DEVELOPMENT OF THE THYMUS 449 



deeply stained clumps which lie in a clear space that has almost 

 the same size and shape as a normal nucleus. 



The cytoplasmic syncytium of the epithelial anlage has the 

 same general structure as that in a 23 mm. embryo. The vacuoles 

 (fig. 1, V) are, however, more numerous and the vacuolation of 

 the cytoplasm has reached its greatest height in this stage. No 

 basement membrane is present but the anlage is quite sharply 

 defined from the surrounding mesenchyme which closely invests 

 the thymus. As in earlier stages some of the protoplasmic 

 processes of the mesenchymal cells are apparently fused with 

 the outer surface of the cytoplasmic syncytium of the epithelium. 



The point of greatest interest and importance in this devel- 

 opmental stage is the presence of lymphocytes (fig. 1, L.L.) 

 in the thymus anlage. They are present in small numbers 

 in the superficial thymus, thymus head, and in the thoracic 

 segment. None were found in the mid-cervical segment which 

 in this and younger stages is not as far advanced as the head 

 and thoracic segments. Lymphocytes, however, occur in the 

 thymus before the 30 mm. developmental stage. In a 25 mm. 

 embryo a single lymphocyte was found in the thymus head. 

 None were seen in the thymus of a 26 mm. embryo. In a 27 

 mm. embryo one lymphocyte was found in one of eleven sections 

 prepared from the thoracic segment. In a 28 mm. embryo 

 only a few could be demonstrated. In the stage being described 

 they are present in small but appreciable numbers, hence, this 

 stage was chosen for the discussion of the origin of the lympho- 

 cytes in the thymus. Their location in the lobules varies. Some 

 are found in or near the center of the lobules while others lie 

 near the periphery. All the lymphocytes that were found in 

 the thymus in this and somewhat later stages are large lympho- 

 cytes. No small lymphocytes such as make up the bulk of the 

 organ in late developmental stages, are present. The large 

 lymphocytes are characterized by a generous amount of non- 

 granular cytoplasm which is distinctly basophilic in its character. 

 This basophilic character of the cytoplasm enables one to dis- 

 tinguish it unmistakably from the cytoplasm of the epithelial 

 cells. On account of their power of undergoing amoeboid 



