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For example, numerous invertebrate species release a larva before development is complete; at the end of the larval period, an animal for the first time comes to resemble an adult similar to its parent or parents. Although invertebrate embryology is similar in some ways for different invertebrate animals, there are also countless variations.
Human embryology is the study of this development during the first eight weeks after fertilization. The normal period of gestation (pregnancy) is about nine months or 36 weeks. The germinal stage refers to the time from fertilization through the development of the early embryo until implantation is completed in the uterus .
In a large study based on 5 birth cohorts in Brazil, Guatemala, India, the Philippines and South Africa, faster linear growth at 0–2 years was associated with improvements in adult stature and school performance, but also an increased likelihood of overweight (mainly related to lean mass) and a slightly elevated blood pressure in young adulthood.
All the developmental processes listed above occur during metamorphosis. Examples that have been especially well studied include tail loss and other changes in the tadpole of the frog Xenopus, [32] [33] and the biology of the imaginal discs, which generate the adult body parts of the fly Drosophila melanogaster. [34] [35]
First attested in English in the mid-14c., the word embryon derives from Medieval Latin embryo, itself from Greek ἔμβρυον (embruon), lit. "young one", [1] which is the neuter of ἔμβρυος (embruos), lit. "growing in", [2] from ἐν (en), "in" [3] and βρύω (bruō), "swell, be full"; [4] the proper Latinized form of the Greek term would be embryum.
Comparative vertebrate embryology. The embryonic disc becomes oval and then pear-shaped, the wider end being directed forward. Towards the narrow, posterior end, an opaque primitive streak , is formed and extends along the middle of the disc for about half of its length; at the anterior end of the streak there is a knob-like thickening termed ...
The development of the reproductive system is the part of embryonic growth that results in the sex organs and contributes to sexual differentiation.Due to its large overlap with development of the urinary system, the two systems are typically described together as the genitourinary system.
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.