DEVELOPMENT OF THE THYMUS 447 



a magnification of 1300 diameters five were found in a single 

 microscopic field. Those with only a slightly basophilic cyto- 

 plasm are quite numerous. 



Other cellular elements such as giant cells, the megaloblasts 

 of Maximow, normoblasts and definitive erythrocytes also 

 occur. These elements and the granular cells found in the 

 thymus in later developmental stages are discussed further on 

 in this paper. 



The occurrence of lymphocytes in the mesenchyme before they 

 are present in the thymus is of the greatest significance from a 

 histogenetic view point, for Bell ('06) in describing the thymus 

 of a 45 mm. pig embryo says: 'There are no lymphocytes in 

 the connective tissue around the thymus or in the blood at this 

 stage," and of a 70 mm. embryo he says: "There are a few lym- 

 phocytes outside the thymus in the interlobular tissue in this 

 region; * * * * j have never seen lymphocytes outside 

 the thymus where there were none inside it, but they appear 

 outside shortly after they are formed here." The results of 

 my observations are contradictory to those of Bell. In a 17 mm. 

 embryo an occasional lymphocyte can be found in the mesen- 

 chyme while in the 23 mm. embryo an occasional lymphocyte 

 was found in the blood. His failure to detect lymphocytes in 

 the mesenchyme in stages less than 70 mm. in length was per- 

 haps due to the use of Zenker's fixing fluid which, as stated 

 above, destroys the basophilic character of the cytoplasm. 

 Beard ('02), in his work on the smooth skate, positively asserts 

 that the first lymphocytes in the body are found in the thymus. 



Embryo of 26 mm. In this stage the lobes of the superficial 

 thymus, the thjmius bead, and the thoracic segments have greatly 

 enlarged. Lobules are beginning to grow out from them. The 

 cervical segment, which enters its period of rapid develop- 

 ment a little later than the thymus head and the thoracic seg- 

 ment, is now quite pronounced and is studded over with short, 

 blunt epithelial buds. No lobules have yet started to grow out 

 from the intermediary and cervico- thoracic cords. The sur- 

 face of the thymus is quite definitely marked from the mesen- 

 chjTiie but no basement membrane is present. The vacuoles 



