A. SEXUAL REPRODUCTION. Sexual reproduction is a type of reproduction that involves two parents. The offspring are different from the parents. Sexual reproduction involves the fusion (joining) of two sex cell nuclei. Sex cells (egg and sperm) are formed from a type of cell division called meiosis. The sex cell nuclei fuse during the process of fertilization. The fertilized egg is called the zygote. The zygote divides by mitosis and develops into a new organism. The large variation (differences) in organisms is due in part to sexual reproduction.

B. GAMETOGENESIS. Gametogenesis is the development of mature sex cells called gametes (Figure 20-1). The formation of gametes involves meiosis in immature sex cells. Gametogenesis includes the processes of spermatogenesis and oogenesis. The process by which sperm develop in the testes of the male is called spermatogenesis. The process by which eggs develop in the ovaries of the female is called oogenesis. Gametogenesis takes place in reproductive organs called gonads. The male gonads are the testes and the female gonads are the ovaries. In some organisms, called hermaphrodites, both the male and female gonads are located on the same animal. The earthworm is an example of a hermaphrodite.

The male gamete is called the sperm. Sperm are monoploid and motile (able to move around). Many sperm are produëed in comparison to few eggs. Each human gametogenesis produces four sperm to every one egg. The female gamete is called the egg (ova). The egg is larger than the sperm. It contains stored nutrients in the form of yolk. The egg is not capable of movement (nonmotile). One large egg and three smaller cells, called polar bodies, are formed during each oogenesis. The egg survives and if fertilized becomes the new organism. The polar bodies disappear.


C. FERTILIZATION. Fertilization is the union (joining) of the egg and sperm nuclei. A monoploid or (n) sperm nucleus unites with a monoploid or (n) egg nucleus to form a diploid or (2n) zygote. Fertilization restores (brings back) the normal species chromosome number (2n)  Each organism has a different species

chromosome number, The new organism will not be identical to either parent because it contains genetic information from both the sperm cell and the egg cell. Half of the  new organism’s charactenstics (traits) come from the female parent and the other half come from the male parent. Fertilization can be either external or internal, External fertilization takes place outside the body of the female. Sperm and eggs are released into the water, and the sperm swim to the eggs. To be sure that at least some of the eggs are fertilized; large number of sperm and eggs are deposited into the water at the same time. Many aquatic vertebrates, such as fish and amphibians, fertilize externally. Internal fertilization takes place inside the body of the female. The male deposits its sperm inside the reproductive tract of the female. The female reproductive tract has moist tissues to provide an environment for the sperm to swim toward the egg. Most terrestrial (land-dwelling) vertebrates, such as reptiles, birds, and mammals, fertilize internally.




D. EMBRYONIC DEVELOPMENT. Embryology is the study of the development of an organism from fertilization until birth. Once fertilization has occurred, the fertilized egg, or zygote, undergoes repeated mitotic cell divisions called cleavage. Cleavage results in a developing organism called the embryo (Figure 20-3).

The cleavage process begins when the zygote divides by mitosis into two identical cells. These two cells divide into four cells, which in turn divide into eight cells. This process continues and a solid-ball of cells, called the morula, is formed. As the cells continue to divide, the center of the ball becomes hollow. The hollow-ball stage is called the blastula. The blastula is made up of a single layer of cells.

The blastula stage is followed by the formation of the gastrula. During the formation of the gastrula, one side of the blastula pushes inward. This results in the formation of a second, inner, layer of cells. The inner cell layer is called the endoderm. The outer cell layer is called the ectoderm. A third cell layer, the mesoderm, forms between the endoderm and ectoderm. The endoderm, ectoderm, and mesoderm layers continue dividing and form all the tissues,

organs, and organ systems of the animal (Table 20-1). The three cell layers are called primary germ layers.




Nervous system, skin, hair, nails


Muscles, circulatory system, skeleton, excretory system, testes or ovaries


Lining of digestive and respiratory tracts, parts of the liver and the pancreas






E. EXTERNAL DEVELOPMENT. After the formation of the three germ layers, the embryo begins to grow and develop. Growth involves an increase both in the number and size of cells. The growth and development of the embryo can take place outside the body of the parent or inside the parents body. External development involves the growth of the embryo outside the body of the parent. External development takes place in water or on land.The eggs of many fish and amphibians are fertilized externally and develop externally in a water environmentThe females lay their eggs in the water and the males deposit sperm over the eggs. Many eggs and sperm must be      laid so that a few fertilized eggs reach adulthood. Most will be eaten by predators or lost through dryness and other factors. The developing embryo gets nourishment from yolk stored in the egg.

