DEVELOPMENT OF THE THYMUS 453 



out in sharp contrast to the dark epithelial nuclei. Now, what 

 structural features of the dark epithelial nuclei give them a 

 semblance to small lymphocytes? As the amount of cytoplasm 

 around the nucleus of a small lymphocyte is meager and often 

 difficult to demonstrate, a comparison between the two must 

 be confined largely to the structure of the nucleus. Prenant 

 observed that the nucleoplasm of the small dark epithelial 

 nuclei stained so intensely as to mask its internal structure. 

 Bell makes no mention of the affinity of the nucleoplasm for 

 basic stains. They apparently contain no greater amount of 

 chromatin than do the normal epithelial nuclei. Their diffused 

 dark color is due to the affinity of the nucleoplasm for a basic 

 stain. This is not the case of the nuclei of the small lympho- 

 cyces. Their dark color is due to the deeply stained chromatin 

 granules which, in proportion to the size of the cells, is much 

 greater in amount than that found in the epithelial nuclei. The 

 nucleoplasm of the small l5aTiphocytes, which is meager in amount, 

 but with a high magnification easily demonstrable, is quite 

 as clear as that ordinarily found in nuclei, for example in normal 

 epithelial nuclei of the thymus anlage. This fact alone, namely, 

 clear nucleoplasm and an abundance of chromatin in the nucleus 

 of a small lymphocyte in contrast to the deeply stained nucleo- 

 plasm and a smaller amount of chromatin in the small dark 

 epithelial cells, is sufficient to cause one immediately to doubt 

 the identity of the two kinds of cells. Prenant writes of degen- 

 erated epithelial nuclei in the thymus of a 28 mm. and later 

 stages of sheep embryos but apparently saw no connection be- 

 tween them and the darkly stained nuclei. Bell does not men- 

 tion degenerating nuclei. 



Most of the investigators who have made a detailed study of 

 the histogenesis of the thymus and who adhere to the trans- 

 formation theory of the formation of the lymphocytes lay a 

 great de^l of stress on the vacuolation of the cytoplasmic syncy- 

 tium as a factor in the histogenesis of the small lymphocytes. 

 This is particularly true of Prenant and Bell. The latter calls 

 the small dark epithelial nu(;lei, while still imbedded in the 

 cytoplasm of the epithelial cells, lymplioblasts. During the 



THE AMERICAN; JOURNAL OF ANATOMY, VOL. 17, NO. 4 



