Ectoderm, mesoderm and endoderm

This is tomorrow's entry on my Baby Centre Pregnancy app (week 5 baby!!) and I think its really fascinating:

Deep in your uterus your embryo is growing at a furious pace. At this point, your baby-to-be is about the size of a sesame seed and looks more like a tiny tadpole than a human. He's now made up of three layers:

the ectoderm
the mesoderm
the endoderm

These layers will later form all of your baby's organs and tissues.

In the top layer, the ectoderm, the neural tube is starting to develop. Eventually your baby's brain, spinal cord, nerves and backbone will sprout from here. This ectoderm will also go on to form your baby's:

skin
hair
nails
mammary and sweat glands
tooth enamel

Your baby's heart and circulatory system begin to form in the middle layer, or mesoderm. These are developing very quickly. This week, his tiny heart begins to divide into chambers and will start to beat and pump blood.

The mesoderm will also form your baby's:

muscles
cartilage
bone
subcutaneous (under skin) tissue

The third layer, or endoderm, will form your baby's:

lungs
intestines
urinary system
thyroid
liver
pancreas

In the meantime, early versions of the placenta and umbilical cord are already at work.