DEVELOPMENT OF THE THYMUS 463 



The clear epithelial zone around the periphery of the thymus 

 is present in all the stages excepting the 180 mm. embryo. Around 

 the thymus head of the 165 mm. embryo it is at its highest de- 

 velopment. This is contradictory to the observations of Bell 

 who states that this zone has disappeared in a 140 mm. embryo. 

 Mitoses of epithelial cells in this zone are quite numerous and in 

 no stage is it entirely free from lymphocytes. The limiting 

 membrane could no longer be distinguished around the thymic 

 lobules in the 180 mm, stage. It apparently has become blended 

 with the thin capsule that invests the thymus of this and later 

 stages. 



In all these stages the medulla contains a relatively much 

 larger number of lymphocytes than in the 65 mm. embryo, which 

 makes it appear less conspicuous. This is especially the case 

 in the 180 mm. embryo, but even in that stage in suitably stained 

 preparations it is still quite sharply defined from the cortex. 

 Mitoses of all types of lymphocytes occur here as in the cortex. 

 In the 180 mm. embryo Hassall's corpuscles are more numerous 

 than in the earlier stages, while some are still in the process of 

 formation. The reticulum on the whole is much coarser than 

 in the cortex and hence more easily demonstrable. 



In the 180 mm. embryo deeply stained (degenerating) epithe- 

 lial nuclei can be found only after prolonged searching while in 

 the earlier stages they are of more frequent occurrence in both 

 the cortex and medulla. Debris of degenerated cells, some of 

 which in these stages is undoubtedly composed of nuclei ex- 

 truded from normoblasts, also occurs. 



A discussion of the red blood-cells and granular leucocytes 

 will be considered later. 



Embryo 270 mm. (full term). Since one of the main objects 

 of the investigation of the developing thymus was to determine 

 the origin and fate of the superficial thymus its histological 

 structure will, therefore, be considered. The lobules are now 

 closely packed together. The cortex has greatly increased in 

 thickness over that of the 180 mm. embryo. The lymphocytes 

 are no more closely packed together than in the previous stage, 

 room having been made for the additional number by an increase 

 in the volume of the organ. While the small lymphocytes are 



