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In psychology, developmental stage theories are theories that divide psychological development into distinct stages which are characterized by qualitative differences in behavior. [1] There are several different views about psychological and physical development and how they proceed throughout the life span.
Ancient Greeks discussed whether only the male had a seed which developed into the embryo within the female womb, or both the male and the female each had a seed that made a contribution to the developing embryo. The difficulty that one-seed theorists confronted was to explain the maternal resemblance of the progeny. One issue that two-seed ...
A part of the dorsal ectoderm becomes specified to neural ectoderm – neuroectoderm that forms the neural plate along the dorsal side of the embryo. [3] [4] This is a part of the early patterning of the embryo (including the invertebrate embryo) that also establishes an anterior-posterior axis.
In seed plants, the embryo will develop one or more "seed leaves" . By the end of embryogenesis, the young plant will have all the parts necessary to begin its life. Once the embryo germinates from its seed or parent plant, it begins to produce additional organs (leaves, stems, and roots) through the process of organogenesis.
The central nervous system (CNS) is derived from the ectoderm—the outermost tissue layer of the embryo. In the third week of human embryonic development the neuroectoderm appears and forms the neural plate along the dorsal side of the embryo. The neural plate is the source of the majority of neurons and glial cells of the CNS.
At the beginning of the ninth week, the embryo is termed a fetus (spelled "foetus" in British English). In comparison to the embryo, the fetus has more recognizable external features and a more complete set of developing organs. Human embryology is the study of this development during the first eight weeks after fertilization.
Other seed components include the endosperm, which is tissue rich in nutrients that will help support the growing plant embryo, and the seed coat, which is a protective outer covering. The first cell division of a zygote is asymmetric, resulting in an embryo with one small cell (the apical cell) and one large cell (the basal cell). [22]
The theory of recapitulation, also called the biogenetic law or embryological parallelism—often expressed using Ernst Haeckel's phrase "ontogeny recapitulates phylogeny"—is a historical hypothesis that the development of the embryo of an animal, from fertilization to gestation or hatching (), goes through stages resembling or representing successive adult stages in the evolution of the ...