E:T. Bell 55 
one node of the syncytium. They are never concentric, and never form 
lamella. Reticular areas often occur in other forms of corpuscles. In 
this way leucocytes are often involved in the corpuscle, since they lie in 
the spaces of the reticulum. Lymphocytes often get into a corpuscle in 
the lymphoblast condition, being cut off by the formation of lamelle 
outside them (Plate III, Fig. 22). The leucocytes shut in the cor- 
puscle in this way during development may not degenerate. ‘They prob- 
ably persist and help to remove the corpuscle in its final stages of degen- 
eration. 
The amount of expansion of the cytoplasm before or during the colloid 
transformation is probably small in the irregular reticular corpuscles, 
since it does not obliterate the spaces of the syncytium. In the compact 
type, the spaces of the syncytium are obliterated and there is evidence 
of some expansive force (note the arrangement of the nuclei in the 
upper part of Fig. 23, Plate III). In the figure referred to, the number 
of nuclei in any part of the corpuscle is less than in an equal area of the 
adjacent reticulum. These facts indicate that there is an expansion of 
the cytoplasm. That this expansive force does not produce a concentric 
form is due primarily to the fact that there is no expansion of a nucleus 
and distinct center of formation as is present in concentric corpuscles of 
the ordinary type. The absence of the onion-like structure in irregular 
corpuscles is due to the fact that the colloid is not laid down in lamelle. 
Significance of the corpuscles of Hassall. It has been shown in the 
preceding pages that the corpuscles of Hassall in the pig are not epithe- 
lial remnants, and also that they are not formed from blood-vessels. 
There is no evidence connecting their development with the involution 
of the thymus, for they begin to form before the lymphoid transforma- 
tion is complete and are most numerous when the thymus is at the height 
of its development. I have not been able to see the decrease in the rate 
of growth of the medulla described by Ammann, and even if such did 
occur it is difficult to understand how it could cause the formation of a 
corpuscle. 
The above theories are, therefore, inconsistent with the facts of devel- 
opment in the pig. It seems to me that the formation of a corpuscle is 
not to be regarded as a process of degeneration. The fact that the for- 
mation of colloid is an essential feature in the development of every 
corpuscle is a strong argument that it is a form of secretion such as 
occurs in its neighboring branchial derivative, the thyroid. The fact 
that the corpuscles differentiate in an apparently uniform syneytium is 
further evidence against a theory of degeneration. Since the lympho- 
eyte-forming function of the thymus is probably secondary, it is not 
