If actinomycin D, an antibiotic that inhibits RNA synthesis, is added to newly fertilized frog eggs, there is no significant effect on protein synthesis (levels of specific proteins) in the eggs. Similar experiments have shown that actinomycin D has little effect on protein synthesis in embryos up until a particular embryonic developmental stage, the gastrula stage. After the gastrula stage, however, protein synthesis is significantly inhibited by actinomycin D, and the embryo does not develop further. Why is protein synthesis not affected before gastrulation but is affected after gastrulation?