EH. T. Bell 41 
Fig. 2. In various parts of the section lymphocytes (/) are completely 
formed. The great majority of the small round nuclei are in the lympho- 
blast (Jb) condition, i. e., they are not yet completely separated from the 
syncytium. There are a few lymphocytes outside the thymus in the 
interlobular tissue in this region; around the head and the thoracic 
segment at 7 cm. they are numerous, these parts being in a later stage 
of transformation. I have never seen lymphocytes outside the thymus, 
' where there were none inside it; but they appear outside shortly after they 
are formed here. Those formed next the interlobular septa seem to pass 
out very early. Of course the lymphoblasts, which are distinguishable 
from the lymphocytes only by being imbedded in the syncytium, are to 
be seen in the thymus long before any appear outside. 
At the stage shown in Fig. 5, a great many nuclei are in mitosis. [ 
have not seen at any stage, the amitoses and nuclear couples described 
by Prenant for the sheep. In some parts of the section comparatively 
large solid epithelial areas occur. These are found as often in the 
central as in the peripheral part. Many of the pale nuclei are smaller 
than those shown in Fig. 2. The blood-vessels are somewhat larger and 
more numerous than those at 4.5 cm. 
It is to be noted that the epithelial anlage does not at any stage become 
converted entirely into small round cells as many observers have stated. 
Distinctly pale angular reticular nuclei can always be seen. 
In the mid-cervical segment at 8.5 em. (Plate I, Fig. 4), a great many 
lymphocytes (/) are formed. These lie between the persisting epithelial 
cells which are now arranged in irregular cords and islands. In these 
epithelial masses, lymphoblasts may still be seen indicating that the 
formation of lymphocytes is still in progress. Many of the pale nuclei 
are now small. The heavy hematoxylin stain in this case makes the 
nuclei darker than they would appear with an ordinary stain. A few 
nuclei are in mitosis. 
This figure shows also the first appearance of the medulla (md). The 
medulla is formed directly, as shown in the figure, from persisting parts 
of the epithelial syncytium. Certain centrally situated masses of this 
syncytium undergo changes of such a nature that they stain readily with 
cytoplasmic stains such as Congo red. Im sections stained with hema- 
toxylin and Congo red, the medulla is first recognized as a_ brightly 
colored area situated usually about the center of the lobule. These 
epithelial masses that give rise to the medulla seem to increase in size 
about the time of the change in staining capacity. The first differenti- 
ation of the medulla is chemical rather‘than morphological, for there are 
other persisting epithelial masses even larger than it in the same section 
