42 The Development of the Thymus 
that do not react in the same way with the cytoplasmic stains. The 
medulla appears in the head and the thoracic segment at 7.5 em. to 8 cm. 
All the gland except this small central area forms the cortex. Blood- 
vessels now reach all parts of the gland, but are still few in number. I 
cannot distinguish any wall except the endothelium on those actually 
inside the gland. 
In a 9.5-em. pig, the medulla is larger. It contains pale nuclei of 
various sizes, large dark nuclei, and lymphoblasts. Its spaces are smaller 
than those of the cortex. The early stages in the formation of the cor- 
puscles of Hassall appear as soon as the medulla begins to form. The 
epithelial cords in the cortex have become less conspicuous, but are still 
forming lymphocytes. A few nuclei are in mitosis. Many blood cap- 
illaries may now be seen penetrating the gland from the periphery. 
These vessels run in the epithelial masses and have a wall of large 
endothelial cells which gives them the appearance of radiating cords. 
When these vessels first appear, as at this stage, they have only an 
endothelial wall. The blood-vessels grow in as small capillaries which, 
after their entrance, increase in size and branch; they do not break in as 
large vessels surrounded by mesenchymal tissue. I am fairly sure that 
aside from the endothelial cells few or no mesenchymal cells come into 
the thymus. Around the greater part of the periphery of the gland is 
a solid zone of syncytium two or three nuclei deep which is in trans- 
formation like the epithelial cords inside. This zone, described by Pren- 
ant as a zone of proliferation, grows rapidly, as the frequent mitoses 
indicate. Its inner boundary is forming lymphocytes and reticulum cells. 
In a 14-em. pig, the lymphoid transformation is practically at an end 
except in the medulla. The peripheral zone of proliferation has dis- 
appeared. The cortex has about the same structure as at 24 cm., as 
previously described. In the medulla, lymphoblasts, large pale nuclei, 
and the large dark intermediate types are still present. ‘There are a few 
mitoses here. It is very probable therefore that the formation of lym- 
phocytes is still in progress in the medulla. The medulla at 24 cm. 
shows a similar structure except that there are fewer lymphoblasts. These 
facts persuade me to regard the medulla as a center for lymphocyte 
formation at least as late as birth. Connective tissue fibrillee begin to 
appear in the gland along the large blood-vessels and the interlobular 
septa as early as 10.5 cm. They are only a little farther in at 16 cm.; 
but near full term they are present in nearly all parts of the stroma. 
(See Plate I, Fig. 6.) 
The above account may be summarized as follows: In the pig the 
epithelial syneytium of the thymic anlage becomes loosened up by the 