The fertilized eggs of birds, many reptiles, and a few mammals (duckbill platypus and spiny anteater), develop externally on land. The embryos get nourishment from yolk stored in the egg. These organisms are adapted to external development on land because of a protective shell and membranes that help to provide a favorable environment for embryonic development (Figure 20-5). The shells of bird and reptile egg protect

the soft inner parts and help to prevent the loss of water from the eggs. The

shells are porous and allow the exchange of respiratory gases.

 The embryonic membranes of a bird are as follows:

•Chorion. A membrane that lies directly under the she and completely encloses the embryo and the other membranes,

•Amnlon (Amniotic Sac). The membrane that lies just inside the chorion. It encloses the amniotic fluid.

•Amniotic Fluid. Cushions the embryo against shock. Together with the ehorion keeps the embryo from drying out. Also prevents the

embryonic tissues from sticking to the shell.

• Yolk Sac. Surrounds the yolk. Is a smaller membrane that grows from the digestive tract of the embryo. Blood vessels that penetrate

the yolk sac carry food (yolk) to the embryo.

•Allantols. Respiratory membrane and storage area for uric acid, a nitrogen-containing waste,

F. INTERNAL DEVELOPMENT. During internal development the embryo grows inside the body of the female parent. Embryos that develop internally get greater protection from the environment than embryos that develop externally. Some fish and reptiles, and most mammals develop internally.

C. PLACENTAL MAMMALS. Fertilization in mammals is internal. It takes place in the oviducts of the female. Most mammals develop internally in a placenta specialized organ called the uterus. oviduct Mammal eggs contain little yolk and are very small. Therefore, they cannot provide food. Developing mammals wall of uterus get food through a placenta inside the mother’s uterus. The placenta is umbilical formed from both maternal and embryonic tissues (Figure 20-6). The enituyo exchange of nutrients, respiratory am gases, and wastes between the mother and the embryo takes place through the amman placenta. There is no direct connection between the circulatory systems of mother and the embryo. The exchange

Human Reproduction

A. PUBERTY. Human reproduction is controlled by hormones secreted by the gonads (testes and ovaries). Your reproductive process begins with the production of eggs and sperm (gametes) when you reach the age of puberty. Puberty is the time when hormones produce physical changes in the human body that enable the body to produce gametes. These changes are called secondary sex characteristics. Males usually reach puberty at about age 12-14 and females reach puberty between the ages of 10-14. Some male and female secondary sex characteristics are as follows:

Female Secondary Sex Characteristics  Male Secondary Sex Characteristics

Development of breasts (mammary glands)        • Growth of beard and other body hair

• Changes in body form                                    • Changes in body form

• Growth of body hair                                       • Lowered voice pitch

Other reproductive processes that are hormone controlled are the preparation of the female body for pregnancy, sexual intercourse, fertilization, pregnancy, and the production of milk for nourishment of the infant.

B. HUMAN MALE REPRODUCTIVE SYSTEM. The male reproductive system performs two major functions: the production of sperm and the deposition of sperm inside the female reproductive tract. The production of sperm takes place in a pair of male gonads called the testes (Figure 21-1). The testes are held in a sac called the scrotum. The scrotum is an outpocketing of the abdominal wall. It keeps the temperature of the testes one or two degrees (C) cooler than normal body temperature.     This      lower

temperature is necessary for sperm production and storage.       Sperm leave the testes and

pass through tubes to the urethra. The urethra is a tube inside the penis. The penis is adapted for internal fertilization.       As the sperm passes through tubes to the urethra, fluids are secreted into the tubes by glands. The fluids provide the sperm with the proper

FIGURE 21-1. HUMAN MALE REPRODUCTIVE SYSTEM, alkaline pH (basic environment) and supply glucose for energy. They also provide a liquid medium in which the sperm can swim, an adaptation for life one land. The mixture of fluid and sperm is called semen. The process by which sperm pass out of the body is known as ejaculation.

C. HUMAN FEMALE REPRODUCTIVE SYSTEM. The female reproductive system produces eggs (ova) and is the site (place) of fertilization and embryonic development (Figure

21-2). The production of eggs takes place in paired female gonads called ovaries. The ovaries are located inside the lower portion of the body cavity. The ovary produces eggs in structures called follicles. Follicles are tiny cavities surrounded by cells. The release of a mature egg from a follicle is called ovulation. Following ovulation, the egg passes through an oviduct. Humans, the oviduct is also called the fallopian tube. From the fallopian tube, the egg moves to the uterus. The uterus, or womb, is shaped like a pear and has thick walls. The embryo develops in the uterus. The lower end of the uterus, the cervix, opens into a muscular tube called the vagina. The vagina is a muscular tube that leads from the uterus to the outside. The vagina receives semen during intercourse.

