an The Development of the Thymus 
adherence of the nuclei to the reticulum; (2) the first free nuclei often 
appear in the center of the gland when there are no other free nuclei in 
the periphery at that level; (3) there is good evidence that lymphocytes 
emigrate from the thymus in large numbers. If we examine the thymus 
of a 7-em. pig in serial sections we find that the lymphoid transformation 
is less advanced in the mid-cervical segment than in the head. In the 
mid-cervical segment there are a few lymphocytes in the interlobular 
tissue. In the lower end of the head where there are more lymphocytes 
inside the gland, lymphocytes pack the interlobular tissue and form a 
thin zone around the periphery of the gland. In the middle of the head 
where the transformation is far advanced, lymphocytes pack the inter- 
lobular tissue and form a thick zone around the entire gland. Indeed, 
in some sections, there are more lymphocytes in the zone outside than 
are present inside the gland. If this zone of lymphocytes be passing into 
the gland, it is not easy to understand why it is formed from within out- 
wards, and why it is thickest where the greatest number of lymphocytes 
are already present inside. No satisfactory suggestion has yet been made 
as to why lymphocytes should thus suddenly pour into the thymus at a 
time when if present at all elsewhere they are rare. They do not come 
to break up the thymic epithelium, for that is already a reticulum before 
free cells are present (Fig. 2, Plate I). Where lymphocytes invade 
intestinal epithelium as in the tonsil they eat paths through it leaving 
spaces. The epithelial reticulum of the thymus is not formed in that 
way. On the other hand it is not difficult to believe that this zone of 
lymphocytes is formed by cells passing out the periphery of the thymus 
and that the gland thus contributes a great number of lymphocytes to the 
organism ; (4) I have not been able to find lymphocytes in the connective- 
tissue around the thymus before they are present inside. An invasion by 
way of the blood-vessels may be excluded, since the thick zone of lympho- 
cytes formed around the gland shows that these cells either enter or leave 
it through the preiphery. 
THE CORPUSCLES OF HASSALL. 
These bodies were first mentioned by Hassall (10) in 46. He speaks 
of them as being composed of mother cells which enclose the newly- 
formed daughter cells and nuclei. He thought the central mass was 
formed by the outer enclosing layers. He found bodies which he regarded 
of the same nature in fibrous coagulations in the heart. 
Virchow (29), 51, in a discussion of endogenous cell formation, com- 
pares Hassall’s corpuscles to carcinoma pearls. He had about the same 
