The process of union of the mature spermatazoa with mature ovum is called fertilization.When ovum is fertilized , a zygote is formed which constitutes the commencement of the development of new individual for the propogation of the species.The mature ovum enters the fallopian tube with the help of fimbria .Fertilisation takes place in the lateral or ampullary part of the tube., into which spermatozoa gains access after traversing the vagina and cavity of uterus .It is believed that spermatozoa may remain alive in the tube for 7 days , though it’s fertilizing power may not last more than 48 hours.
The sperm attached by the ovum penetrates through corona radiata into the yolk , the point of entry being subsequently closed.It looses it’s tail while entry and head and body are transformed into two centromeres and a male pronucleus.Male and female pronuclei migrate towards each other , meet near the centre of ovum and form first segmentation nuleus
In the pronuclei chromosomes are reduced to half.
After fertilisation zygote is formed.
Sereis of repeated division of zygote form 2,4,6,8 ,16..a mass of cells.This is called morula.
Morula is converted into blastocyst., outer layer trophoblast and inner layer embryonic cell mass.This is by the end of first week.
Trophoblast gives nutrition to the fetus.
Implantation of the blastocyst into decidua:
The blstocyst travels from the tube and about on sixth day reaches the cavity of the body of uterus.
The mucous membrane of the uterus changes into decidua.
Important changes take place in trophoblast.
The trohoblastic wall of blastocyst lined internally with mesoderm is known as chorion.
The placenta :
The organ in which blood vessels of the embryo and fetus are brought into intimate relationship with the blood of mother is called placenta.maternal portion is composed of decidua basalis containing intervillous spaces. The fetal portion of the placenta consists of the secondary chorionic villi and chorion at placental site.(chorion frondosum )
Umbilical cord :
It is covered externally by the amniotic ectoderm and contains a core of mesoderm in which there are two umbilical arteries and one vein.It is attached by it’s proximal end to embryo and by distal end to chorion which forms placenta.The cord is twisted due to unequal lengths of the arteries , by third month it is 4 inches and by the ninth month it is 20 inches.
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- Fertilisation (asingaporeanbiologist.wordpress.com)
- The Menstrual Cycle: Ovulation (naturallyfertile.wordpress.com)