It may be concluded that, under the conditions of the experiments:

1. Lymphocytes and large mononuclear cells can live and increase greatly in numbers in blood serum, while fibroblasts are not capable of doing so.

2. While living in serum, lymphocytes and large mononuclear cells manufacture and secrete substances which may be used as food material by the fibroblasts.

3. It is probable that lymphocytes and large mononuclear cells synthetize from the nitrogenous compounds contained in serum the substances which fibroblasts and epithelial cells require for their multiplication.

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