Three layers of cells form called germ layers.
- Ectoderm (________ layer) differentiates into the ________ and nervous system.
- Endoderm (inner layer) forms the digestive tract.
- Mesoderm (middle layer) differentiates into the _________, skeleton, circulatory & reproductive organs.
Neurula
Next the embryo develops a _________ cord and brain from a long, thickened fold in the ectoderm.
The embryo at this stage of development is called a neurula.
Video: Embryonic germ layers
Activity: Embryology Picture Book!
Vocabulary Words:
Zygote Cleavage
Blastula
Ectoderm
Endoderm
Gastrulation
Fraternal Twins
Identical Twins
Mesoderm
Morula
Neurula
Essential Questions:
- What are the stages of embryonic development immediately after the egg is fertilized?
- What do the first three layers of our cells differentiate into?
- How are identical and fraternal twins formed?
Internal Vs External EMbryonic Development
- Embryonic development is controlled by genes- the _______ provided by the egg and _______.
- A single cell contains all of the genes necessary to construct the ________ body!
- Cells differientiate (divide into specialized body parts) as it develops. To do this, some genes are turned on and some remain silent.
External Embryonic Development

Illustration of chicken egg
- External embryonic development occurs ________ the mother's body
- Common in birds, reptiles, amphibians and _______
- The ________ protects the egg from drying out.
- The chorion lies just beneath the shell and functions in _____ exchange.
- The allantois collects and stores nitrogenous _________.
- The yolk stores _______. As time goes on, the yolk gets smaller while the _________ gets larger.
- The amnion cushions the embryo and provides a ________ environment.
How is Human Embryonic Development different from Chicken egg development?
Internal Embryonic Development
- The baby develops ________ the mother
- Common in __________
- Placenta- the lining of the uterus feeds the baby.
- It gets food and eliminates _________ though the placenta
- The ___________ of the mother NEVER mixes with the baby
- The umbilical cord connects from the baby's belly button to the __________
- The amniotic fluid is a __________ solution that surrounds the fetus and protects it. It is held in by the amnionic sac.
Humans
- A human pregnancy lasts 9 months (from the woman's last period). The gestation period is 38 weeks from conception.
- The first ___ weeks of development are called embryonic development.
- Fetal development occurs during the remaining months, after 8 weeks of growth
- Refinement of the major organs occurs.
Stages of development- Slide show, Video
Week 1
- Fertilization occurs in the upper 1/3 of the _______________.
- Cleavage begins as the embryo passes through the oviduct to the uterus.
- The morula reaches the uterus in about 3 to 3.5 days.
- By the end of the first week, the morula becomes a ____________, a hollow, fluid-filled structure.
Week 2
- The embryo begins to implant in the ________.
- The corpus luteum secretes _______________ which maintain the uterine lining, preventing _____________.
- The ectoderm, endoderm and ___________ form.
- The yolk sac forms -it does not function to provide food. It creates ________ cells.
- The amnion forms to surround the embryo to protect it from bumps and _____________ change.
Week 3
- The nervous system begins to form.
- The _______ begins to form but does not beat yet.
- The allantois develops as an extension of the gut. It will form the blood vessels of the umbilical cord.
- Limb buds are small paddle-like structures that will eventually produce ______ and ______.
Week 4
28 day old embryo
- Nutrients are exchanged across the membranes of the ___________.
Mom and baby blood don't mix.
- The placenta secretes __________ to prepare the mother for breast feeding.
Week 6
- Brain, heart and ______ first grow.
Week 10
- _________! Arm and legs form. Genitals form.
Week 14
- All organs and muscles form.
- Fetus can _______!
Week 18
Week 26
- Eyes ______.
- Can hear your ________.
Week 34
- May turn ________ down in uterus
Week 38
- _______ breaks. Ready to roll!
Birth Defects
Anything that can cause _______ in a developing fetus is a birth defect. If can be caused by:
- __________ disorder
- ______________
- Negative _________ of the mom.
Sort out the following into the correct type of birth defect
Down’s Syndrome
Spina Bifida
Cleft Palate
Anencephaly
Fetal Alcohol Syndrome
Cerebral Palsy
Vocabulary Words:
Embryo
Fetus
Gestation
Placenta
Amnion
Chorion
Allantois
Yolk Sac Yolk
Placenta
Umbilical Cord
Differentiation
Growth
Essential Questions:
- Are the differences between human and chicken embryo development?
- What are the functions of the different parts of the chicken egg?
- What features of the chicken egg allow birds to successfully mature outside their mother’s body?
- At what time period is a human embryo considered a fetus?
- Does the blood of the mother and baby intermix?
- What is the role of the yolk sac in human embryos?
- Where does the umbilical cord run to and from?
- How do fingers form?
- When is the embryo planted in the uterus?
- How is the uterine lining maintained?
- What is the gestation period of a human?
- Know the three types of birth defects and examples of each.
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