The sperms share in secreting the Hyaluronic enzyme which dissolves part of the ovum coat through which one sperm only enters (the sperm and the neck only).

4. The sperms can stay alive about 2 – 3 days inside the female genital system.

After fertilization, the ovum surrounds itself with a coat, that prevents the entrance of any other sperm.

Pregnancy and embryonic development:

By about 24 hours after fertilization, the zygote converts (by mitotic division) to two cells (two blastomeres) in the upper part of fallopian tube, then to four cells in the next day.

The cellular division speeds up to form a mass of small cells, known as morula. Morula is pushed along the uterine tube by ciliary action and muscle contraction till it reaches the uterus.

In the uterus, morula is implanted among the folds of the thick-walled uterus (the endometrium), at the end of the first week. The endometrium is characterized by rich blood supply necessary for the development of embryo along the nine months of pregnancy.

The embryonic membranes:

The rate of embryo growth is increased. Organogenesis, and tissue differentiation gradually takes place. Two embryonic membranes are formed which are the Chorion, and the inner one is the Amnion:

1. The Amnion:

It surrounds the embryo with a fluid that serves to protect the embryo against shocks, and dryness.

2. The Chorion:

It surrounds the amnion, its function is to protect the embryo. Finger-like projections grow from the chorion membrane to be inserted in the endometrium in which the capillaries of both embryo and mother touch, these intermingle forming the Placenta.

The Umbilical cord:

The embryo is connected with the Placenta by the Umbilical cord; its length is about 70 cm and increases to give more freedom for the motility of the embryo. The Umbilical cord is a tissue rich in blood vessels which transfer digested food, Vitamins, water, salts, and Oxygen from the Placenta to the embryo’s intestine, and it transfer the excretory wastes and Carbon dioxide from the embryo’s intestine to the Placenta.

The importance of the Placenta:

1. It transfers digested food, water, Oxygen, and Vitamins form mother’s blood to the embryo’s blood by diffusion, and get rid of the embryo’s excretory wastes. Foetus blood does not normally mix with mother blood.

2. It secretes the Progesterone hormone at the beginning of the 4^th. month of pregnancy when the Corpus luteum degenerates and Placenta becomes the source of the Progesterone hormone.

3. It transfers the drugs, and harmful substances such as Alcohol, Nicotine, Viruses from mother’s blood to the embryo, these cause great harms, serious deformities, and diseases to the embryo.

Stages of the embryonic development:

The period of the embryonic development is classified into three stages, which are:

1. The first stage:

It includes the 1^st. three months of pregnancy, in which:

- The nervous system and the heart start its development. (in the 1^st. month)

- The hands and eyes become differentiated.

- The two sexes become differentiated. (the testes are developed in the 6^th. week, and

the ovaries are developed in the 12^th. week)

- Response to stimuli becomes established.

2. The second stage:

It includes the middle three months, in which:

- The development of the heart is completed, and its beats can be heard.

- Ossification and support of the skeletal system takes place.

- The sense organs are completed.

- The growth of the embryo in size increases.

3. The last stage:

It includes the last three months, in which:

- The development of the brain is completed.

- The growth slows down.

- The development of the other internal organs is completed.

Events of the ninth month:

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