472 J. A. BADERTSCHER 



With this brief review of the origin of the erythrocytes in the 

 interlobular septa we are prepared to consider their source in the 

 cortex and medulla of the thymus. In every stage from the 55 

 mm. to the full term embryo that was examined, red blood- 

 cells were found singly and in groups in the thymus. In the 

 developmental stages approaching maturity a larger number of 

 the lobules contain groups of red cells than in younger stages. 

 Some lobules contain two or three groups some of which are 

 quite large. Also red cells lying singly in the thymus are more 

 numerous in the later than in the earlier stages. The super- 

 ficial thymus and the thymus head were found to contain a 

 relatively larger number than the mid-cervical segment. Un- 

 fortunately, the thoracic segment of late developmental stages 

 was not collected, so I was unable to make a comparison of their 

 number with that of the other segments of the thymus. The 

 superficial thymi and the thymus heads of two full term fetuses 

 (270 and 280 mm in length) contained a comparatively larger 

 number of red blood-cells than the corresponding segments of 

 somewhat earlier stages. In the thymus of the 280 mm. fetus 

 the red blood-cells were about equally distributed in the two 

 segments while the red cells in the superficial thymus of the 270 

 mm. embryo were much more numerous than in the thymus 

 head. In full term embryos groups of red blood-cells are found 

 in both the cortex and medulla of the thymus. In the younger 

 stages no groups of red blood-cells were found in the medulla 

 although they may be found lying singly in that region. 



An occasional nucleated red blood-cell can be found in the 

 thymus of embryos 35 to 50 mm. in length. Erythrocytes in 

 these stages are very seldom found. They do not come from 

 the blood for blood capillaries have not yet penetrated the lobules 

 at this stage. In an embryo 55 mm. in length, in which only a 

 few capillaries are found in the lobules, they are much more 

 numerous than in the preceding stages. Nucleated and non- 

 nucleated red cells can be found singly among the lymphocytes 

 which at this stage are already quite numerous but the striking 

 feature is that they are present in groups (fig. 6). They vary 

 somewhat in size as do those in the interlobular septa but the 



