DEVELOPMENT OF THE THYMUS 445 
the intermediary and cervico-thoracic cords where only a few 
vacuoles are present. Those lying near the surface are quite 
regularly arranged. The long axis of the oval ones is usually 
perpendicular to the surface. The more centrally located cells 
have no regular arrangement. A very distinct nuclear mem- 
brane is present. They possess a quite rich supply of chromatin 
which is distributed mostly in the form of fine threads, giving it 
a reticular structure. About one-half of the nuclei possess two 
nucleoli while the other half contains but one. Occasionally 
one can be found with three nucleoli. They are large and occupy 
no definite position in the nucleus. In the oval nuclei they may 
lie near the ends or near the center, while in the round nuclei 
they may occupy an eccentric position. The nuclei at this 
stage do not all stain with the same intensity. In the enlarged 
segments of the thymus some stain much more intensely than 
the majority and with only moderately high magnification 
could easily be mistaken for transforming stages leading to the 
development of lymphocytes, which, however, is not the case. 
A consideration of their real significance will be given in connec- 
tion with a discussion of the origin of the lymphocytes in the 
thymus. Nuclear division at this stage goes on rapidly in the 
enlarged segments of the thymus. Even with a magnification 
of 1300 diameters often three nuclei in mitotic division can be 
brought into a microscopic field. 
To determine whether or not cells migrate into the thymus 
it is necessary to make a study of the connective tissue, at least 
in the earlier stages, as painstaking as the study of the thymus 
itself. The mesenchymal cells are of the spindle or stellate 
type. Their protoplasmic processes often unite with those 
of neighboring cells. Many fine fibers are scattered in the 
meshes between the cells giving the appearance of a network. 
The mesenchyme so closely invests the thymus that in places 
the cytoplasmic processes of the mesenchymal cells appear to 
be fused with the cytoplasm of the epithelial cells. The nuclei 
are large and spherical or oval in shape, and contain about as 
much chromatin as the epithelial nuclei of the thymus. 
