The growth-inhibiting action of serum has been shown to be due largely to the lipoids.

Serum from which the lipoids have been removed is much less inhibiting to the growth of fibroblasts in vitro than is the original serum, and only slightly more inhibiting than Tyrode solution.

The lipoids extracted from the serum are toxic and more inhibiting to the growth of fibroblasts than the original serum.

Lipoids extracted from chicken brain, chicken liver, egg, and embryonic tissue have likewise an inhibiting action.

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