ABSTRACT
Chick blastoderms of the definite primitive streak stage were cultured in vitro for about 24 hours with radioactive DL-methionine-S35 added to the medium. It was shown by autoradiography that the neural tube and especially its roof and also adjacent areas of the ectoderm (neural crest) take up more activity than other structures. Mesodermal structures such as the notochord, somites, and myoepicardium also reach a high level of uptake, presumably correlated with their differentiation.
Tissues of the host, the induced axis, and self-differentiating (non-labelled) grafts take up activity from the culture medium at the same rate.
If a labelled organizer is grafted on to a host blastoderm, the induced neural tube takes up a substantial amount of labelled molecules but the host does not. Experiments involving dilution with unlabelled methionine or grafting of labelled peripheral organizer tissue, produced evidence for the view that the activity is carried to the induced tissues by diffusion or exchange of free aminoacid molecules; transfer of macromolecules does not occur on a scale detectable by the methods used but it cannot be excluded that it might occur on a smaller scale.
In cases of induction caused by organizer graft the notochord, mesenchyme, and, occasionally, tissues lining the foregut associated with the induced axis are contributed by the graft.