DEVELOPMENT OF THE THYMUS 459 



rence. Only an occasional deeply stained epithelial cell can 

 be found. Completely degenerated epithelial nuclei can be 

 seen scattered here and there throughout an entire section. 

 The connective tissue of the interlobular septa now contains 

 numerous lymphocytes of all sizes. The deep portions of some 

 of the septa are so completely gorged with them that it is diffi- 

 cult to distinguish clearly where the septa end. An especiallj^ 

 favorable place for lymphocytes to collect seems to be along 

 the course of blood vessels of the septa. They can be found 

 strung along in rows on one side of the vessels or the accumula- 

 tion may extend entirely around it. The vacuoles of the syn- 

 cytium are not as numerous as in the preceding stage. Most 

 of them have become occupied with lymphocytes. In later 

 stages they are altogether absent. 



An almost uninterrupted zone of epithelial syncytium (7.pr.) 

 extends around the periphery of the thymus. It is from one 

 to three epithelial nuclei deep and, on account of the few lym- 

 phocytes which it contains, appears quite clear in contrast to 

 the deeper portion of the syncytium in which are found many 

 lymphocytes. It is not pronounced along the interlobular 

 septa. Mitoses of the epithelial nuclei are more numerous in 

 this zone than they are in the deeper portions of the lobules, 

 hence, Prenant called it the zone of proliferation. According to 

 him both lymphocytes and reticulum cells are formed from this 

 zone. This, however, cannot be the case for the transition forms 

 from epithelial nuclei to lymphocytes are not present. Con- 

 sidering the fact that the epithelial zone is most pronounced 

 only on the convex peripheral surface of the lobules, and that 

 it is present only during the period of rapid growth of the thy- 

 mus, it can rightly be regarded as a zone of proliferation for 

 epithelial cells but not for lymphocytes. It is mainly from this 

 zone that the reticulum of the peripheral margin of the cortex 

 is formed while the thjanus is rapidl}^ growing in thickness. This 

 interpretation of the significance of this zone is in accord with 

 that of Maxim ow. 



This develomental stage marks the appearance of the medulla. 

 Longitudinal sections through the thymus head show that the 

 epithelial syncytium of ahnost the entire central stem has under- 



