UCT~OlNl
Sexual Reproduction is the process that provifes variation in a
species. Gametes are sex cells. How does a human develop from two sex
cells? What are the stages it"). human development?
Sexual Reproduction
The sexual differences between male and female hymans become
obvious as a child develops into an adult. How are these changes
controlled? Males produce sex cells called sperm; females produce sex
cells called eggs. How does an egg and a sperm become a baby?
Reproductive Organs
Humans reproduce by sexual reproduction. Usually, one offspring is
born at a time. Sometimes, twins or mutiple births of three or more occur.
The time when a human is first able to reproduce is called pUberty. At
puberty, the pituitary gland located at the base of theibrain releases
hormones. These hormones stimulate the production of male and female
sex hormones by the reproductive organs. The reproductive organs·and the
body of the young person then mature.
The sex cells in humans, as in all complex organisms, are sperm and
eggs. Sperm are produced in the two testes of males. The testes are oval
shaped male sex glands. They are enclosed in an external sac located
behind the penis. The sac is called the scrotum. The testes esch contain
over 200 meters of threadlike tubes in which sperm are produced.
Bladder
Uterus
a
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Organs of the
male (a) and female (b) repro­
ductive systems.
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Testes
;
b
.
Vagma
The testes produce a hormone that controls the production of sperm
within these tubes. The sperm are then formed constantly for the life of
the male. Sperm pass into a larger coiled tube on top of the sex gland. The
sperm mature while in this tube. As sperm begin to be released from the
body of the male, they pass through a series of other sex glands. These
glands produce a whitish fluid that mixes with the sperm and form semen.
Contractions of the glands force semen a tube to the penis. The semen
passes out of the male's body through the urethra. Urine also passes out of
the body through the urethra. However, semen and urine do not pass out of
the body at the same time. Hundreds of millions of sperm are released
from the body in semen. Sperm that are not released are absorbed into the
body.
Eggs are produced, usually one at a time, by two ovaries in the body of
the female. The ovaries are female sex organs located in the lower
abdominal cavity. An ovary releases one egg into a tube called an oviduct.
The two oviducts are also called fallopian tubes. Eggs move along the
oviducts to the uterus. The uterus is a hollow, thick-walled organ with a
narrow opening that leads to the vagina. The vagina is a canal that opens
to the outside of the female's body. In female's, the urethra through which
urine passes is a seperate passsage from the vagina and the reproductive
organs.
Menstrual Cycle
The reproductive cycle of a human female averages 28 days. However,
the cycle can vary from 20 to 40 days. The female reproductive cycle is
called the menstrual cycle. The menstrual cycle is controlled by
hormones from the pituitary gland and the ovaries.
At one stage of the menstrual cycle, an egg develops inside the ovary.
At the same time, the walls of the uterus become thicker and the blood
flow to the uterus increases. Then ovulation occurs. Ovulation is the
release of an egg from the ovary. Ovulation usually occurs 14 days before
the onset of menstruation. Menstruation is a discharge of blood and
tissues that are shed from the uterus through the vagina.
An egg is alive for about 24 to 48 hours after it is released from the
ovary. During this time, it travels along the oviduct. A female can become
pregnant at this time in her menstrual cycle. The egg can be fertilized if
live sperm are present in the oviduct 48 hours before or after ovulation. It
takes only one sperm to fertilize an egg and produce a zygote.
If the egg is not fertilized by a sperm, the menstrual cycle continues.
The soft tissues lining the uterus break down. Then menstruation occurs.
Discharge of blood and tissues lasts four to six days. At the same time,
another egg begins to form in the ovary. The menstrual cycle continues.
All the stages in the menstrual cycle are controlled by hormones.
Most human females first menstruate between the ages of ten and
sixteen. A female menstruates until she reaches menopause, usually
between the ages of 45 and 55. Menopause is the time in a women's life
when ovulation ceases.
If the egg is fertilized, the zygote passes along the oviduct to the
uterus. The zygote divides and becomes attached to the wall of the uterus.
Ovulation and menstruation stops for the length of the pregnancy that
follows. The menstrual cycle resumes a few weeks after the birth takes
place.
The menstrual
cycle in human females aver­
ages 28 days. As the egg ma­
tures in the ovary, the lining of
the wall of the uterus thickens in
preparation for pregnancy. The
lining is shed during menstrua­
tion if fertilization does not
occur.
Lming of uterus
breaks down
Lining of uterus thickens
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Ovulation