[1]  This led them to believe that the cells in the early embryo are undetermined in the sense that each cell has the potential to develop in a variety of different ways. [2] Later biologists found that the situation was not so simple. It matters in which plane the embryo is cut. [3]

[4] Some scientists think they know why. Which embryo cells are determined, just when do they become irreversibly committed to their fates, and what are the “morphogenetic determinants” that tell a cell what to become? But the debate could not be resolved because no one was able to ask the crucial questions in a form in which they could be pursued productively. [5] During gastrulation cells migrate to the interior of the blastula, subsequently forming two (in diploblastic animals) or three (triploblastic) germ layers. Now investigators think they know at least some of the molecules that act as morphogenetic determinants in early development. [6] They have been able to show that, in a sense, cell determination begins even before an egg is fertilized.

1 Studying sea urchins (any of numerous echinoderms (class Echinoidea) that are usually enclosed in thin brittle globular tests covered with movable spines海胆), biologist Paul Gross found that an unfertilized egg contains substances that function as morphogenetic determinants. 2 They are located in the cytoplasm of the egg cell [7]. 3 When the egg is fertilized, the substances become active and, presumably, govern the behavior of the genes they interact with. 4 Since the substances are unevenly distributed in the egg, when the fertilized egg divides, the resulting cells are different from the start and so can be qualitatively different in their own gene activity.[8]

1 Once synthesized, the histones move into the cell nucleus, where section of DNA wrap around them to form a structure that resembles beads, or knots, on a string. 2 The substances that Gross studied are maternal messenger RNA’s[9] —products of certain of the maternal genes. 3 The beads are DNA segments wrapped around the histones; the string is the intervening DNA. 4 He and other biologists studying a wide variety of organisms have found that these particular RNA’s direct, in large part, the synthesis of histones, a class of proteins that bind to DNA. [10] [11]  

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