DEVELOPMENT OF THE THYMUS 473 



majority in the thymus have a smaller average diameter than 

 those in the latter place. Their contour is often very irregular 

 which is due to their lying closely together when found in groups 

 or wedged in between lymphocytes and epithelial cells when 

 they occur singly. The nuclei of some have the characteristic 

 coarsely granular structure of erythroblasts (Erb.) and young 

 normoblasts, while in others the nuclei are pyknotic. The red- 

 dish hemoglobin-containing cytoplasm of the nucleated red cells 

 varies in its amount in different cells but is easily recognized 

 even with moderately high magnification in the larger and 

 medium sized cells. Some of the nucleated red cells have two 

 nucleoli {A. Erb.) which stain intensely, are round, and of equal 

 size. These apparently are undergoing amitotic division. Ery- 

 throcytes {Ere.) are quite numerously scattered among the 

 erj^hroblasts. Aside from the irregular outline of some of the 

 nucleated red cells found in the thymus of this stage they com- 

 pare favorably in all other respects with the free erythroblasts 

 and normoblasts found in the septa. Also they have the same 

 origin, namely, from the lymphocytes. 



The lymphocytes in the thymus in which the origin of the 

 red blood-cells was just considered belong almost entirely to 

 the large and medium-sized type. Only a very few small ones 

 are present. It is, therefore, necessary to consider the origin 

 of the numerous free erythrocytes in the thymus of late develop- 

 mental stages in which the large majority of all the lymphocytes 

 belong to the small type. The superficial thymus of a 270 

 mm. (full term) fetus was selected for this purpose because the 

 red blood-cells in the thymus of this stage are more numerous 

 than in any other examined. The great majority lie in groups 

 which, in a section, appear as smaller or larger bright red irregu- 

 lar patches or as long tortuous streamers. The proportion of 

 lymphocytes to the red cells varies in different groups. In 

 some the lymphocytes are most numerous, in others they are 

 about equal in number, while in still others the red cells greatly 

 predominate. Also, in some groups the erythrocytes make 

 up nearly the entire num.ber of red blood-cells, only a few nu- 

 cleated red cells being present. In some groups many nucleated 



