymphocytes first present in the thymus, 

 namely, that they have ixiigrated into the thymus from the 

 surrounding mesenchyme. The lymphocyte bordering on the 

 surface of the thymus was apparently about to enter it when the 

 material was fixed. Many similar conditions exist, indicating 



