By the end of the second month of life, all the organs necessary to sustain life will have formed. From now on until the age of about 25 years the organs merely develop through an increase in size and a gradual refinement in their functioning. This is the reason that the embryo (from the Greek word meaning to swell) is now called a foetus (from the Latin word for a young child). The foetus is already recognizable as a human being. The size of this miniature baby will not, however, exceed three centimetres.
The genes determine which cells will grow at what stage. At about six weeks, for example, both the ears start to develop. One week later it is already possible to see certain familial characteristics.
The rate of cell growth is also responsible for the ‘movement’ of structures. The eyes, for example, form at the side of the head, but their ultimate position is at the front. Of course the eyes do not really move. This phenomenon is caused by a relatively slow development of the area between the eyes in comparison to the quick development of the rest of the head. The rate of growth of the different parts of the body is not always the same. The growth of the brain for example accelerates at about the thirtieth day so that within two days there has been an increase in size of more than 25 per cent.
One of the first areas to develop that gives a more human appearance to the embryo is the formation of
the jaws. These develop from the first of five structures called pharyngeal arches. The arches are thickenings of tissue that grow round the head of the embryo. At this point there is only a ridge of tissue along the edge of the jaws, called the dental lamina. In the following weeks ten dental buds emerge on both the upper and lower jaws. From these the milk teeth will develop. Meanwhile the nose and the nasal cavity are separated from the mouth by the the formation of the upper lip and the palate. Both nostrils are plugged up at this stage.
By eight weeks most of the basic structures of the eyes, including a primitive pigmented retina, have formed. Because the eyelids are just beginning to develop at the end of the second month, the eyes cannot yet be closed.
The ears form externally by about six weeks, although the inner part of the ear concerned with hearing and balance is still a simple cavity.
The development of the upper limbs starts at the fifth week. In just one day tiny limb buds emerge. One day later they have already grown to such an extent that primitive arms and hands can be distinguished. Some days later the outline of the fingers will become evident. The lower limbs start to grow about a week later. This illustrates an important principle in embryonic development: the head and those parts of the body nearest to the head develop first.
By the eighth week muscles are formed that allow for tiny movements – not vigorous enough, however, to be felt by the mother. The skeleton is derived from cartilaginous tissue, which begins to be formed from the fifth week. The ossification of tissue into bone starts in the eighth week and is generally accepted as marking the beginning of the foetal period.
The heart develops from a simple tube into a muscular, chambered structure. The blood pumped by the heart is oxygenated in the placenta and not in the lungs. This process takes place through an exchange of oxygen across a thin membrane between the maternal and embryonic circulation in the placenta. The lungs therefore receive relatively little blood, and are consequently fairly slow to develop.
The growth of the reproductive and urinary systems are closely related. At first, the reproductive organs develop along identical lines whether the embryo is male or female, and at eight weeks the sexes still cannot be told apart by external appearance. The internal sexual organs, however, can be distinguished from seven weeks onwards.
The ducts that form the urinary system develop alongside the genital organs. A primitive kidney emerges, but will not be fully functional before the eighth month.
The brain begins to take on its ultimate form in the fifth week. The forebrain enlarges and makes up the main part of the brain. It becomes increasingly complex with the development of specialized regions. At the same time, the embryo’s trunk becomes less flexed, so that the head, rather than the back of the neck, forms the upper part of the embryo. Thus during the second month the embryo gains a definite human appearance, with most organs of the body formed and some developed enough to be functional.