In the third week of development the blastocyst has now burrowed into the wall of the uterus and cells on the outside are beginning to form early parts of the placenta.
The clump of cells inside the blastocyst has also begun to change and now looks like 2 discs of cells. It looks like 2 dinner plates stacked on top of each other. The picture shows looks at this stack from the top. Cell from the top plate move down to form a new layer. The embryo now looks like stacked 3 dinner plates.
We use stages as a way of measuring changes in the embryo. Stages are like different years at school: each year something new will occur, each stage something new will happen. Stages are also a way of comparing changes that occur in different animals. (More?) (If all animals have the same stages why do they end up looking different?)
click the small picture to see a big picture.
The Embryo is a disc with a streak of cells on the surface. The connecting stalk that will be part of the placenta is on the left. There are no other visible features.
The Embryo is now several different layers. The connecting stalk that will be part of the placenta is on the left.
The Embryo layers have now folded to make a visible head-tail end and left-right sides.
The Embryo has now begun to form small balls of cells along the middle that will make all the muscle and bone. The dark line in the middle is the beginning of the nervous system.
In the picture the head is at the right, tail is at the left.
We use stages as a way of measuring changes in the embryo. Stages are like different years at school: each year something new will occur, each stage something new will happen. Just like School, the embryo is growing and changing with each stage.
School Years: K, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12 = 13 stages of School.
Embryo Stages: 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20, 21, 22, 23 = 23 Carnegie stages.
Stages are also a way of comparing changes that occur in different animals. The time that these changes occur will be different for different animals, using stages means that we can compare the development of different animals. The stages used here were originally identified and named last century at a research institue that gives the stage names "Carnegie Stages". Embryologists use many different ways of staging animal development including days of development.
Most of the early important stages of embryo growth occur before the mother knows she is pregnant.
Movie of Human Embryo Growth (this shows a human embryo growing, all images are to scale)
Movie of Limb Growth(this shows development of the human arm from bud to paddle, to hand with arm)
Movie of Mouse Embryo Growth (this shows a mouse embryo growing)
Movie of Rotating Embryo(this shows an early embryo from all angles)
All Moviesthis shows many different ways of looking in at changes in different embryos.
Week 1 | Week 2 | Week 3 | Week 4 | Week 5 to 8 | Week 9 to 12 | Week 13 to 15 | Week 16 to 20 |
Home Page
Embryo stages
Fetus
Dev Notes 1
Dev Notes 2
System Notes
Acknowledgements
K12 Home
Week 1
Week 2
Week 3
Week 4
Week 5 to 8
Arm and Leg Development
Week 9 to 12
Week 13 to 15
Week 16 to 20
Week 21 to 25
Week 26 to 29
Week 30 to 34
Week 35 to 38
Birth
Movies
Class Notes
Serial Images
K12 Students
Abnormal
Embryology in the News
Histology
Site Map
School of Med Sci
UNSW Medicine
University of NSW
UNSW Cell Biology
NCBI Books
Dev Biol (Gilbert)
Teachers Note- These notes are still not yet complete.
It is also difficult to make notes suitable to cover the entire K -12 age group.
For younger students I suggest showing the movies of embryo growth and other movies. Also the Other Embryos pages also has many pictures of different embryos.
Please email Dr Mark Hill if you wish to make a comment about this current project.