a. The Placenta dissociates gradually, thus Progesterone decreases in its secretion, and the attachment between the foetus, and the uterus becomes loosened preparing for birth.

B. Labor begins with series of contractions in the uterine wall till the foetus is expelled outside. The baby starts his life with a distinct cry, which stimulates the respiratory system.

c. The Placenta separates from the uterine wall, and moves outside the body, then the umbilical cord is cut from the baby side, and the food supply of the newborn changes to the mother’s milk. A pituitary hormone stimulates the production of milk in the mammary glands of the mother, milk flows out to feed the baby with the most valuable nutritive and emotional supply, protecting him from many somatic and psychic diseases in his future.

N.B.:

The suitable female age for pregnancy ranges between 18 and 35 years. If the age decreases or exceeds that range, both the mother and the embryo will be subjected to serious problems, and the possibility to produce deformed babies will increase. On the other hand, if the father age is too old, similar results will occur to his babies.

2. The time interval of pregnancy differs according to the living organism:

- It is 21 days in case of rats.

- It is 150 days in case of sheep.

- It is 270 days in case of Man.

- It is 330 days in case of cattle.

- It is 900 days in case of elephant.

Means of contraceptive:

Several contraceptive methods are used in birth control:

1. The pills:

They contain a combination of synthetic Estrogen and synthetic Progesterone hormones. A woman starts using them after the menstrual cycle and continues for 3 weeks. These pills prevent pregnancy by inhibiting ovulation.

2. The intrauterine device (the coil):

It is inserted into the lumen of the uterus. It prevents the fertilized ovum to be implanted in the uterus.

3. Condum:

It prevents the sperms from entering the vagina.

4. Surgical sterilization:

It involves the ligation of the two fallopian tubes in the woman and cutting them. So, fertilization does not occur for the ova produced by the ovary. In a similar way, each vas deferens is tied, separated from other structures and is cut in case of males.

Multiple births:

Usually, one baby is born in each birth. Sometimes, multiple births take place reaching six babies in the same time. The most common is the twins. The international percentage of twins is once in about 86 births. Multiple births are rare. There are two types of twins:

1. Fraternal (Dizygotic) twins:

Two matured ova are liberated (from one or both ovaries) at the same time. The two ova are fertilized (with two sperms), and each one will develop to an independent embryo. The two embryos differ genetically from each other. Each embryo has its own embryonic sac, and separate placenta in the uterus. They never exceed brothers and sisters of the same age.

2. Identical (Monozygotic) twins:

This type of twins results from the fertilization of one ovum by a single sperm. The developing cells separate into two masses of cells, each of which develops independently. They form two embryos, identical genetically and share one embryonic sac and mostly one placenta. They have the same genetic characters. These twins may be born partially attached to each other in some place in the body, a case known as Siamese twins which can be surgically separated in most cases.

Test tube babies:

A matured ovum is obtained from a wife’s ovary, and being fertilized externally with the husband’s semen inside test tube in a certain nutritive medium till reaches to the morula stage. Then it is implanted in the wife’s uterus to complete its embryonic development till birth.

The embryo culture experiments:

Experiment No. 1:

Experiments have been carried out on rabbits, and mice. Cells from early embryonic stages were separated, and each embryonic cell was cultured in a female uterus to produce a complete individual. This can be considered as Asexual reproduction.

Experiment No. 2:

Two early embryos (8 cells stage) were removed from two different mice. The two embryos were cultured together in a nutritive medium, then they were pushed to fuse into single embryo. Then it was implanted in the uterus of another female mouse. It was developed and born normally. This newborn is produced from two mothers, two fathers, besides the foster mother.

Experiment No. 3 (Renucleation):

Very fine surgical instruments were used to remove nuclei from an Amphibian (toad) embryonic cells at different stages of development, and to transplant them into unfertilized eggs of the same species, whose nuclei had been removed or destroyed by radiation. Normal development was proceeded giving rise to individuals identical in characters to the individual from which the cultured nuclei were taken. This proves that the early embryonic cell nucleus is capable to direct the embryonic development in a manner similar to that of the zygote nucleus itself.

Gamete banks:

These banks are used to store selected animal gametes especially those of cattle and horses to keep them available for reproduction till the time of need. The gametes are stored in a frozen conditions (- 120° C) for up to 20 years. After that period, they can be used in artificial fertilization, even after the death of their producer individuals, or if some rare animal species are liable to extinction. Some people desire to store their gametes in such banks, to ensure the continuity of their generations, even after their death with several years.

On farm animals, researches are carried out to control the sex of their newborn. It is possible to separate the sperms with (X) chromosome from sperms with (Y) chromosome by laboratory means such as centrifugation, or exposure to a limited electric field. This technique is applied on cattle to produce only males for meat, or females for reproduction, and milk production as required by breeders.

Дата добавления: 2015-11-16