DEVELOPMENT OF THE THYMUS 461 



pig, for only very seldom can degenerated cells be found which 

 may be degenerated epithelial nuclei, and as to whether they 

 migrate into the cortex it is indeed difficult to establish in fixed 

 material when no circumstantial evidence is present indicative 

 of their migration. A more plausible interpretation seems to 

 be that during the hypertrophy of the epithelial cells the medul- 

 lary portion of the thymus greatly increases in volume through 

 the enlarging of both the nuclei and anastomosing processes of 

 the syncytium thus separating the lymphocytes farther apart. 

 Just as many are present in the rapidly newly formed medulla 

 as there were in the syncytium from which the medulla was 

 formed only they are scattered over a larger area making them 

 to appear less numerous. This interpretation is made plausible 

 when the great rapidity of its initial development is considered, 

 e.g., in the thymus of a 60 mm. embryo no traces of the medulla 

 were present while in a 65 mm. embryo it has reached a stage of 

 development as described above. 



The reticulum of the cortex also is formed from the cyto- 

 plasmic syncytium of the epithelial cells. Its development, 

 unHke that of the medulla, is gradual. The change from the 

 rather coarse syncytial network of younger stages to fine and 

 greatly attentuated threads making up the reticulum in the 

 fully developed thymus is due to the lymphocytes constantly 

 increasing in numbers in its meshes thus gradually separating 

 the cell bodies of the reticulum farther apart. In all of the 

 developmental stages studied mitosis of the epithelial nuclei 

 could be found, being, however, more numerous in younger 

 than in later developmental stages. 



In this and slightly earlier stages (55 mm.) nucleated and non- 

 nucleated red blood-cells lying free in the parenchyma of the 

 thymus are of frequent occurrence. While some are scattered 

 about singly they usually occur in groups. An occasional 

 eosinophile cell can also be found. In the interlobular septa 

 phagocytes can be found without much searching. 



The thymus head and superficial thymus were so oriented 

 on the microtome that sections of both of these regions were 

 made by a single stroke of the knife. This made a comparison 



