1. Copyright Pearson Prentice Hall
Biology
Copyright Pearson Prentice Hall

2. 39–4 Fertilization and Development
Photo Credit: ©David M. Phillips/Photo Researchers Inc.
Copyright Pearson Prentice Hall

3. Copyright Pearson Prentice Hall
Development
When an egg is fertilized, human development begins.
In this process, a single cell undergoes a series of cell divisions that results in the formation of a new human being.
Copyright Pearson Prentice Hall

4. Copyright Pearson Prentice Hall
Fertilization
Fertilization
During sexual intercourse, sperm are released when semen is ejaculated through the penis into the vagina.
Sperm swim through the uterus into the Fallopian tubes.
if an egg is present in one of the Fallopian tubes, its chances of being fertilized are good.
Copyright Pearson Prentice Hall

5. Copyright Pearson Prentice Hall
Fertilization
The egg is surrounded by a protective layer that contains binding sites to which sperm can attach.
When a sperm attaches to a binding site, its head releases enzymes that break down the protective layer of the egg.
The sperm nucleus enters the egg, and chromosomes from the sperm and egg are brought together.
Copyright Pearson Prentice Hall

6. Copyright Pearson Prentice Hall
Fertilization
What is fertilization?
Copyright Pearson Prentice Hall

7. Copyright Pearson Prentice Hall
Fertilization
The process of a sperm joining an egg is called fertilization.
Copyright Pearson Prentice Hall

8. Copyright Pearson Prentice Hall
Fertilization
After the two haploid (N) nuclei fuse, a single diploid (2N) nucleus is formed.
A diploid cell has a set of chromosomes from each parent cell.
The fertilized egg is called a zygote.
Copyright Pearson Prentice Hall

9. Copyright Pearson Prentice Hall
Early Development
Early Development
While still in the Fallopian tube, the zygote begins to undergo mitosis.
Four days after fertilization, the embryo is a solid ball of about 64 cells called a morula.
Copyright Pearson Prentice Hall

10. Copyright Pearson Prentice Hall
Early Development
What are the stages of early development?
Copyright Pearson Prentice Hall

11. Copyright Pearson Prentice Hall
Early Development
The stages of early development include implantation, gastrulation, and neurulation.
Copyright Pearson Prentice Hall

12. Copyright Pearson Prentice Hall
Early Development
Implantation 
As the morula grows, it becomes a hollow structure with an inner cavity called a blastocyst.
6–7 days after fertilization, the blastocyst attaches to the uterine wall.
The embryo secretes enzymes that digest a path into it.
This process is known as implantation.
Copyright Pearson Prentice Hall

13. Copyright Pearson Prentice Hall
Fertilization
Fertilization and Implantation
If an egg is fertilized, a zygote forms and begins to undergo cell division (mitosis) as it travels to the uterus.
Copyright Pearson Prentice Hall

14. Copyright Pearson Prentice Hall
Early Development
Blastocyst cells specialize due to the activation of genes.
This process, called differentiation, is responsible for the development of the various types of tissue in the body.
Copyright Pearson Prentice Hall

15. Copyright Pearson Prentice Hall
Early Development
A cluster of cells, known as the inner cell mass, develops within the inner cavity of the blastocyst.
The embryo will develop from these cells, while the other cells will differentiate into tissues that surround the embryo.
Copyright Pearson Prentice Hall

16. Copyright Pearson Prentice Hall
Early Development
Gastrulation 
The inner cell mass of the blastocyst gradually sorts itself into two layers, which then give rise to a third layer.
Copyright Pearson Prentice Hall

17. Copyright Pearson Prentice Hall
Early Development
The third layer is produced by a process of cell migration known as gastrulation.
Amniotic cavity
Primitive streak
Mesoderm
Gastrulation results in the formation of three cell layers. The diagram shows the primitive streak, a line that forms in the center of the blastocyst. The movement of cells away from the primitive streak, shown in the diagram on the right, forms the mesoderm.
Ectoderm
Endoderm
Copyright Pearson Prentice Hall

