Be cS Bell 39 
of the pharynx. Before any blood-vessels or connective tissue have in- 
vaded the organ, changes begin in.the central part. These changes con- 
sist in the breaking up of the cytoplasm so that the cells become 
branched and connected by delicate processes. These processes finally 
break apart leaving a nucleus surrounded by a thin layer of protoplasm— 
a lymphocyte. The peripheral epithelial layer multiplies rapidly, forming 
nuclei somewhat smaller and darker than their own. These nuclei be- 
come gradually changed into the nuclei of lymphocytes and break away 
from the other cells. All transitions are present between the large, clear 
epithelial nuclei and the lymphocytes. Blood-vessels and connective 
tissue grow in from the outside. 
It appears from a survey of the literature that, of those who have 
studied the origin of lymphocytes in the mamalian thymus, His, Stieda, 
and Gulland have advocated the idea that they invade the gland from 
without, and that the original epithelial anlage persists only as remnants, 
the corpuscles of Hassall. They also consider the stroma of mesenchymal 
origin. On the other hand, Kolliker, Hermann and Tourneux, and Pren- 
ant, have described the lymphocytes as derived directly from the epithelial 
cells of the anlage. Hermann and Tourneux and Prenant ascribed a 
similar origin to part of the reticulum. 
Neither His, Stieda, nor Gulland made a detailed histological study 
of the changes that take place in the thymus during the transformation. 
They did not see the vacuolization of the cytoplasm, the changes in the 
epithelial nuclei, ete.—processes which undoubtedly occur. Gulland 
made nearly all his observations on the tonsil and then from a superficial 
examination of the thymus concluded that the process is the same there. 
The conclusions of these men are therefore not to be compared on this 
point with those obtained by the thorough and careful work of Prenant. 
On amphibians, Maurer hesitatingly agrees with His, and Ver Hecke 
accepts the mesenchymal origin of the leucocytes; while on fishes Maurer, 
Beard, and Nusbaum and Prymak believe in the epithelial origin of 
lymphocytes. Maurer’s work on reptiles is in agreement with his work 
on teleosts. 
As to the origin of leucocytes in the lymphoid organs of the alimentary 
canal, opinion is divided. Retterer, v. Davidoff, Rudinger, Klaatsch, 
and others have described the leucocytes as arising from epithelium and 
being invaded by mesenchymal elements forming adenoid tissue. Stohr, 
Gulland, Tomarkin, and others describe them as penetrating the epithe- ~ 
lum from without. 
I shall now discuss my own observations on the lymphoid transform- 
ation in the thymus of the pig. From a very early stage (11 mm.), the 
