456 J. A. BADERTSCHI5R 



« 



the entrance of lymphocytes into the thymus at the time of the 

 fixation of the material. 



Usually there are no indications on the surface of the lobules 

 to mark the place where lymphocytes have entered it. On 

 account of the plasticity of the cytoplasmic syncytium we can 

 assume that the gaps formed in it by the entrance of the lympho- 

 cytes immediately close up. Not infrequently, however, places 

 can be found where the surface of a lobule is dented in and a 

 lymphocyte located in the thymus near the depression (fig. 2, 

 L.L., lower border to the left). Also occasionally a lymphocyte 

 can be found in a lobule some distance away from the periph- 

 ery with a trail (fig. 2, T.) leading to the surface of the lobule. 

 This trail apparently marks the path that a very active lympho- 

 cyte took in its migration from its place of entrance to the posi- 

 tion it now occupies. To similar microscopic pictures as repre- 

 sented above, Maximow has given a like interpretation. The 

 first lymphocyte present in the thymus then must come from 

 the mesenchyme and not from transformed epithelial nuclei. 



Another type of cells which are comparatively few in number 

 and found only in the earlier developmental stages deserves 

 mention before passing on to a later developmental stage. These 

 cells (fig. 1, X.) are characterized by rather deeply stained nu- 

 clei which resemble closely the degenerating epithelial nuclei 

 discussed above. The cell wall, if present, is indistinct. Their 

 cytoplasm can be distinguished from the cytoplasm of the epithe- 

 lial cells only by its darker color. The majority of the cells are 

 long and drawn out and usually lie near the surface of the thy- 

 mus anlage with the long axis of the cell nearly perpendicular 

 to the surface. These were most numerous in the 17 mm. 

 stage and entirely absent from the 40 mm. and later develop- 

 mental stages. This type of cells was also observed by Maxi- 

 mow ('09 b) who derived them from epithelial cells which for 

 a time assume such form then revert to the usual type of epithe- 

 lial cells. Their origin and significance are unknown to me. 



Embryo ^2 mm. (fig. 3). The lobes of the superficial thymus, 

 the thymus head, and of the mid-cervical and thoracic segments 

 have greatly increased in size. A few lymphocytes are now 



