1. Aim: How do humans develop from a zygote to a fetus?
Devolpment
Aim: How do humans develop from a zygote to a fetus?

2. A) Fertilization
Fertilization may occur within a few days of ovulation
The fusion of the sperm nucleus and the egg nucleus result in a diploid zygote.
Fertilization occurs in the _______________.

3. B) Cleavage and Implantation
In the first week after fertilization, the zygote undergoes a series of mitotic divisions known as cleavage.
Cleavage produces many smaller cells within the zygote.

4. Cleavage continues as the zygote moves through the Fallopian tube toward the uterus.
When the zygote is a solid ball of cells, it is called a morula.
By the time it reaches the uterus, the zygote is a hollow ball of cells called the blastocyst.
The blastocyst attaches itself to the lining of the uterus in an event called implantation.

6. C) Differentiation
Once the blastocyst has been implanted into the uterus, differentiation occurs.
Differentiation (also called spcialization) is when the cells of the blastocyst develop into the various types of tissues in the body.

7. What causes differentiation to occur?
Differentiation is caused by the activation of certain genes.

8. D) Gastrulation The next stage of development is gastrulation.
During gastrulation, three distinct cell layers form.
The ectoderm
The mesoderm
The endoderm

9. Development of Organs and Organ Systems
ECTODERM
MESODERM
ENDODERM
Nervous system
Bones and muscles
Lining of digestive tract
Epidermis of skin, sweat glands, hair and nails
Blood and blood vessels
Lining of trachea, bronchi and lungs
Reproductive and excretory systems
Liver, pancreas