hh. T. Bell 43 
formation of vacuoles in it. These vacuoles increase in number and 
size, converting the anlage into a cellular reticulum. While this vacuoli- 
zation is in progress, the nuclei, which at first are of one kind with a 
medium amount of chromatin, differentiate into large clear, large dark, 
and small dark (lymphoblast) forms. The large dark nuclei probably 
divide by mitosis and form the lymphoblasts. The lymphoblasts grad- 
ually break loose from the syncytium, passing into its spaces and becoming 
lymphocytes. Shortly after lymphocytes begin to be formed, some of 
them pass out of the gland into the surrounding connective tissue. The 
lymphoid transformation begins in embryos of 2.5 cm. to 3 cm. and con- 
tinues in the cortex until 12 cm. or 13 em. In the medulla it is not com- 
plete at birth. Since the thymus increases greatly in size during this 
period the epithelial syncytium must grow rapidly. Lymphocytes are 
constantly being formed at the expense of the growing syncytium. A 
peripheral zone of proliferation is present from about 8 cm. to 12 em. 
The medulla is formed as a chemical differentiation of certain centrally 
situated areas of the epithelial syncytium. ‘The histological changes 
occur earlier in the head and thoracic segment than in the mid-cervical 
segment and very much earlier than in the cords. The reticulum of both 
cortex and medulla is practically all of epithelial origin. Some branched 
cells around the blood-vessels in the cortex may be of mesenchymal 
origin. 
My reasons for regarding the lymphocytes as of epithelial origin are 
as follows: 
A. The lymphoblasts are true epithelial nuclei, because (1) there are 
numerous transition forms between them and the large dark nuclei which 
later cannot be regarded as invading lymphocytes; (2) they are closely 
embedded in the syncytium and show no evidence of having eaten their 
way through the protoplasm; (3) they are present from a very early 
stage and increase in number as development proceeds; (4) they are 
present before blood-vessels invade the gland and have no constant rela- 
tion to blood-vessels or to the surface of the gland that indicates an inva- 
sion from either of these directions; (5) they are present before lympho- 
cytes appear in the connective-tissue around the thymus. 
B. Some observers admit that the small dark nuclei (lvmphoblasts) 
are of epithelial origin but do not admit that they form lymphocytes. 
The considerations that lead me to believe that the lymphoblasts do form 
the lymphocytes are: (1) the small dark nuclei (lymphoblasts) show 
every possible relation to the syneytium from being completely embedded 
in it to lying free in the syncytial spaces. A comparison with later 
stages shows that this appearance is not due to poor fixation or to the 
