Reproduction in Animals - K.C.S.E BIOLOGY NOTES

Reproduction in Animals

• Sexual reproduction involves the fusion of gametes.
• In animals two individuals are involved, a male and a female.
• Special organs known as gonads produce gametes.
• In males testes produce sperms while in females ovaries produce ova.
• The fusion of male gamete and female gamete to form a zygote is called fertilisation.

There are two types of fertilisation. External and internal. 
External fertilization

• Example in amphibians takes place in water.
• The male mounts the female and shed sperms on the eggs as they are laid.
• Eggs are covered by slippery jelly-like substance which provides protection.
• Many eggs are released to increase the chances of survival.

Internal fertilisation

• This occurs in reptiles, birds and mammals.
• Fertilisation occurs within the body of the female.
• Fewer eggs are produced because there are higher chances of fertilisation since sperms are released into the female body.

Reproduction in Humans

Structure of female reproduction system,  
​The female reproduction system consist of the following: Ovaries

• Are two oval cream coloured structures found in lower abdomen below the kidneys.

Oviducts.

• They produce the ova.
• Are tubes which conduct the ova produced by the ovaries to the uterus.
• Fertilisation occurs in the upper part of the oviduct.

Uterus

• The uterus is a hollow muscular organ found in the lower abdomen.
• The embryo develops inside the uterus.
• The inner lining endometrium supplies nutrients to embryo.
• The embryo is implanted into the inner uterine wall- the endometrium which nourishes the embryo.
• The thick muscles of the uterus assist in parturition.

Cervix

• Has a ring of muscles that separates the uterus from the vagina.
• It forms the opening to the uterus

Vagina

• Is a tube that opens to the outside and it acts as the copulatory and birth canal through the vulva.

Structure of male reproductive system
The male reproductive system consists of the following: 
Testis:

• Each testis is a mass of numerous coiled tubes called semniferous tubules.
• Each is enclosed within a scrotal sac that suspends them between the thighs.
• This ensures that sperms are maintained at a temperature lower than that of the main body.

Seminiferous tubules

• The lining of seminiferous tubules consists of actively dividing cells which give rise to sperms.
• Between the seminiferous tubules are interstitial cells which produce the male hormones called androgens e.g. testosterone.
• The seminiferous tubules unite to form the epididymis, which is a coiled tube where sperms are stored temporarily .
• Vas deferens (sperm duct) is the tube through which sperms are carried from testis to urethra.
• Seminal vesicle produces an alkaline secretion which nourishes the spermatozoa.  

Prostate gland

• Produces an alkaline secretion to neutralise vaginal fluids.

Cowpers' gland

• Secretes an alkaline fluid.
• All these fluids together with spermatozoa form semen.

Urethra

• Is a long tube through which the semen is conducted during copulation.
• It also removes urine from the bladder.

Penis

• Is an intro-mittent organ which is inserted into the vagina during copulation.

Fertilisation in Animals

• Fertilisation is preceded by copulation in which the erect penis is inserted into the vagina.
• This leads to ejaculation of semen.
•  The sperms swim through the female's genital tract to the upper part of the oviduct.
•  The  head of the sperm penetrates the egg after the  acrosome_ releases lytic enzymes t dissolve the egg membrane.
• The tail is left  behind.
•  Sperm nucleus fuses with that of the ovum and a zygote is formed.
• A fertilisation membrane forms around the zygote which prevents other sperms from penetrating the zygote.

Implantation:

• After fertilisation the zygote begins to divide mitoticaly as it moves towards the uterus.
•  It becomes embedded in the wall of the uterus a process called implantation.
• By this time the zygote is a hollow ball of cells called blastocyst or embryo.
• In the uterus the embryo develops villi which project into uterus for nourishment later the villi and endometrium develop into placenta.

Embryonic membranes

• Embryonic membranes develop around the embryo.
• The outermost membrane is the chorion which forms the finger-like projections (chorionic villi) which supply nutrients to the embryo.
• The amnion surrounds the embryo forming a fluid filled cavity within which the embryo lies.
• Amniotic cavity is filled with amniotic fluid.
• This fluid acts as a shock absorber and  protects the foetus against mechanical injury.
• It also regutates  temperature.
• The chorionic villi, allantois together with the endometrium from the placenta.
• The embryo is attached to the placenta by a tube called umbilical cord which has umbilical vein and artery.
• The maternal blood in the placenta flows in the spaces lacuna and surrounds capillaries from umbilical vein and artery.
• The umbilical cord increase in length as the embryo develops.

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