HE. T. Bell 37 
be an accumulation of small round nuclei. At about the same period 
blood-vessels and connective tissue grow into the epithelial anlage.” 
His (12 b), 80, and Stieda (26), 81, claimed that the corpuscles of 
Hassall are the only remnants of the epithelial anlage, that the lymph- 
ocytes, reticulum, ete., are of mesenchymal origin. 
Maurer (17 a), 86, described the leucocytes as arising directly from the 
cells of the epithelial anlage in the thymus of teleosts. In the amphibian 
thymus (17 b), 88, he thinks that the leucocytes are probably of mesen- 
chymal origin. He was unwilling to conclude that they arose from the 
epithelium because he could not find transition forms. In lizards (17 ¢), 
99, he records that even before the separation of the epithelial anlage of 
the thymus from the pharynx, changes begin. The peripheral cells are 
closely crowded together and show many mitoses. There arises between 
the central cells, or is formed in vacuoles in their protoplasm, a fluid which 
accumulates until the nuclei surrounded by a thin zone of protoplasm are 
connected only by protoplasmic threads. A loose medulla is thus formed 
which looks lke a cellular reticulum. The cortex is still sohd. The 
lymphocytes are formed from the epithelial cells; none come from with- 
out. Later, blood-vessels and connective tissue grow in. He believes 
that the reticulum is of mesenchymal origin in all forms. Maurer 
(17 d), 02, still holds that in amphibians the lymphocytes are probably 
of mesenchymal origin. 
Hermann et Tourneux (11), 87, find that in man and other mammals 
the epithelial anlage of the thymus is gradually converted into leucocytes 
and reticulum cells. Vacuoles appear during the transformation which 
seem to be formed by the absorption of large cells. In a sheep embryo 
of 130 mm., the clear epithelial cells have all disappeared, giving rise 
to small round cells and reticulum cells. Prolongations of connective 
tissue, each containing a blood-vessel, grow into the anlage during the 
transformation. They are not sure that all the thymic elements are 
epithelial in origin, being especially in doubt about the origin of some of 
the reticulum cells. 
Gulland (8), 91, describes the development of the tonsil in the rabbit. 
Leucocytes first appear in the connective tissue around the thymus. Later 
they appear in the connective tissue around the tonsil. They infiltrate 
the tonsillar epithelium. No leucocytes are of epithelial origin. After 
studying the tonsil he examined the thymus in a few specimens and 
concluded that the same process of leucocyte infiltration occurred there. 
He does not give the details of their infiltration, and did not see any of 
the transition forms of nuclei in the thymus at that period. 
Prenant (22), 94, made a careful study of the development of the 
