K12 Week 2

From Embryology

Week 2

In the second week of development the dividing zygote forms a solid ball of cells and is now called a blastocyst.

<html5media height="470" width="500">File:Human_blastocyst_day_3-6.mp4</html5media>

This movie shows human development between day 3 and day 6 after fertilization.

The movie finishes with the blastocyst hatching just before the beginning of week 2 in development.

Week 2 is then about this blastocyst sticking (attaching) and burrowing (implanting) into the uterus wall.

During this second week the blastocyst will stick to the inside of the uterus and will no longer float free. This sticking process is required to allow the blastocyst to begin to get food from the mother.

  • The cells on the surface of the ball will not form part of the embryo and will go on to make the placenta and membranes that surround the embryo.
  • The cells on the inside form a small clump beside a fluid filled space. This small clump of cells (embryoblast) will form all the parts of the embryo.


This movie shows implantation that occurs during week 2 of human development.

The uterus wall is shown by the white cells (left) and the space where the blastocyst was floating is to the right.

By the end of the second week the embryo is completely within the mother's uterus wall.

The green cells will then form part of the placenta.

<html5media height="580" width="500">File:Week2_001.mp4</html5media>

Question: You start as a single cell, how many cells are you now?

K12 Links: Start Here | Week 1 | Week 2 | Week 3 | Week 4 | Week 5 to 8 | Arms and Legs | Heart | Fetus | Brain Growth | Eyes and Ears | Animal Development Times | Humans and Animal Embryology | Comparative Embryology | Thalidomide

Words Used

blastocyst - the name given to all the new cells formed from the first cell (the zygote).

implanting - this means to burrow into the uterus wall.

placenta - the structure formed by both the mother and baby to allow growth. Egg-laying animals like birds, fish, frogs do not have placentas.

Cite this page: Hill, M.A. (2024, May 28) Embryology K12 Week 2. Retrieved from https://embryology.med.unsw.edu.au/embryology/index.php/K12_Week_2

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© Dr Mark Hill 2024, UNSW Embryology ISBN: 978 0 7334 2609 4 - UNSW CRICOS Provider Code No. 00098G