Harvey: On the Generation of Animals

How does a chick grow inside an egg? William Harvey, the doctor who discovered that blood goes round and round the body, wanted to know. So he did something simple and patient: he took eggs from a sitting hen, one each day, and looked inside very carefully, day after day, writing down what he saw.

At first there is only a tiny white spot on the yolk. By the fourth day Harvey saw a tiny red dot that jumped, again and again, like the smallest spark. It was the chick's heart, already beating! Then came blood vessels spreading over the yolk like red threads, then eyes, then a beak, then legs, wings and feathers. On about the twenty-first day, the chick pecks its way out.

Harvey's big idea was that the chick is built up step by step, not already there as a tiny finished bird that just gets bigger. And he thought that every animal begins from some kind of egg.

Who, when and why

William Harvey (1578–1657) published Anatomical Exercises on the Generation of Animals in 1651, when he was over seventy, more than twenty years after his book on the heart and blood (the rung before this one). He had worked on it for decades. Many of his notes were lost in 1642, when his rooms in London were ransacked during the English Civil War. The book's engraved frontispiece shows Jupiter (the Greek Zeus) opening an egg from which animals come out, with the words ex ovo omnia, "all things from an egg".

The problem Harvey faced

How does a new living thing form? Aristotle had written that some animals come from eggs, some are born alive, and some small creatures arise by themselves from mud or rotting matter. Harvey's teacher Fabricius had studied the chick in the egg but, Harvey thought, misread where it begins. Harvey wanted to watch the process itself, as it happened, and to compare egg-laying animals with those born alive.

Idea 1: watch it day by day

Harvey's method was to open hens' eggs at known stages of incubation and record each change. He found that development begins at the small white spot on the yolk (the cicatricula, now called the germinal disc), and not, as Fabricius thought, in the twisted white cords that hold the yolk in place. The first thing he saw move was the heart.

Harvey called it the punctum saliens, the "leaping point". He noted that with sharp eyes, bright sunlight or a magnifying glass, it could be seen as early as the end of the third day.

Idea 2: built step by step

Some thinkers believed a living thing already exists, complete but tiny, from the very start, and only grows (later called preformation). Harvey saw something else: the parts appear one after another, first the blood and the heart, then the body around them. He argued for building up part by part, which later writers named epigenesis. The debate went on for over a century; careful microscope work in the 1700s and 1800s settled it Harvey's way.

Idea 3: every animal from an egg

Harvey also studied deer in King Charles I's parks, to see how young deer first form. He could not find a mammal's egg; it is far too small to see without a good microscope. But he guessed that every animal starts from some first beginning like an egg.

Getting the history right

• "Ex ovo omnia" is on the frontispiece, not quite a sentence of Harvey's. The famous short form omne vivum ex ovo ("every living thing from an egg") was coined by later writers summing up his view.
• Harvey still allowed spontaneous generation. He wrote that some small creatures might arise from other matter, as Aristotle taught. Francesco Redi's experiments with covered meat (1668) showed that maggots come from flies' eggs, and Louis Pasteur (1859–1862) showed that even microbes come only from other microbes.
• The mammal egg was found much later. Karl Ernst von Baer first saw the egg of a mammal in 1827, confirming Harvey's guess almost two hundred years on.
• He was an observer, not a lucky guesser. Many of his descriptions of the developing chick are accurate, though he lacked a microscope. The modern timing is earlier than he saw: the chick's heart starts beating at about two days.

What later science kept, and what it changed

Kept: development as a sequence of stages that can be watched and described (the science of embryology), the idea of building up part by part, and the egg as the common start of animal life. Changed: we now know that the egg is a single cell, that a new animal gets its inheritance from both parents through genes, and that development is guided by those genes. Harvey's "plastic force", a shaping power in the egg, was replaced by cells, genes and chemistry.

The patience of looking. Harvey's book shows how much can be learned by a careful, honest observer with simple tools, day after day. Compare the children's activity above: what makes daily observation different from a single look?

Two big ideas, one man. In his two great books Harvey asked how life works (the circulation) and how life begins (generation). He used the same habits for both: watch living things closely, count when you can, and test your teachers' claims.

How later books answered. Charles Darwin, on this ladder, used embryos as evidence in On the Origin of Species (1859): the early embryos of very different animals look remarkably alike. Later biology added genes and cells to the story.

Big questions. Does a living thing grow from a plan, or build itself step by step? Can both be true at once?