D is the best answer. According to the author, early investigators arrived at the conclusion that the cells of the embryo are undetermined because they “found that if they separated an invertebrate animal embryo into two parts at an early stage of its life, it would survive and develop as two normal embryos” (lines 1-6). However, later biologists discovered that when an embryo was cut in places different from the one used by the early investigators, it did not form two whole embryos. Because the earlier biologists apparently arrived at their conclusion without attempting to cut an embryo in different planes, it would appear that they assumed, erroneously, that different ways of separating the embryos would not affect the fate of the two embryo parts.