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Human reproduction naturally takes place as internal fertilization by sexual intercourse. During this process, the man inserts his erect penis into the woman's vagina and then either partner initiates rhythmic pelvic thrusts until the man achieves orgasm , which leads to ejaculation of semen containing sperm into the vaginal canal.
When eight blastomeres have formed, they start to compact. [6] They begin to develop gap junctions, enabling them to develop in an integrated way and co-ordinate their response to physiological signals and environmental cues. [7] When the cells number around sixteen, the solid sphere of cells within the zona pellucida is referred to as a morula ...
Until about the ninth week of gestational age, [4] the external genitalia of human males and females look the same, and follow a common development. This includes the development of a genital tubercle and a membrane dorsally to it, covering the developing urogenital opening , and the development of the labioscrotal fold .
The process begins with fertilization, where an egg released from the ovary of a female is penetrated by a sperm cell from a male. The resulting zygote develops through mitosis and cell differentiation , and the resulting embryo then implants in the uterus , where the embryo continues development through a fetal stage until birth .
Human fertilization is the union of an egg and sperm, occurring primarily in the ampulla of the fallopian tube. [1] The result of this union leads to the production of a fertilized egg called a zygote, initiating embryonic development. Scientists discovered the dynamics of human fertilization in the 19th century. [2]
[140] [141] It results in a baby born without signs of life. [141] Worldwide prevention of most stillbirths is possible with improved health systems. [141] [142] About half of stillbirths occur during childbirth, and stillbirth is more common in the developing than developed world. [141]
However, the mother's body does provide gas exchange. [23] The young of ovoviviparous amphibians are sometimes born as larvae, and undergo metamorphosis outside the body of the mother. [24] The fish family Syngnathidae has the unique characteristic whereby females lay their eggs in a brood pouch on the male's chest, and the male incubates the ...
Its reproduction is typically parthenogenetic and viviparous and females produce unfertilized eggs which they retain within their bodies. [30] The embryos develop within their mothers' ovarioles and the offspring are clones of their mothers. Female nymphs are born which grow rapidly and soon produce more female offspring themselves. [31]