DEVELOPMENT OF THE THYMUS 471 



from the mitoses that some are undergoing, the large lympho- 

 cytes through repeated divisions become smaller and form 

 small lymphocytes, /. Whether or not the small lymphocytes 

 have the power to grow and again form large lymphocytes, as 

 claimed by some investigators, is difficult to demonstrate. The 

 point of interest and importance is the development of erythro- 

 cytes and granular cells from the lymphocytes. In some of 

 the transition stages between the large and small lymphocytes, 

 or for convenience, the large and medium-sized lymphocytes, 

 changes occur in both their nucleus and cytoplasm. The latter 

 stains a faint brick-red indicating the presence of hemoglobin 

 while the nucleus becomes granular. These are the mega- 

 loblasts of Maximow or erythroblasts, g. In some cells, h, the 

 nucleus has the characteristic granular structure of the erythro- 

 blasts while the cytoplasm still retains its basophilic character, 

 or is dimmed only slightly by a faint trace of hemoglobin. These 

 are the younger forms of erythroblasts and aid in tracing the 

 source of the older ones. They may be found lying singly but 

 usually occur in groups. Mitoses of erythroblasts can be found 

 without much searching. Diagram i represents a normoblast. 

 The cells of this type are on the whole a little smaller than the 

 erythroblasts from which they are derived. Through the 

 extrusion of their nuclei they are transformed into erythrocytes, 

 j. That the nuclei are extruded is indicated by deeply stained 

 degenerating nuclei or fragments of them lying free among the 

 cells in a group made up of a mixture of both erythrocytes and 

 normoblasts. Thus the free erythrocytes of the interlobular 

 septa, as stated by Maximow, are derived from the lymphocytes, 

 their ultimate source being from transformed mesenchymal cells. 

 Whether or not they enter the circulation will be considered 

 presently. While they are found in the septa in quite early 

 stages they are most numerous in this region in embryos rang- 

 ing from 115 to 165 mm. in length, the greatest number 

 being present at about the 125 mm. stage. The superficial and 

 head thymus of a 270 mm. (full term) fetus contained a few, 

 singly and in groups, in the deeper and looser portions of the 

 septa. 



