James Lefanu describes the almost incomprehensible natural phenomenon culminating in the birth of each one of us.
Seeing the Christmas story through the eyes of small children is to be reminded of the remarkable accessibility of Christianity to even young minds. They may not grasp the implications of God Made Man but this central tenet could not be portrayed more evocatively than by the nativity scene with at its centre the baby in a manger. For St Ambrose the humble circumstances of Christ’s coming into the world was a sign from God intended to “pierce our hearts” and open them to religious belief.
And so too the revelation over the past 150 years of the utterly astonishing process of embryonic transformation that can “pierce our hearts” to the same end. For millennia the details of what happens at the moment of conception were “enveloped in the most profound and hopeless obscurity”. Until in 1875 a German physician, Oskar Hertwig, placed a drop of milky seminal fluid on a sea urchin’s egg. Soon after, peering down his microscope, he noted the darkly staining nuclei of sperm and egg fusing together – demonstrating for the first time that the genes of both father and mother contribute equally to the formation of their offspring. Dr Hertwig’s discovery focused biologists’ attention on the profounder question as to how the now fertilised egg transforms itself in a few short months into a billion-cell living being. “Nothing could compare with the fascination of observing the beauty of those rapid embryonic transformations working themselves out in front of their eyes,” writes science historian William Coleman.
Some idea of those unfolding events can be imagined by placing the hand palm down and supposing it represents the single flat sheet of the embryonic disc just 0.1 mm in diameter – from which the embryo will emerge. First a tremor of movement, the “primitive streak” runs up the back of the hand defining the main axes of the embryo-to-be. Then the cells to right and left begin to divide and quite suddenly the single layer of cells has become three piled on top of each other: the ectoderm that will become the skin and vertebral column, beneath the mesoderm destined to be the skeleton, muscles and heart and beneath that the endoderm that will form the internal organs.
The ectoderm builds up forming two ridges which, leaning towards each other, form a tube that will become the “keel of the ship”, the skull and vertebral column. The fingers of the hands now curl forward to form the head, the incipient heart, lungs and gut nestle in the undersurface of the palm and the limb buds emerge from either side. A month later the first hint of the eyes and ears can be discerned and soon after a beating heart will be impelling blood through what will become the circulatory system. So it goes on, minute by minute, day by day until three months following conception, the foetus – now just an inch long – is almost complete made up of 4,500 different parts.
The extraordinary nature of these events is underpinned by four fundamental processes as described by the late embryologist Lewis Wolpert in his book The Triumph of the Embryo. The most basic is growth through cell multiplication – a 3,000-fold increase that, by analogy, would see an adult human finger become an 800-foot skyscraper. This requires each individual cell together with all its complex protein-making machinery and the 3 billion molecules of DNA in its nucleus to faithfully replicate itself in its entirety two or three times in every 24 hours. Next is specialisation where the initially featureless embryonic cells differentiate into more than 350 distinct types each with its own unique function – red blood cells carrying oxygen, bone cells providing mechanical support and so on. Third comes the acquisition of form where combinations of those now specialised cells conjure from apparently nowhere all those different parts, each perfectly designed to fulfil their ultimate purpose – the eyes to see, the ears to hear, the limbs to move, the hands to grasp.
The fourth of those fundamental processes is migration where, for example, the motor nerves emerge from the spinal cord to travel across what to them are enormous distances wiring the body together – entering the limb buds and branching off at just the right point to connect up with the individual muscles. “They rarely make a mistake”, notes Prof Wolpert.
And behind it all is an overarching sense of purpose – the relentless drive towards the realisation of a preconceived plan. There must presumably be some powerful unifying physical or chemical force that choreographs their association together. But almost 150 years after Dr Hertwig peered down his microscope it remains completely elusive. Rather, embryonic transformation remains what the Jewish philosopher Maimonides classed as a “natural miracle”. It is to be sure a natural phenomenon but lies so far beyond our comprehension it might as well be a miracle.
James Le Fanu is a retired General Practitioner, journalist and author.
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