Every human being begins life as a single cell, formed when father's sperm fertilises mother's egg. Fertilisation normally takes place in the mother's Fallopian tube, which connects the uterus (womb) with the ovary. The uterus is the size and shape of a large pear: it is made of muscle and it stretches to allow the baby's growth throughout the months of pregnancy.
A woman ordinarily has two tubes and two ovaries, one at each side of her uterus. Every month one of the ovaries in turn releases an egg (ovum) which passes slowly along the tube towards the womb cavity
If the egg is not fertilised within 12 hours or so of being released, it dies; it cannot develop further. But if the woman has ...view middle of the document...
Human body cells normally contain 46 distinctively human chromosomes. But an egg and a sperm cell contain only 23 chromosomes each, to allow for their adding together at fertilisation: sperm and ovum are termed gametes (from a Greek word for "marriage partners"). When they "marry" they make one completely new cell - the human embryo, zygote or conceptus - with 46 chromosomes carrying a fresh, unique combination of genes. At fertilisation this human embryo is about 0.1mm in diameter. Since characteristics come from both parents the zygote is never the same as, or part of, the mother, but is a genetically distinct individual. The colouring of hair, skin and eyes, the sex of the new human being, and factors influencing height and build, are determined at fertilisation by information on the DNA.
A baby's sex is determined at fertilisation. A chromosome from the father's sperm determines whether the child is male or female. If an X chromosome is present the baby is a girl; if a Y chromosome is carried by the sperm instead, the baby is a boy.
Occasionally two eggs are released by the ovary and fertilised. This results in fraternal twins who are different in appearance and may be of different sexes because their genes form from two eggs and two sperm cells.
Rarely, one embryo splits into two and both cells develop separately, as identical twins, similar in appearance.
"They have the same genetic make-up and apparently the whole genetic message is the same in both of them. Nevertheless, they are obviously different human beings."2
Blueprint, builder and house
The embryo is not simply a set of instructions for making a new human being, like a blueprint for building a house. A blueprint is inert and cannot carry out instructions, but the embryo is active and begins work at once. A house needs builders, carpenters, electricians and plumbers to complete it; but the embryo has the ability to grow spontaneously, moving on to other phases of development and constructing the skeleton, flesh, nerve connections and a waste disposal system of the human body. After a house is built, a blueprint remains separate; but the embryo - already an essential human by virtue of the genes - is blueprint, builder and "house" together.
After fertilisation the single cell splits into two, then the two cells double to four, four to eight, eight to sixteen and so on. Because the cell cluster looks superficially like a berry it is called the morula (Latin for "mulberry"), but the new life is always biologically human (species Homo sapiens).
The journey along the Fallopian tube continues slowly for about four days. Growth increases. By the time the womb cavity is reached, the cell cluster becomes hollow and fluid-filled, and is referred to as the blastocyst. However, this is not an inert clump of cells but a busily developing human individual: differentiation (organisation into different parts and functions) is already taking place. Meanwhile...