PLATE 2 



EXPLANATION OF FIGURES 



3 Camera lucida drawing of portions of two lobules of the left thymus head 

 in an embryo 42 mm. in length. In the thjmius of this stage many lympho- 

 cytes are present most of which are large and medium-sized. Only a few small 

 lymphocytes are present. Mitoses of both epithelial nuclei and lymphocytes 

 occur. The lymphocytes in the interlobular septa are quite numerous. 1.5 mm. 

 App. objective, X 4 Comp. ocular; reduced one-fourth. 



4 Camera lucida drawing of portions of two lobules of the left thymus head 

 in a 65 mm. embryo. The thymus of this stage contains numerous lymphocytes 

 most of which are small ones. Mitoses of lymphocytes are comparatively numer- 

 ous. Many lymphocytes are found in the interlobular septum. 1.5 mm. App. 

 objective, X 4 Comp. ocular; reduced one-foiu-th. 



490 



