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An embryo is the initial stage of development for a multicellular organism. In organisms that reproduce sexually , embryonic development is the part of the life cycle that begins just after fertilization of the female egg cell by the male sperm cell .
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.
1 - morula, 2 - blastula 1 - blastula, 2 - gastrula with blastopore; orange - ectoderm, red - endoderm. Embryology (from Greek ἔμβρυον, embryon, "the unborn, embryo"; and -λογία, -logia) is the branch of animal biology that studies the prenatal development of gametes (sex cells), fertilization, and development of embryos and fetuses.
Scientists in Israel have created a model of a human embryo from stem cells in the laboratory, without using sperm, eggs or a womb, offering a unique glimpse into the early stages of embryonic ...
The embryo, meanwhile, proliferates and develops both into embryonic and extra-embryonic tissue, the latter forming the fetal membranes and the placenta. In humans, the embryo is referred to as a fetus in the later stages of prenatal development. The transition from embryo to fetus is arbitrarily defined as occurring 8 weeks after fertilization.
Embryo measures 30–80 mm (1.2–3.1 in) in length. Ventral and dorsal pancreatic buds fuse during the 8th week; Intestines rotate. Facial features continue to develop. The eyelids are more developed. The external features of the ear begin to take their final shape. The head comprises nearly half of the fetus' size. The face is well formed.
In embryology, Carnegie stages are a standardized system of 23 stages used to provide a unified developmental chronology of the vertebrate embryo.. The stages are delineated through the development of structures, not by size or the number of days of development, and so the chronology can vary between species, and to a certain extent between embryos.
The difference between a mammalian embryo and an embryo of a lower chordate animal is evident starting from blastula stage. Due to that fact, the developing mammalian embryo at this stage is called a blastocyst, not a blastula, which is more generic term. There are also several other differences from embryogenesis in lower chordates.