454 • J. A. BADERTSCHER 



process of vacuolation some of the lymphoblasts become free 

 and lie in vacuoles. They then are called lymphocytes. My 

 observation on the histogenesis of the thymus in the numerous 

 pig embryos examined warrants no distinction in the nomen- 

 clature between the two. Referring again to figures 1, 2 and 5, 

 it will be seen that some of the small dark epithelial nuclei lie 

 entirely in the cytoplasm and some in contact with vacuoles. 

 Occasionally one can be found that lies free in a vacuole (none 

 of the latter happened to be present in the portions of the sec- 

 tions from which the figures were drawn) . The structure of the 

 nucleus is the same whether they lie in the vacuoles or in the 

 cytoplasm. This also is true of the large lymphocytes (L.L.). 

 Some are entirely imbedded in the cytoplasm of the epithelial 

 cells while others are in contact with or entirely in the vacuoles. 

 The structure of all is the same and there is no reason why they 

 should not all bear the same name. The significance of the 

 vacuoles and their mode of formation in the thymus is unknown 

 to me. They are already present in embryos of 17 mm. in length, 

 and slightly increased in size and number in later stages (30 

 to 40 mm.). 



In considering the genetic relationship of the small dark 

 epithelial nuclei, the 'lymphoblasts' of Bell, to the large dark 

 epithelial nuclei, Bell expresses a doubt by saying that ''the 

 large dark nuclei probably divide by mitosis and form the lympho- 

 blasts." I was unable to find any of the dark epithelial nuclei 

 in mitosis although I earnestly searched for them. To me it 

 appears that the small and large dark nuclei are derived respec- 

 tively from small and large normal epithelial cells, their degree 

 of darkness in stained preparations depending on the extent of 

 degeneration. Also it cannot be that the small dark nuclei are 

 formed through a contraction of the large ones for the structure 

 of both is the same. Prenant, in the developing thymus of the 

 sheep, described and figured direct cell division. I w^s unable 

 to find amitotic cell division in the thjonus of the pig. 



The true source of the lymphocytes first present in the thy- 

 nms will now be considered. Reference has already been made 

 to the presence of lymphocytes in the thymus anlage of a 30 mm. 



