In the tunicate Styela, immediately after fertilization, different colored cytoplasms become localized to specific parts of the embryo. Cells containing differently colored cytoplasm become determined to have a different fate. In particular, a yellow cytoplasm is segregated into a crescent just before the first cleavage. Cells that come to contain this cytoplasm become muscle. Keep in mind that the yellow pigment is only an endogenous marker of the determinant, and not the determinant itself, since there are closely related, but unpigmented ascidians that develop in exactly the same way. Evidence that the yellow pigmented cytoplasm contains a muscle determinant comes from experiments in which the yellow cytoplasm is distributed to different cells; this alters their fate. The critical component of the yellow cytoplasm appears to be muscle-specific mRNAs.