Whether or not he was the first to study the embryology of the chicken, his description was surely better than any that had come before:
In hens, it is three days before the first visible sign [of life]: it takes longer in bigger and shorter in smaller birds. It is during this period that movement of the yolk actually occurs, upwards to where the egg’s origin is, the sharp end where the egg hatches. The heart is in the white, the size of a spot of blood. This speck beats and moves as if it were alive. From it, as it continues to develop towards each end of the covering which envelops it, lead two interleaved blood-vessel tubes containing blood. At this stage a membrane with bloody fibrous material actually envelops the white, leading off from the blood-vessel tubes. A little later the body too can actually be distinguished, extremely small to start with and white. The head is visible and in it the eyes, extremely prominent . . .*
Aristotle studied the development of the chick because he could. Fish embryos are tiny; mammal embryos lie concealed in the womb; but to see the chick all you have to do is crack open an egg. He also describes the development of many other creatures, albeit in much less detail. A fish embryo, as far as he can make out, is very much like a bird’s except that it has only one kind of yolk and no allantois. Even the embryos of live-bearers (mammals, the smooth dogfish) are quite similar to those of egg-layers (birds, most fishes and reptiles): both are protected from the outside world (by the eggshell or the uterus), both are surrounded by an amniotic sac (the khōrion) and both get nutrition via umbilical cords from either the yolk or the mother’s blood. Aristotle knows that cows, sheep and goats have uteri studded with kotylēdones (cotyledons or caruncles) but that most other animals do not.* Still, sometimes, when he’s in generalizing mood, it’s hard to know whether Aristotle is talking about a chicken or a man.
CHICKEN EMBRYO
When doing so he isn’t being careless. Rather he’s saying something quite important:
It is not the case that a human being, horse, or any other particular sort of animal is formed at the moment when an animal is formed. The final stage of each animal’s development is its goal and what is distinctive about it . . .
Which is to say that when an embryo initially forms, you can see only the general features that make it an animal – the fact that it’s got a heart and the basic outlines of its organs. The specific features – the features that make a man a man rather than a horse – appear last in development.
That’s a beautiful observation. It was made again, in far greater detail, by Karl von Baer who, in his great Über Entwicklungsgeschichte der Thiere, 1828, called it his ‘First Law’ of comparative embryology. It would become one of the great generalizations of evolutionary developmental biology.*
VON BAER’S FIRST LAW OF EMBRYOLOGY IN VERTEBRATES, ILLUSTRATED LEFT TO RIGHT: DOGFISH, SALMON, AXOLOTL, SNAKE, CHICKEN, CAT, HUMAN EMBRYOS TOP TO BOTTOM: EARLY, INTERMEDIATE, LATE STAGES
Aristotle’s anatomy, although certainly the product of his own dissections, also has much in it that he learnt from fishmongers and butchers, hunters and travellers, physicians and soothsayers. His embryology, however, is obviously about things that he saw for himself. Who but a biologist, eager to uncover the secrets of generation, would spend so much time peering at tiny embryos? If, with a nod to perhaps-Polybus, we acknowledge that Aristotle was not the first to study the chicken embryo, he was certainly the first to see in it the solution to the problem of development.
LXVII
WHEN THE MENSES contact semen they congeal into an embryo or an egg. Aristotle uses homespun analogies to explain how this works: ‘The case resembles that of fig-juice which curdles milk, for this too changes without becoming any part of the curdling masses.’ Or, elsewhere, ‘this acts in the same way as rennet acts upon milk’. This is all about making cheese. When rennet, a substance derived from the stomachs of unweaned calves, is mixed with milk it causes them to separate into solid and liquid parts: curds and whey. Aristotle supposes that seminal pneuma does the same thing to menstrual fluid, coagulating the earthy stuff out of it, leaving a fluid behind. He probably thought the analogy particularly apt. The active ingredients (semen, rennet, fig-juice) all draw their power from being charged with vital heat; their substrates (menstrual fluid, milk) are both very closely related derivatives of blood.*
The result of all this cheese-making is an embryo enveloped by a membrane floating in a fluid. And now the real business of pneuma begins: the manufacture of the embryo’s parts. Aristotle claims, and repeats the claim with the vigour of a man who thinks he’s made a major discovery, that the heart is the first organ to appear in the embryo. It’s a reasonable one if we allow that he means the first visible functioning organ and so exclude the somites and notochord that form well before the heart does. It isn’t just a fact for Aristotle: it fits with theory too. The heart must be the first organ to develop because nutrition and so the growth of all the other organs depends on it.
