The influence of the area opaca on the development of the young chick embryo

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Abstract
The effects exerted by the area opaca upon the area pellucida have been tested experimentally by interfering with the normal relationships between them and then growing the blastoderm in tissue culture by the technique of New. If the entire area opaca was removed from a blastoderm at the full-length primitive streak stage, the embryo that formed was smaller than the controls and poorly differentiated. New evidence supporting the hypothesis that the area opaca influences the development of the embryo by exerting a tension upon the area pellucida is presented. Estimates of the mitotic rates provided no evidence to support the idea that this centrifugal tension promoted a high rate of cell division. With reduction in tension, however, cell death increased. No evidence could be found to suggest that the isolated area pellucida failed to grow and differentiate because its cell population was inadequate. When three area pellucidae were cultured in close contact with one another, growth and differentiation were no better than when one was cultured alone. The hypothesis that the proximal part of the area opaca exerted a nutritive effect on the area pellucida was tested. This was supported by the fact that when the area pellucida was explanted on top of the area opaca, differentiation of the embryo was good. Furthermore, some digestion of the area opaca endoderm appeared to take place. On the other hand, such a relationship cannot be essential for development at this stage, since equally good development could occur when the proximal part of the area opaca had been removed.