18. Copyright Pearson Prentice Hall
Early Development
The result of gastrulation is the formation of three cell layers—the ectoderm, the mesoderm, and the endoderm.
Amniotic cavity
Primitive streak
Mesoderm
Ectoderm
Endoderm
Copyright Pearson Prentice Hall

19. Copyright Pearson Prentice Hall
Early Development
The ectoderm develops into the skin and nervous system.
The endoderm forms the digestive lining and organs.
Mesoderm cells differentiate into internal tissues and organs.
Copyright Pearson Prentice Hall

20. Copyright Pearson Prentice Hall
Early Development
Neurulation 
Gastrulation is followed by neurulation.
Neurulation is the development of the nervous system.
Copyright Pearson Prentice Hall

21. Copyright Pearson Prentice Hall
Early Development
Shortly after gastrulation is complete, a block of mesodermal tissue begins to differentiate into the notochord.
Neurulation is the formation of the central nervous system. The ectoderm near the notochord thickens and forms the neural plate. The raised edges of the neural plate form neural folds. The neural folds gradually move together and fuse to form the neural tube. One end of the neural tube will develop into the brain; the other end develops into the spinal cord. Cells of the neural crest migrate to other locations and develop into nerves.
Copyright Pearson Prentice Hall

22. Copyright Pearson Prentice Hall
Early Development
As the notochord develops, the neural groove changes shape, producing neural folds.
Neural crest
Neural fold
Notochord
Copyright Pearson Prentice Hall

23. Copyright Pearson Prentice Hall
Early Development
Gradually, these folds move together to create a neural tube from which the spinal cord and the nervous system develop.
Neural crest
Neural tube
Ectoderm
Notochord
Copyright Pearson Prentice Hall

24. Copyright Pearson Prentice Hall
Early Development
Extraembryonic Membranes
As the embryo develops, membranes form to protect and nourish the embryo.
Two of these membranes are the amnion and the chorion.
Copyright Pearson Prentice Hall

25. Copyright Pearson Prentice Hall
Early Development
Amniotic sac
The amnion develops into a fluid-filled amniotic sac, which cushions and protects the developing embryo.
Placenta
Umbilical cord
Uterus
Amnion
Fetus
Copyright Pearson Prentice Hall

26. Copyright Pearson Prentice Hall
Early Development
Fingerlike projections called chorionic villi form on the outer surface of the chorion and extend into the uterine lining.
Maternal portion of placenta
Fetal portion of placenta
Chorionic villus
Amnion
Umbilical cord
The placenta is the connection between the mother and the developing embryo or fetus. It is through the placenta that the embryo gets its oxygen and nutrients and excretes its waste products. Notice how the chorionic villi from the fetus extend into the mother’s uterine lining (indicated by the overlapping brackets).
Maternal artery
Umbilical arteries
Maternal vein
Umbilical vein
Copyright Pearson Prentice Hall

27. Copyright Pearson Prentice Hall
Early Development
The chorionic villi and uterine lining form the placenta.
The placenta connects the mother and developing embryo.
Copyright Pearson Prentice Hall

28. Copyright Pearson Prentice Hall
Early Development
What is the function of the placenta?
Copyright Pearson Prentice Hall

29. Copyright Pearson Prentice Hall
Early Development
The placenta is the embryo's organ of respiration, nourishment, and excretion.
Copyright Pearson Prentice Hall

30. Copyright Pearson Prentice Hall
Early Development
The placenta acts as a barrier to some harmful or disease-causing agents.
Some disease causing agents, such as German measles and HIV can cross the placenta.
Some drugs, including alcohol and medications also can penetrate the placenta and affect development.
Copyright Pearson Prentice Hall

31. Copyright Pearson Prentice Hall
Early Development
After eight weeks, the embryo is called a fetus.
After three months, most major organs and tissues are formed. During this time, the umbilical cord also forms.
The umbilical cord connects the fetus to the placenta.
Copyright Pearson Prentice Hall

