Human evolution begins by fertilization , a process through which spermatozoon from man is fused with ovum from woman to produce the zygote.Although fertilization takes place in a short period,it includes several stages, in a very brief classification , sperm passes through three stages to complete fertilization :
1 - it passes a route to reach ovum and penetrate through its corona.
2 - penetration to the translucent pant of the ovum .
3 - amphimixis ) the union of the sperm and ovum ).
Following fertilization , the zygote moves towards uterus and along its route it bears consecutive clevages the result of which is augmentation of the number of embryonic cells.The first embryonic division takes place about 30 hours post fertilization. After about 3 days and in 12 to 16 cell stages zygote reaches the uterus . After spending a few days in a free condition in the uterus,in days 5th and 6th it begins to implant in the uterine mucous memberan and at this stage the cellular mass which is called Blastocyst is completely lined under the inner surface of the uterus and the penetration zone is repaired.Complete implantation usually takes about 2 weeks to occur.
The required time for the evolution of zygote to a complete fetus is called the Pregnancy Period.This period for a quite matured fetus is 280 days or 40 weeks after last menstruation or exactly 266 days or 38 weeks after fertilization pregnancy stages are usually divided into three general periods :
1 - Preembryonic period : During this period which includes the three weeks post-fertitization , implantation is completed.At the end of this period embryo resembles a flat plane in the length of one millimeter.This period is the beginning of differentiation of organs and tissues.
2 - Embryonic period : This period lasts up to the eighth week. At this period the embryo has a spherical form.The face begins to form;eyes,ears,and nose appear and fingers and thumb develop. At the beginning of this stage the embryo has a tail that is gradually lost through fetal development.All body organs form and differentiate from each other.At about the middle of the third week that human embryo is no more able to provide its alimentary needs through diffusion , the circulatory system appears.During the 4th to 7th week heart,with its four cavities appear.At the end of this period , the embryo measures about 2.5 cm. In length and weighs about one gram and has a complete human shape and is called fetus.At this stage,primary signs of pregnancy appear in the mother.
3 - The fetal period : This period is extended from the beginning of 9th week until delivery and is characterized by rapid growth and maturation of organs and viscera. Longitudinal growth is considerable during the 3rd,4th,and 5th months , ) about 5 cm per month ( while during the lost two months of pregnancy weight gain is more conspicuous.At the beginning of this period the embryo presents a human figure however, the size of its head is disproportionately larger than other organs and due to lack of hypodermal fat it shows a slimbony appearance . At the 5th month of pregnancy fetal movements are easily perceived by the mother . At 8th month,fat accumulates under the skin and the fetus looses its bony appearance.At the end of this period the fetus is quite developed and pregnancy is going to be ended.Soon labour begins. It is worth mentioning that sometimes due to concomitant exit of two ovums and their fertilization by two sperms, twins are born whose morphological and genetic specifications show no particular similarities and resemble two quite different offsprings though they develope synchronously and are born at the same time. Sometimes however , at a very early stage a zygot is divided into two quite separate sections and in this case identical twins are born who being originated from the same cell quite similar to each other.