Extracts of homologous adult tissues detemine an increase in the mass of pure cultures of chicken fibroblasts nourished thereon comparable to that resulting from embryonic tissue juice. But the effect of these extracts differs markedly from that of the latter, since cell multiplication does not continue indefinitely. After a few passages, the fibroblasts cultivated in adult tissue extracts grew more slowly than in Tyrode solution. The cytoplasm became dark and filled with fat granules, and death followed. It is possible that the tissues of adult animals contain, as does the serum, substances which are toxic for the homologous cells, and which progressively overcome the effect of the growth-activating substances. The effect of heterologous adult tissue extracts did not differ markedly from that of homologous tissues. The chicken connective tissue increased slightly in mass, but died sooner than the controls in Tyrode solution.

By contrast, tissue juices derived from the embryos of mice, guinea pigs, and rabbits acted on chicken fibroblasts in the same manner as chick embryo juices. The increase in mass of the cultures was regular and rapid. They doubled in size every 48 or 72 hours, and the rate of growth did not decrease after 30 days. It appears that embryonic tissue juices are not necessarily toxic for heterologous fibroblasts, and that they can be used in the building up of protoplasm in the tissues of a different species. In experiments made long ago, the action of tissue juice was described as being specific. The premature death of the fibroblasts cultivated in heterologous juices at that time would now appear to have been due to spontaneous changes in the pH and the deterioration that even normal chick embryo juice at a pH of 7.8 undergoes spontaneously. In the recent experiments, when freshly prepared homologous and heterologous juices were used, their action on chicken fibroblasts in pure culture was identical. However, the fibroblasts produced in cultures nourished by rabbit juice grew better when transferred to rabbit serum than did ordinary chicken fibroblasts. It has not been determined as yet whether this effect is due to an immunization of the fibroblasts against rabbit humors, or to some decrease in the specificity eventuating in cells intermediate between rabbit and chicken fibroblasts.

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

1. Pure cultures of chicken fibroblasts increase in mass under the influence of extracts of adult homologous tissues. But they ultimately die while the fibroblasts cultivated in embryonic tissue juices live indefinitely.

2. The increase in mass of chicken fibroblasts cultivated in the juices of mouse, guinea pig, rabbit, and chick embryos is about identical.

3. Chicken fibroblasts produced in cultures nourished by rabbit embryonic tissue juice are less sensitive to the inhibiting action of rabbit serum than ordinary chicken fibroblasts.

4. Cultures of chicken fibroblasts in extracts of adult tissues of mice, guinea pigs, and rabbits increase slightly in mass, but the increase is temporary and death occurs after a few passages.

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