DEVELOPMENT OF THE THYMUS 443 



HISTOGENESIS 



To determine the origin of the different cellular elements 

 that are found in the fully developed thymus it is necessary to 

 begin with stages in which the thymus is purely epithelial, and 

 to use a differentia stain by which one can definitely distinguish 

 a l3rmphocyte from an epithelial cell. The latter fact was em- 

 phasized by Maxim ow ('09 b) who accomplished this differen- 

 tiation by fixing the tissue in Helly's fluid and staining with 

 eosin-azure. A similar differential staining was accomplished 

 by me by fixing the tissue, as stated above, in Helly's fluid and 

 staining with Hasting's Nocht's blood stain. The material 

 prepared for the histogenesis of the thymus begins with an 

 embryo 17 mm. in length. Of the many stages that were pre- 

 pared and examined there are chosen for description only a few 

 series of successively older stages, each of which is decidedly 

 advanced in development over the previous stage and yet closely 

 enough connected with it so that the developmental history 

 will be continuous. 



The histogenesis of the thymus may conveniently be divided 

 into epochs, each of which is characterized by more or less 

 distinct developmental features. They are: (1) a purely epithelial 

 epoch which extends from its earliest development as an out- 

 pocketing from the third pharyngeal pouch and the forma- 

 tion of the cervical vesicle to the appearance of the first lympho- 

 cytes in the epithelial anlage of the organ; (2) The epoch of 

 lymphocyte infiltration and lymphocyte proliferation, and the 

 formation of the reticulum. This epoch begins with the appear- 

 ance of the first lymphocytes in the thymus. The invasion 

 of the thymus by lymphocytes from the surrounding mesenchyme 

 continues probably up to stages 180 mm. in length while the 

 proliferation of the lymphocytes in the thymus still continues 

 in full term embryos and doubtless after birth. During this 

 epoch the cortical and medullary portions of the lobules appear 

 first in stages ranging from 65 to 75 mm. in length. The reticu- 

 lum, which according to the nature of its development is formed 

 gradually, is fully developed in embryos 180 mm. in length; 



