Early experiments in embryology indicated that long-range influences help determine pattern formation in embryos. Sander examined the consequences of isolating parts of the embryo by tying off portions so as to prevent chemical communication between two groups of cells. Similar experiments showed that the anterior portion of the egg exerted an influence over the pattern development in the remainder of the egg. Material removed from the anterior pole and microinjected elsewhere behaved as though it contained the source of the pattern maker. Also, destroying material at the anterior end of the egg eliminated head structures and yielded an embryo with a posterior replacing the head structures.
Removal of about 10% of the cytoplasm from the anterior end of an egg also affects the resulting pattern development. Usually the abdomi-nal region of the resulting organism is defective. Removal of an equal amount of cytoplasm from other parts of the egg has relatively little effect on the developing embryo. Dominance experiments can even be done with microinjection. Injecting cytoplasm from the posterior region into the anterior region suppresses head development. Similarly, ante-rior cytoplasm taken from an embryo somewhat later in development represses posterior development if it is injected into the posterior region. In either case, removal of cytoplasm from one end reduces the tendency to develop structures characteristic of that end, and injection of cyto-plasm from the other end of the embryo can reverse the identity of structures near the end. In this way two-headed or two-tailed embryos can be formed.
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