36 The Development of the Thymus 
osing cells, though of course no cell boundaries are distinguishable. The 
nuclei are poor in chromatin, rounded, and usually 4.5 to 5.5 in 
diameter. ‘The amount of cytoplasm around the nuclei and connecting 
them is usually very small. At some nodes there is a greater amount of 
cytoplasm giving the appearance of a large reticulum cell. In connection 
with the blood-vessels, which are numerous in the cortex, are often found 
branched cells with pale nuclei and cytoplasm that stains more intensely 
than that of the rest of the reticulum. By a modification of Mallory’s 
method used by Jackson (13, 8. 39), I have been able to demonstrate 
numerous fibrille in the reticulum. In some cortical areas at this 
stage there are a great many erythroblasts. Masses of free erythrocytes 
are often found, usually near a comparatively large vessel, but such cells 
occur singly anywhere in the cortex. 
In the medulla, the syncytial character of the stroma is much more 
pronounced. ‘The cytoplasm is much more abundant than in the cortex, 
and the spaces are smaller and not so numerous. There is not so much 
room for lymphocytes as in the cortex, hence the lighter color of the 
medulla in stained preparations. The nuclei of the syncytium are either 
pale or dark, both types showing wide variations in size. By Jackson’s 
method (13, 8. 39), fibrilla may be readily demonstrated in the syn- 
cytium. In Plate I, Fig. 6, is shown the arrangement of these fibrille 
(s f) in the medulla at 24 em. They often may be traced into the 
areas which are forming the concentric corpuscles. In some parts of the 
medulla the fibrille are very numerous; in a few places, entirely absent. 
In both cortex and medulla eosinophile cells are often found. These 
occur in groups in the interlobular tissue around the blood-vessels, around 
some of the corpuscles of Hassall, and singly in the reticulum. These 
have been described by Schaffer (24). Free erythrocytes are rarely found 
in the medulla. In the medulla are also found the corpuscles of Hassall. 
Since the structure of these bodies depends largely upon their age, it may 
be better understood from the consideration of their development. 
Tur LYMPHOID TRANSFORMATION. 
Kolliker, in 79, first advanced the idea that the leucocytes are formed 
directly from the epithelial cells of the thymic anlage. According to 
Minot (in human embryology, p. 878), “he records for the rabbit that 
between the twentieth and twenty-third days the cells of the thymus be- 
come smaller and their outlines disappear, so that the organ appears to 
>This term will be used to include those changes that occur in the thymus 
during its passage from an epithelial to a characteristic lymphoid structure. 
