Article: Mona McAlpine from Your Pregnancy magazine

The fish embryo developing outside its mother carries its nourishment in the yolk sac under its belly; the human embryo has a yolk sac but uses it as a blood cell factory. Here we see the embryo at four weeks with a body, a trunk and a tail, and it seems to have its heart in its mouth. On the side of the body the first buddings of an arm and a leg are visible.

This results in a groove leading from front to rear. The groove closes to form a tube (the neural tube) ? the folds meet and fuse, starting from the 'waist' and continuing out towards the ends. The top of the tube then swells to form the brain, and nerve fibres begin to grow out from the brain and from the primitive spinal cord.

Blood is pumped out to the placental root threads through the two umbilical arteries. In the placenta, the blood receives oxygen and nourishment from the fresh arterial blood of her mother and then starts its journey back to the foetus.

The first signs of hair appear in the beginning of the third month. These are more like whiskers which appear on the upper lip, the eyebrows and curiously enough, near her palms and soles. By about four months, the hair is replaced by woolly lanugo which grows all over her body like down. The lanugo follows the whorled pattern of the skin.

Nobody is as yet quite sure what purpose the lanugo plays because almost all of it is shed before birth. Babies born prematurely, before 30 weeks, can be covered in lanugo. It is thought that lanugo may help to regulate body temperature, or it may be there to hold the protective vernix caseosa in place.

Large amounts of vernix are found in hairy areas such as the eyebrows, scalp and upper lip. At the time of delivery the amniotic fluid is quite muddy with loosened vernix, and many newborn babies are slippery at birth because of vernix. Vernix stops the baby from getting waterlogged in the womb. It is also good protection against skin infections.

At birth the cord is usually about the same length as the newborn, and is long enough to allow the newborn to suckle at her mother's breast, whilst still attached to the placenta.

• The fish embryo and human embryo, both vertebrates, develop the same primitive features, for example the six arches between the mouth and the heart. In the picture above we see the 'double chins' on the human embryo which are known as the brachial arches. For fish, gills develop from the arches whereas we develop a lower jaw, a tongue bone (hyoid bone) and a larynx.
• The chicken needs only two days for its heart to start beating, whereas a human heart needs about four weeks before it starts to tick.