1. Fertilization and Development
Chapter 39 Section 4
Fertilization and Development

2. When an egg is fertilized, human development_____________.
In this process, a single cell undergoes a series of cell divisions that results in the formation of a new human being.
Fertilization
During sexual intercourse, sperm are _____________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.
begins
released

3. The egg is surrounded by a protective layer that contains binding sites to which sperm can____________.
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_____________.
attach
together

4. A ___________cell has a set of chromosomes from each parent cell.
The process of a sperm joining an egg is called_________________.
After the two haploid (N) nuclei fuse, a single diploid (2N) nucleus is formed.
A ___________cell has a set of chromosomes from each parent cell.
The fertilized egg is called a___________.
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___________
fertilization
diploid
zygote
morula

5. The stages of early development include implantation, gastrulation, and neurulation.
As the morula grows, it becomes a hollow structure with an inner cavity called a________________.
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____________________.
blastocyst
implantation

6. 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.

7. Blastocyst cells specialize due to the activation of genes.
This process, called____________________, is responsible for the development of the various types of tissue in the body.
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.
Gastrulation 
The inner cell mass of the blastocyst gradually sorts itself into two layers, which then give rise to a ____________layer.
differentiation
third

8. 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

9. 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

10. The _______________develops into the skin and nervous system.
The ______________forms the digestive lining and organs.
Mesoderm cells differentiate into internal tissues and organs.
Neurulation 
Gastrulation is followed by neurulation.
____________________is the development of the nervous system.
ectoderm
endoderm
Neurulation

11. 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.

12. As the notochord develops, the neural groove changes shape, producing neural folds.
Neural crest
Neural fold
Notochord

13. Gradually, these folds move together to create a neural tube from which _____________________and the nervous system develop.
The spinal cord
Neural crest
Neural tube
Ectoderm
Notochord

14. The chorionic villi and uterine lining form the _____________.
Extraembryonic Membranes
As the embryo develops, membranes form to protect and nourish the embryo.
Two of these membranes are the amnion and the chorion.
The chorionic villi and uterine lining form the _____________.
The placenta connects the mother and developing embryo.
Placenta

15. Early Development
Amniotic sac
The amnion develops into a fluid-filled amniotic sac, which cushions and
_____________the developing embryo.
Placenta
Umbilical cord
protects
Uterus
Amnion
Fetus

16. The placenta is the embryo's organ of respiration, nourishment, and excretion.
The placenta acts as a _____________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.
barrier

17. After eight weeks, the embryo is called a___________.
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.
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.
fetus

18. During the last _______________months, the organ systems mature.
Later Development
4–6 months after fertilization:
The _____________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.
During the last _______________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.
heart
three

19. These muscles begin rhythmic contractions known as_____________.
Childbirth
About nine months after fertilization, the fetus is ready for birth.
A complex set of factors affects the onset of childbirth.
The mother’s posterior pituitary gland releases the hormone__________________, which affects involuntary muscles in the uterine wall.
These muscles begin rhythmic contractions known as_____________.
The contractions become more frequent and more powerful.
oxytocin
labor

20. Contractions force the baby out through the___________________.
The opening of the cervix ________________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___________________.
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.
expands
vagina

21. Multiple Births Early Years
If two eggs are released during the same cycle and fertilized by two different sperm, __________________twins result.
A single zygote may split apart to produce two embryos, which are called ____________________twins.
Early Years
The first two years of life are called________________. 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.
fraternal
identical
infancy

22. Adulthood Development continues during adulthood.
Adults reach their ________________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.
highest