32. Control of Development
The fates of many cells in the early embryo are not fixed.
The inner cell mass contains embryonic stem cells, unspecialized cells that can differentiate into nearly any specialized cell type.
Researchers are still learning the mechanisms that control stem cell differentiation.
Copyright Pearson Prentice Hall

33. Copyright Pearson Prentice Hall
Later Development
Later Development
4–6 months after fertilization:
The heart can be heard with a stethoscope.
Bone replaces cartilage that forms the early skeleton.
A layer of soft hair grows over the fetus’s skin.
The fetus grows and the mother can feel it moving.
Copyright Pearson Prentice Hall

34. Copyright Pearson Prentice Hall
Later Development
During the last three months, the organ systems mature.
The fetus doubles in mass.
It can now regulate its body temperature.
The central nervous system and lungs completely develop.
Copyright Pearson Prentice Hall

35. Copyright Pearson Prentice Hall
Childbirth
Childbirth
About nine months after fertilization, the fetus is ready for birth.
A complex set of factors affects the onset of childbirth.
Copyright Pearson Prentice Hall

36. Copyright Pearson Prentice Hall
Childbirth
The mother’s posterior pituitary gland releases the hormone oxytocin, which affects involuntary muscles in the uterine wall.
These muscles begin rhythmic contractions known as labor.
The contractions become more frequent and more powerful.
Copyright Pearson Prentice Hall

37. Copyright Pearson Prentice Hall
Childbirth
The opening of the cervix expands until it is large enough for the head of the baby to pass through it.
At some point, the amniotic sac breaks, and the fluid it contains rushes out of the vagina.
Contractions force the baby out through the vagina.
Copyright Pearson Prentice Hall

38. Copyright Pearson Prentice Hall
Childbirth
The baby now begins an independent existence.
Its systems quickly adapt to life outside the uterus, supplying its own oxygen, excreting waste on its own, and maintaining its own body temperature.
Copyright Pearson Prentice Hall

39. Copyright Pearson Prentice Hall
Multiple Births
Multiple Births
If two eggs are released during the same cycle and fertilized by two different sperm, fraternal twins result.
A single zygote may split apart to produce two embryos, which are called identical twins.
Copyright Pearson Prentice Hall

40. Copyright Pearson Prentice Hall
Early Years
Early Years
The first two years of life are called infancy. It is a period of rapid growth and development.
Childhood lasts from infancy until puberty.
Adolescence begins with puberty and ends with adulthood.
Puberty produces a growth spurt that will conclude in mid-adolescence.
Copyright Pearson Prentice Hall

41. Copyright Pearson Prentice Hall
Adulthood
Adulthood
Development continues during adulthood.
Adults reach their highest levels of physical strength and development between the ages of 25 and 35.
Most people begin to show signs of aging in their 30s.
Around age 65, most body systems become less efficient, making homeostasis more difficult to maintain.
Copyright Pearson Prentice Hall

42. Copyright Pearson Prentice Hall
39–4
Copyright Pearson Prentice Hall

43. Copyright Pearson Prentice Hall
39–4
Fertilization takes place in the
ovary.
Fallopian tube.
cavity of the uterus.
cervix.
Copyright Pearson Prentice Hall

44. Copyright Pearson Prentice Hall
39–4
The process in which a blastocyst attaches to the wall of the uterus is called
fertilization.
implantation.
gastrulation.
neurulation.
Copyright Pearson Prentice Hall

45. Copyright Pearson Prentice Hall
39–4
The central nervous system develops during which phase of early development?
gastrulation
neurulation
implantation
fertilization
Copyright Pearson Prentice Hall

46. Copyright Pearson Prentice Hall
39–4
The placenta is a structure that
belongs entirely to the mother.
belongs entirely to the fetus.
brings blood from the mother and fetus close together.
provides an impermeable barrier between the mother and the fetus.
Copyright Pearson Prentice Hall

47. Copyright Pearson Prentice Hall
39–4
Which of the following is not a primary germ layer?
neural tube
endoderm
ectoderm
mesoderm
Copyright Pearson Prentice Hall

48. END OF SECTION