Chapter I
Eggs
The oldest detailed study of an embryo to survive is Aristotle's. He opened hens' eggs on successive days and saw the heart appear as a beating spot of blood before any other organ. He concluded that the parts of an embryo form one after another, in order. He called the process generation. It is now called epigenesis.
Two thousand years later William Harvey, physician to Charles I, took up the question. He dissected the king's deer at intervals after the rut, expecting to find an egg in the womb, and did not. He found only a formless fluid in which an embryo later appeared. He still concluded, from the chick and from reasoning, that every animal begins as an egg. His book of 1651 carried the motto "ex ovo omnia", everything from an egg.
Chapter II
Preformation
The microscope made the question harder, not easier. In 1672 Marcello Malpighi drew chick embryos in eggs that had hardly been warmed and saw structures already in place. Others looking at sperm imagined tiny people curled inside. Preformation, the idea that the body exists complete in miniature and only grows, became the orthodox view. It had a philosophical appeal: if God had made every body at the Creation, nested one inside the next, then matter did not need the power to organise itself.
Caspar Friedrich Wolff disagreed. In 1759 he argued that blood vessels form where there were none, and in 1768–69 he showed that the chick's gut begins as a flat sheet that folds into a tube. The body is built, not unpacked. The great physiologist Albrecht von Haller rejected Wolff's argument, and it took another two generations of better microscopes before epigenesis won.
Chapter III
Layers and Laws
In 1817 Christian Pander, working in Würzburg, saw that the early chick embryo is made of distinct layers. His friend Karl Ernst von Baer traced the layers through many vertebrates and found that they give rise to the same organs in each. In 1827 he found the mammalian egg itself, a speck in the ovary of a dog, finishing what Harvey had begun. The next year he stated his laws of development. General features, such as a backbone, appear before special ones, such as feathers. The embryo of one animal never resembles the adult of another, only its embryo.
Von Baer did not accept evolution. But once Darwin published in 1859, the resemblance of embryos became some of the best evidence for common descent. Ernst Haeckel went further. His biogenetic law of 1866 held that each embryo replays the history of its ancestors, so that a human embryo's throat grooves are the gills of a fish ancestor. His plates of embryos in rows, fish to human, were reprinted for a century. They were also accused, from 1868 onwards, of exaggerating the likeness.
Chapter IV
A Closer Look: From One Cell to Thirty Trillion
An adult human is made of roughly 30 trillion cells, about , according to a careful 2016 estimate, and about five in six of them are red blood cells. All come from one fertilised egg. If every cell divided in step, how many rounds of doubling would it take?
About 45 doublings. That is surprisingly few. The number of divisions is much larger, because every doubling of cells takes divisions, so building cells needs about divisions in all. Real development is not in step either. Some lineages stop dividing early, others keep going for life, and many cells die on purpose along the way.
The first divisions are special. A frog egg is a huge cell, about 1.2 mm across, stocked with yolk. After fertilisation it divides without growing, a process called cleavage, so each division halves the size of the cells:
| Divisions | Cells | Average volume of each cell | Average diameter of each cell |
|---|---|---|---|
| 0 | 1 | 1 | 1.2 mm |
| 3 | 8 | 1/8 | 0.6 mm |
| 6 | 64 | 1/64 | 0.3 mm |
| 9 | 512 | 1/512 | 0.15 mm |
| 12 | 4,096 | 1/4,096 | 0.075 mm |
Because volume goes as the cube of diameter, every three divisions halve the diameter, and after twelve the cells are times narrower, about 75 micrometres, close to the size of ordinary cells. In the frog Xenopus the first division comes about 90 minutes after fertilisation and later ones about every 30 minutes, so the twelfth division happens around minutes, about seven hours in. Only then does the embryo's own genome switch on in earnest. Until that point it has been running on instructions and materials packed into the egg by the mother.
Human embryos are slower. Their first divisions come roughly once a day, so an embryo is only about eight cells three days after fertilisation. The principle is the same: a large cell is cut into many ordinary ones before growth begins.
Chapter V
Beyond Description
By 1880 embryologists could describe development in detail for dozens of species and arrange embryos in evolutionary order. They could not say what caused any of it. Why does a cell become part of the gut and not the skin? Answering needed a new approach: cutting, separating and transplanting parts of living embryos to see what they do. That became experimental embryology. The cells themselves were by then understood through cell theory, which showed that the egg is a single cell and every tissue is made of its descendants.