When multiplying actively in serum, fibroblasts are in a condition of survival, not of cultivation, for they do not synthetize protoplasm from substances contained in the medium. They are able to utilize only the nitrogenous material stored in the tissue itself. The distinction between survival of tissue and active growth is an important one. Innumerable errors in the interpretation of the experimental results have been made during the past years on account of this lack. An active proliferation of fibroblasts taking place during several days in a medium containing a given substance does not mean that this substance is used by the cells, but indicates only that residual cell proliferation is unopposed by a toxic substance. It is not possible to ascertain whether a medium is nutrient or not in the sense that it supports growth unless the mass of the tissues is shown to increase. When the proliferating fragment can be halved again and again, and each half reaches the size of the original whole in 48 hours, it is evident that new protoplasm must have been synthetized from the medium. The experiments described above show that egg white, egg yolk, pure egg albumin, egg white mixed with egg yolk, and bouillon are not utilized by fibroblasts for multiplication. This fact renders more plausible the hypothesis advanced previously that for proliferation the fibroblasts require some nitrogenous substances elaborated by other cells, which are present in embryonic tissue juices.

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

1. The duration of life of pure cultures of fibroblasts in a nitrogen-free medium is about 8 days.

2. Egg white and egg yolk, isolated or together, slightly increase the activity of the fibroblasts, but this effect is temporary and the tissues ultimately die. Pure egg albumin and bouillon are not utilizable by fibroblasts for multiplication.

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