The morphogenetic changes involved with the differentiation of a unicellular fertilized egg to an organised multicellular embryo pass through a cycle of events. Manifestation of differentiation in structural diversity starts from the physiological differences In the tissue. These differences can be demonstrated through biochemical variations. These ideas are examined In the gametophyte-embryo complex of Ginkgo biloba and Pinus resinose. It is shown that the embryo experiences different physiological conditions during the various phases of development. In addition, the potentialities of embryonic cells to grow and differentiate vary during these phases. Alternative developmental sequences can occur if the nutritional environment is altered. Radiosensitivity of the embryo as measured by abortion and abnormal differentiation is shown to be dependent on the stage of embryonic development. / Thesis / Doctor of Philosophy (PhD)