I FERTILIZATION. During sexual intercourse (copulation) the penis becomes modified (erect) for the efficient transfer of sperm into the vagina. After sexual intercourse, the sperm cells swim through the female reproductive tract and enter the oviducts. If they meet with an egg in the oviduct, the egg and sperm cell may fuse (unite). The fusion of a sperm cell nucleus and an egg cell nucleus is known as fertilization. A fertilized egg is known as a zygote. Fertilization generally occurs when the egg is in the upper portion of the oviduct. If the egg is not fertilized within about 24 hours after ovulation, it breaks down and disappears. Cleavage of the fertilized egg begins while the egg is still in the oviduct. Six to ten days later, the resulting embryo may become implanted (attached) in the lining of the

If more than one egg is released and fertilized at the same time, multiple births may occur. Fraternal twins develop when two eggs are released at one time and each egg is fertilized by a different sperm. Identical twins develop from one zygote that separates in half early in cleavage.

It is now possible for fertilization to take place outside the human body (in vitro). The fertilized egg is then implanted in the uterus so that development can occur.



E. PRENATAL DEVELOPMENT AND BIRTH. When the fertilized egg athves in the uterus, it implants itself in the thickened, spongy uterine wall. The gastrula forms, differentiation and growth occur. The developing embryonic membranes become part of the placenta and umbilical cord. After eight weeks of development the embryo is called the fetus. The period of time between the fertilization of the egg and the birth of the offspring is called gestation. In humans the gestation period is nine months or approximately 266 days. During this time, it is essential that the expectant mother provide good prenatal care by eating nutritious foods, avoiding alcohol, tobacco, and drugs, and receiving proper medical attention regularly. Good prenatal care is important in the production of a healthy baby. Using tobacco, alcohol and drugs is thought to cause Low Birth Weight Syndrome and Fetal Alcohol Syndrome as well as other birth defects.

When gestation is completed, the baby is forced from the uterus by muscular contractions controlled by a hormone from the pituitary. The period of time before birth is referred to as prenatal and the time after birth is called postnatal.

F. POSTNATAL DEVELOPMENT. Following birth, the placenta is discarded from the mother’s body and the mother begins producing milk from mammary glands located in the breasts. This milk is normally the best natural food for a newborn baby. The baby should receive regular medical attention and other postnatal care as it grows.

After birth, growth and development continue at different rates. Although it is often assumed that development ends when the individual becomes a mature adult, it actually continues throughout life and ends only with death. Aging is the term that is applied to the developmental changes that occur in an organism from birth until death. The causes of aging are not fully understood. It appears that aging involves both hereditary and environmental factors.

G. THE MENSTRUAL CYCLE. The menstrual cycle usually begins in females between the ages of 10 and 14. It is repeated approximately every 28 days. Hormones control the changes that occur in the ovaries and uterus. The menstrual cycle involves the release of a mature egg from a follicle and the preparation of the uterus for pregnancy. The duration of each cycle may vary considerably, and may be interrupted by illness and other factors. The cycle stops during pregnancy. The menstrual cycle has four stages: the follicle stage, ovulation, the corpus luteum stage, and menstruation (Figure 21-4). The cycle starts at puberty and ends at menopause.

I. Follicle Stage (10-14 days): FSH from pituitary stimulates maturation of egg In follicle Estrogen form follicle stimulates thickening of uterine lining

2. Ovulation: Mature egg released from follicle. FSH production inhibited by estrogen, which stimulates release of LH by pituitary

3. Corpus luteum stage ( 10 – 12 day)s burst follicle develops into Corpus luteum, wich produces progesterone. Progesterone stimulates (further thickening of uterine lining.

4. Menstruation if egg is not fertilized, progesterone secretion decreases, and uterine lining degenerates. Tissue and blood are discharged from body.

H. HUMAN REPRODUCTIVE DISORDERS. In some geographic areas, sexually transmitted diseases have reached epidemic proportions in recent years. They may be transmitted by both males and females and may cause sterility or death if not treated by a competent physician. Early treatment usually leads to recovery. Some sexually transmitted diseases in humans are syphilis, gonorrhea, genital herpes, and AIDS.

It is very dangerous to diagnose and treat yourself if you suspect you have a sexually transmitted disease (STD). If you think you have a sexually transmitted disease, go to a doctor and let her/him give you proper treatment.

Prostate enlargement and cancer are other disorders associated with the human reproductive tract. Prostate enlargement is most common in males over 40 years of age. Breast cancer is a major cause of death of women who are 25-45 years of age.