Early Embryonic Development


After fertilization, cleavage (cell ___________) occurs as the cells divide but the embryo does not become ___________; the cells become smaller with each division.

  • Identical twins form if the zygote splits into two _separate_ cells. Identical twins come from the same egg and sperm and always are the same ________.
  • Fraternal twins form if two different eggs were fertilized from two __________ sperms at the same time so they can be _________ sexes.


The resulting mass of cells is called a morula- a _________ ball of cells.


As cell division continues, the mass of cells __________ out called a blastula.




Next, cells migrate inward producing a hollow embryo with an ____________ to the outside called a gastrula, much like a partially deflated ___________ ball.

Below: Various stages of embryonic development in a sea star (Echinodermata)




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.


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:

Fraternal Twins
Identical Twins

Essential Questions:

    1. What are the stages of embryonic development immediately after the egg is fertilized?
    2. What do the first three layers of our cells differentiate into?
    3. 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.



  • 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.
    • Major organs are formed.
  • 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

  • First _______ felt!

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:

Sort out the following into the correct type of birth defect

Down’s Syndrome
Spina Bifida
Cleft Palate
Fetal Alcohol Syndrome
Cerebral Palsy



Vocabulary Words:

Yolk Sac
Umbilical Cord

Essential Questions:

  1. Are the differences between human and chicken embryo development?
  2. What are the functions of the different parts of the chicken egg?
  3. What features of the chicken egg allow birds to successfully mature outside their mother’s body?
  4. At what time period is a human embryo considered a fetus?
  5. Does the blood of the mother and baby intermix?
  6. What is the role of the yolk sac in human embryos?
  7. Where does the umbilical cord run to and from?
  8. How do fingers form?
  9. When is the embryo planted in the uterus?
  10. How is the uterine lining maintained?
  11. What is the gestation period of a human?
  12. Know the three types of birth defects and examples of each.