462 J. A. BADERTSCHER 



of their histological structure easy as they lay side by side on 

 the sUde. No difference in structure could be distinguished 

 between the two, thus indicating that the histogenetic processes 

 of that portion of the thymus derived from the ectoderm keep 

 pace with that portion derived from the entoderm. 



Embryos 85, 100, 125, 165, and 180 mm. in length. In these 

 developmental stages all the structures found in the fully devel- 

 oped thymus are laid down and will, therefore, be considered 

 only briefly. The average size of the lobules and the thymus 

 as a whole increases in the successively older stages. In the 

 first four stages the thymic septa have still a very loose struc- 

 ture while in the 180 mm. embryo they are quite narrow and 

 correspondingly denser. Many of the septa are broadly ex- 

 panded where the larger interlobular blood vessels are harbored. 

 The thymic septa of the 85 and 100 mm. stages are characterized 

 by the large number of all types of lymphocytes (large, medium- 

 sized, and small) which they contain. The presence of so many 

 lymphocytes in the septa is a feature that is most marked in 

 developmental stages from 65 to about 115 mm. in length. The 

 septa of the 125 and 165 mm. stages contain many lymphocytes 

 but on the whole they are less numerous than in the earher stages 

 cited. In the 180 mm. embryo the lymphocytes are mostly 

 confined to the deep expanded portions of the septa which are 

 often gorged with them. The number of lymphocytes in the 

 connective tissue immediately surrounding the thymus is rela- 

 tively small when compared with the number present in the septa. 

 Mitoses of all types of lymphocytes are of frequent occurrence. 



A marked feature of the cortex in these stages is the large 

 number of small lymphocytes which it contains. Excepting 

 the 85 mm. stage they make up the largest proportion of all the 

 lymphocytes present. Mitoses of all types of lymphocytes are 

 of frequent occurrence while mitoses of epithelial nuclei can occa- 

 sionally be found. The epithelial (reticulum) nuclei are, in gen- 

 eral, smaller than those found in younger stages but their struc- 

 ture has remained unchanged. The reticulum composing the 

 strands are greatly attenuated and only in very thin sections 

 can it be satisfactorily demonstrated. Its meshes are filled 

 with lymphocytes. Vacuoles are no longer present. 



