40 The Development of the Thymus 
epithelium of the third gill pouch is a syneytium. No cell boundaries 
exist. The nuclei, large and irregular in shape, are embedded in a com- 
mon mass of cytoplasm. In the thymus at 20 mm. I find a syncytium 
of dense cytoplasm embedded in which are large nuclei of irregular shape 
and size. No distinct types of nuclei are present yet; all stain with 
medium intensity. A few mitoses are to be seen. 
In a section of the mid-cervical segment at 3.7 cm. (Plate I, Fig. 1), 
I find evidence that the lymphoid transformation has begun. The syn- 
cytium is composed of coarsely reticulated cytoplasm much looser in 
texture than that of the preceding stage. It contains a few irregular 
spaces (s s) which are evidently of the nature of vacuoles. These may 
be formed, as Maurer suggests, by liquefaction of the cytoplasm. There 
is no reason to suppose that cells degenerate and form them as Hermann 
and Tourneux believed. Three types of nuclei may be distinguished ; 
large pale nuclei (/ pn) large dark nuclei (J dn), and small dark nuclei 
(lymphoblasts) (db). Transition forms occur between these types. 
The large dark nuclei are intermediate forms between the pale nuclei 
and the lymphoblasts. A few mitoses (m) occur. No blood-vessels are 
present inside the anlage but they may be seen between the buds just 
outside. At this stage, the head and the thoracic segment have areas 
that are somewhat farther advanced than this. The intermediary and 
cervico-thoracic cords show no changes. 
At a later stage than the above (Plate I, Fig. 2), in the thoracic 
segment of a 4.5-cm. pig, «he spaces of the syncytium (s s) have in- 
creased greatly in number and size. The anlage is now a cellular retic- 
ulum. The large pale nuclei are somewhat less numerous than the 
dark nuclei and many have become angular, adapting themselves to the 
nodes of the syncytium. They contain less chromatin than in the 
preceding stage. Large dark nuclei and lymphoblasts are present; the 
lymphoblasts are much more numerous than in the preceding stage. A 
very few small dark nuclei are completely separated from the syncytium. 
These are lymphocytes. There are no lymphocytes in the connective 
tissue around the thymus or in the blood at this stage. I did not 
examine the tonsil or spleen at any stage. A few small blood-vessels are 
to be seen; their walls consist of endothelium only. There are more 
mitoses than at the preceding stage, but none happened to be present in 
the area shown in the figure. During mitosis, at all stages of develop- 
ment, except the early epithelial condition, the chromosomes are so 
closely packed that it is very difficult to distinguish them individually. 
In a section through the mid-cervical segment of a 7-cm. pig 
(Plate I, Fig. 5), we see a stage somewhat later than the one shown in 
