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Following ingression, a blastopore develops where the cells have ingressed, in one side of the embryo and it deepens to become the archenteron, the first formative stage of the gut. As in all deuterostomes , the blastopore becomes the anus whilst the gut tunnels through the embryo to the other side where the opening becomes the mouth.
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 small, apical cell will eventually give rise to most of the structures of the mature plant, such as the stem, leaves, and roots. [23]
The zygote is the earliest pp developmental stage. In humans and most other anisogamous organisms, a zygote is formed when an egg cell and sperm cell come together to create a new unique organism. The formation of a totipotent zygote with the potential to produce a whole organism depends on epigenetic reprogramming.
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.
In 1651 William Harvey refuted Aristotle's idea that menstrual blood could be involved in the formation of a fetus, asserting that eggs from the female were somehow caused to become a fetus as a result of sexual intercourse. [7] Sperm cells were discovered in 1677 by Antonie van Leeuwenhoek, who believed that Aristotle had been proven correct. [8]
As the zygote divides, it first becomes the embryo (which means 'growing within'), typically between zero and eight weeks, then from the eighth week until birth, it is considered the fetus (which means 'unborn offspring'). The internal genitalia are all the accessory glands and ducts that connect the gonads to the outside environment.
During sexual intercourse, sperm cells are ejaculated into the vagina through the penis, resulting in fertilization of an ovum to form a zygote. [1] While normal cells contain 46 chromosomes (23 pairs), gamete cells contain only half that number, and it is when these two cells merge into one combined zygote cell that genetic recombination ...
The two-cell blastomere state, present after the zygote first divides, is considered the earliest mitotic product of the fertilized oocyte. [2] These mitotic divisions continue and result in a grouping of cells called blastomeres. During this process, the total size of the embryo does not increase, so each division results in smaller and ...