DEVELOPMENT OF THE THYMUS 457 



present in the intermediary and cervico-thoracic cords. No 

 blood vessels are present in the thymus. The walls of the 

 blood vessels of the interlobular connective tissue septa are made 

 up of endothelium only. The mesenchyme around the super- 

 ficial and head thymus and the thoracic segment is much looser 

 in its structure than in the corresponding regions in previous 

 stages. Around the intermediary and cervico-thoracic cords 

 and the mid-cervical segment it has a somewhat denser struc- 

 ture than around the above named regions of this stage. 



As in previous stages completely degenerated epithelial nuclei 

 (fig. 3, D.e.N'.) are present. Epithelial nuclei (D.e.N.) in the 

 first stages of degeneration are also present but they are not as 

 numerous as in the 36 mm. embryo. Mitoses of epithelial 

 nuclei (M.e.N.) are quite numerous. The vacuoles are not 

 as numerous as in stages ranging from 25 to 37 mm. in length. 

 The most striking feature of this stage, however, is the large 

 number of lymphocytes that are present in the thymus. They 

 no longer all belong to the type of large lymphocytes but now 

 and then a small lymphocyte {S.L.) is found. These are char- 

 acterized by a rather small nucleus which is richly laden with 

 chromatin and surrounded bj^ only a very thin layer of cyto- 

 plasm which is often difficult to demonstrate. Intermediate 

 stages between the large and small lymphocytes make up a 

 relatively large proportion of all present. Some in mitotic 

 division can be found without much searching. Mitosis of 

 epithelial nuclei and large lymphocytes can be distinguished 

 from each other without much difficulty. The chromosomes of 

 the lymphocytes are shorter, somewhat thicker, and more 

 closely packed together than those of epithelial cells. The 

 basophilic cytoplasm of the lymphocytes also is sharply out- 

 lined in contrast to the cytoplasm of epithelial syncytium. 

 The absence of blood vessels in the thymus at this stage, the 

 absence of transition forms between epithelial cells and lym- 

 phocytes, the unbroken' series of intermediate stages between 

 the large and small lymphocytes, and the frequent mitoses 

 found among them all, are evidences that undoubtedly point 

 to the conclusion that the large lymphocytes through repeated 



