HISTOLOGY OF THE THYMUS 303 



the closely packed small cells, one finds scattered nuclei of the 

 large type, evidently belonging to the reticular cells, but the 

 protoplasmic reticulum is obscured. 



In the ordinary hematoxylin-eosin preparations, or in sections 

 stained with polychrome methylene-blue eosin, there are regu- 

 larly seen scattered cells with large, rounded eosinophilic granu- 

 lations. These cells are found both in the cortex and the me- 

 dulla, and are often but not always, in close relation with the 

 connective tissue sheaths of the blood vessels. I have been 

 unable to decide from my preparations whether these granulae 

 always belong to leucocytes or myelocytes, or whether some of 

 the reticular cells may not contain eosinophilic granules. 



The foregoing brief description, which agrees in all essentials 

 with that of previous workers on the amphibian thymus, but 

 which ignores entirely the mooted points as to the histogenesis 

 of the different cellular components, will however suffice for the 

 present study. I wish now to describe the appearances found 

 in sections fixed and stained according to Benda's method for 

 the demonstration of mitochondria. 



In successful slides, the nuclear chromatin does not retain the 

 crystal violet, but is stained yellowish brown by the alizarine. 

 When, for reasons which were not determined, the nuclei retain 

 the violet stain, the cytoplasmic granules are much less readily 

 demonstrable. These preparations, however, give excellent pic- 

 tures of the mitotic figures, of the fibrils of the myoid cells, 

 and bring out also certain coarser granules in the reticular cells, to 

 which we shall refer later. 



In good preparations, the nuclei, though unstained with the 

 violet, are distinct. The chromatin of the small cells is in the 

 form of clumps, from one to four in a nucleus, lying in an un- 

 stained clear space. Ragged threads extend from these to the 

 nuclear membrane. The latter shows a tendency to retain the 

 crystal violet, especially when differentiation is not carried too 

 far. The nuclei are thus often bounded by a rather heavy 

 purplish line. It is not possible to distinguish absolutely between 

 the nuclei of the small cells and those of the larger reticular 

 elements. In general, the latter show a more delicate and evenly 



