1S) 
ioe) 
The Development of the Thymus 
thymus in sheep embryos. His results are as follows. At 25 mm. the 
gland is composed of distinct polyhedral cells with nuclei of only one 
kind. A few mitoses and an occasional direct division are to be seen. At 
26 mm. mitoses are numerous (ore nucleus in fifty). Nuclei are reg- 
wlarly rounded or elliptical and some small nuclei occur juxtaposed to 
large nuclei. At 28 mm. many mitoses are present and irregular spaces 
have appeared. ‘These spaces are not blood-vessels nor parts of the 
thymie duct but vacuoles. Some nuclei, noticeably small and darkly 
colored, lie close to the large, clear nuclei and seem to be budded off from 
them. Some nuclei (rare) are broken into three or four chromatic 
bodies. Embryos of 40 mm. have undergone in great part the lymphoid 
transformation. All transitions are found between the large, pale ellip- 
tical nuclei of clear reticular structure and the small, deeply colored 
rounded nuclei whose sap is strongly stained. These last are certainly 
lymphocytes and constitute an immense majority of the cellular elements. 
Large, clear nuclei are found joined to small dark ones—nuclear couples. 
At 85 mm. the medulla appears; the cortex corresponds to the entire’ 
thymic mass of preceding stages. The cortex contains a great many 
lymphocytes separated by islands and rows of pale nuclei. There are 
about thirty lymphocytes to one pale nucleus. Mitoses are numerous in 
the cortex. In the medulla at this stage, the large clear, and small dark, 
nuclei are about equal in number, and mitoses are rarer than in the 
‘cortex. In later embryonic stages a clear peripheral zone is present 
where cell proliferation takes place. Mitoses are now more numerous in 
the medulla than in the cortex. It is probable that a certain number of 
the epithelial cells persist as reticulum cells in the fully-formed organ. 
J. Beard (3 a), 94, (3 b), gg, thinks that the function of the thymus 
is to form the first leucocytes. He finds that in the skate the epithelial 
cells are converted early into lymphocytes which emigrate into the blood. 
There are many breaks in the gland where the lymphocytes escape in 
masses. The thymus is the only source of leucocytes until the other 
lymphoid organs are formed. 
Ver Eecke (28), 99, finds that in the frog the epithehal thymus is 
invaded by lymphocytes and connective tissue. The epithelial cells are 
not destroyed but merely dispersed by the mesenchymal elements. He 
alls the resulting tissue lympho-epithelial. This idea of the comming- 
ling of the two tissues had already been advanced by Retterer. 
Nusbaum and Prymak (20), or, on teleosts, agree with Maurer that 
the lymphocytes are of epithelial origin but disagree on the details of their 
formation. They find that the epithelial anlage is at first composed of 
cells with distinct boundaries. It is not different from the epithelium 
