1. Fertilization and Implantation

3. With ovulation, the secondary oocyte and adhered cells of the cumulus-oocyte complex are freed from the ovary., the oocyte is quickly engulfed by the fallopian tube infundibulum.. Fertilization, must take place within a few hours, and no more than a day after ovulation. Almost all pregnancies result when intercourse occurs during the 2 days preceding or on the day of ovulation

4. Steps of fertilization are highly complex
. Steps of fertilization are highly complex. Molecular mechanisms allow spermatozoa to pass between follicular cells; through the zona pellucida, which is a thick glycoprotein layer surrounding the oocyte cell membrane; and into the oocyte cytoplasm. Fusion of the two nuclei and intermingling of maternal and paternal chromosomes creates the zygote

5. The Zygote
After fertilization, the zygote—a diploid cell with 46 chromosomes—undergoes cleavage, and zygote cells produced by this division are called blastomeres . The zygote undergoes slow cleavage for 3 days while still within the fallopian tube. As the blastomeres continue to divide, a solid mulberry-like ball of cells—the morula—is produced. The morula enters the uterine cavity about 3 days after fertilization. Gradual accumulation of fluid between the morula cells leads to formation of the early blastocyst

6. The Blastocyst
As early as 4 to 5 days after fertilization, blastula differentiates into embryo-producing cells—the inner cell mass, and cells destined to form trophoblast
■ Blastocyst Implantation
Six or 7 days after fertilization, the embryo implants the uterine wall. This process can be divided into three phases: (1) apposition—initial contact of the blastocyst to the uterine wall; (2) adhesion—increased physical contact between the blastocyst and uterine epithelium; and (3) invasion—penetration and invasion of syncytiotrophoblast and cytotrophoblasts into the endometrium, inner third of the myometrium, and uterine vasculature

7. Early Trophoblast Invasion
After gentle erosion between epithelial cells of the surface endometrium, invading trophoblasts burrow deeper. By the 10th day, the blastocyst becomes totally encased within the endometrium. The mechanisms leading to trophoblast invasion are similar to those of metastasizing malignant

8. Human embryogenesis
is a complex process that occurs during the first eight weeks after fertilization. The stages of embryogenesis are described in more detail below.
Stage 1: The process of embryogenesis begins with an egg or ovum being fertilized by a sperm cell to form a zygote. The zygote is formed 24 hours after the egg and sperm nuclei fuse. The zygote is a single diploid cell.

9. Stage 2: Over the next three days, the zygote undergoes a number of cell divisions, a process referred to as cleavage. Once the embryo has reached the 8-cell stage, it undergoes another process called compactation, which involves the tight binding of the cells to create a compact sphere. After compactation, the embryo is made up of 16 cells, which is referred to as the morula.

11. Stage 3: After one more day, a cavity referred to as the blastocele develops in the morula. Cells inside the blastocele compact and flatten to form a structure referred to as the blastocyst.

13. Stage 4: Around 24 hours later, the blastocyst moves towards the womb, where it implants in the lining.
Stage 5: Over the next week, the mass of cells rapidly divides, giving rise to a disc-shaped structure that has two layers. One of the layers goes on to become the embryo and the amniotic cavity, while the other layer develops into the yolk sac.
Stage 6: A process called gastrulation also occurs, where the blood system starts to appear in the placenta and blood cells are also produced by the yolk sac. A streak of cells becomes apparent on the embryonic disc

15. Stage 7: Three weeks after ovulation, two types of cells arise from the streak which changes the two-layered disc into a three-layered one. These layers are referred to as the ectoderm, mesoderm and endoderm. The endoderm eventually gives rise to the tissue that forms a lining in the tonsils, lungs, bladder, digestive system, tongue and urethra. The mesoderm develops into the bones, lymph, heart, lungs, muscles, blood cells, spleen and genital organs. The ectoderm gives rise to the nails, skin, tooth enamel, mammary glands, pituitary gland, anus, mouth, sinuses, eye lens, the lining of the external and internal ear and the entire nervous system

17. Stage 8: Twenty-three days after ovulation, the embryo is pear shaped, with a broader head area and embryonic blood cells start to form blood vessels. The ectoderm develops into the neural plate and neural curve that becomes the nervous system.
Stage 9: Twenty-five days after ovulation the mesoderm condenses into somites which form on either side of the neural groove. Three pairs of somites are visible at this stage.

18. Stage 10: Four weeks after ovulation, the heart fold fuses with the neural folds and up to twelve pairs of somites develop..
Stage 11: Thirty days after ovulation, up to 20 pairs of somites have formed and the central nervous system is developed.

19. Stage 12: Thirty-two days after ovulation, up to 29 pairs of somites have formed and the upper limb buds have developed. The brain and spinal cord are large and compact. Differentiation of the digestive layer also gives rise to buds for the lung, stomach, liver and pancreas.

20. Stage 13: Thirty-four days after ovulation, up to 40 somites are visible and the brain differentiates into the forebrain ,the midbrain and the hindbrain

21. Stage 14: Five weeks after ovulation, the lens pit and optic cup are visible and the head and brain rapidly grow.

22. Stage 15: Thirty-eight days after ovulation, the jaw begins to form and the beginning of the mouth and oral cavity are visible. The lower limb bud becomes more rounded at the top where the foot will start to form.

23. Stage 16: Six week after ovulation, the hemispheres of the brain are well differentiated and the lower and upper jaws are visible.

24. Stage 17: Forty-five days after ovulation, a sense of smell starts to develop and the jaw and facial muscles grow. The diaphragm, teeth buds, trachea, bronchi and larynx all start to develop and the trunk becomes straighter.
Stage 18: Seven weeks after ovulation, the eyes are pigmented, the eyelids start to form and nerves develop in the scalp.

25. Stage 19: Fifty-two days after ovulation, the brain waves can be detected in the brain and the gonads form.

26. Stage 20: Eight weeks after ovulation, the brain is connected to nerves and muscles and movement is possible. The upper limbs bend at the elbows and skin folds between what will be the toes are recognizable

27. .
Stage 22: Sixty days after ovulation, the heart is fully formed, the eyelids and ears are more developed and the upper lip is formed

28. Stage 22: Nine weeks after ovulation, the head is rounded, the ears are fully formed, taste buds develop, the upper and lower limbs are well formed and all toe and finger digits are separated.