Nutrition supplied by the mother, and concocted by her heat, flows into the embryo through the vitelline vessels and is redistributed by the heart and its system of ramifying vessels. He compares the vessels to the roots of a seedling or to irrigation runnels in a field, and the way nutrition seeps through the walls of the vessels to how water seeps through unbaked pottery. In the final stage the judicious application of heat transforms the nutrition into the flesh, sinews, bones and all the other tissues of which a growing embryo is built.
Aristotle thinks that tissue and organs are made out of raw, unformed material, but first he demolishes the obvious rival idea, that the parts of an embryo – perhaps even the entire embryo – already exist in the parents’ seed, but are too small to be seen. His opponents were the Neo-Ionian physiologoi who denied that matter of any kind – even tissues – could be either created or destroyed. A late commentator relates Anaxagoras’ theory: ‘For in the same seed he [Anaxagoras] says that there is hair and nails and veins and arteries and muscles and bones, and they are invisible because of the smallness of their parts but as they grow they are gradually separated out. For how, he says, might hair come to be from what is not hair and flesh from what is not flesh?’ Aristotle, however, picks on Empedocles who, he says, believed that organisms self-assemble from pre-formed organs. (He says – Aristotle often seems to be an unreliable reporter of the Sicilian’s ideas.) In any event, he adduces many arguments against the theory, but doesn’t hesitate to make some rather banal points as well: ‘Also, if the animal’s parts are dispersed within the semen how are they to live? If they coalesced they would form a small animal’ – and that, his reader is left to conclude, is obviously absurd.
Aristotle gives his own vision with the aid of two beautiful metaphors. In one, nature paints the embryo:
The parts are all sketched in outline first and only later acquire colour, softness and hardness. It’s exactly as if nature were an artist working on a painting. Painters sketch out animals as well – before they apply a bit of colour.
In the other, the embryo is woven like a net:
How, then, are the other parts produced by the semen? They . . . either come into being all together or in succession as Orpheus’ poem suggests . . . like the knitting of a net. The former is not the case: some parts are clearly visible already existing in the embryo while others are not . . . the lung is bigger than the heart, and yet appears later than the heart in the original development.
It is the second passage that tells us the real reason that Aristotle doesn’t like the idea of pre-formed organs. Any such theory must invoke tiny chickens or bits of chicken in the semen and Aristotle simply does not believe in the existence of things too small to see. This jaundiced view of the invisible world springs directly from his most fundamental theory of matter. Semen is homogeneous: it is composed of neither molecules nor microscopic fowl.
Having established the pattern, Aristotle feels compelled to explain it. How do the parts come to be one after the other? He considers the possibility that organ
s give rise to each other – that the liver actually grows out of the heart – but rejects this on the grounds that each organ has its own form, and the form of one organ cannot exist in another; all organs are made from more basic matter. His own solution depends on a much more subtle kind of causal chain. Semen, he says, initiates movement in the embryo; and, once it does so, this is what happens:
Imagine A moving B and B moving Γ as in those amazing automatic puppets. Even when their parts are inactive, they retain some sort of potentiality. This means that when some outside force sets one part in motion the next part is immediately activated in actuality. So, in a sense, A does move Γ in the automatic puppets, not because there is any current contact with any part but rather because of its previous contact. The same is true of the semen’s origin. The producer of the semen sets things in motion through a past connection as opposed to any current one.
These are the automata of The Movement of Animals. Using puppets to explain how an animal moves is obvious enough; using them to explain how an embryo develops, isn’t. By ‘A’ and ‘B’ and ‘Γ’ Aristotle certainly means the embryo’s developing organs. The semen’s movements shape the heart, which then shapes other organs which shape yet others, until the picture is painted, the net woven and the embryo complete.
Throughout much of his account, Aristotle seems to be telling us that the making of an embryo is like making a statue: the father is the artist who sculpts and the semen is his hand; the mother is the oven in which her menstrual clay is fired. It now becomes clear that this simile does not capture what he means. He’s already conceded that the menses are in a way alive, that they contain the potential for a nutritive soul. Automaton-causality gives new meaning to the word ‘potential’ for it tells us that the menses have a hidden structure and a latent formative power; that it’s more like a wound-up clock with a catch that semen merely springs.
Automaton-causality also accounts for the diversity of forms. Embryos start out the same but, as they develop, the causal chains that shape them diverge. He’s talking about a creature called the kordylos. It’s an amphibian: it has gills and swims using a tail that resembles a catfish’s but also has legs instead of fins and can live on land.* It is, by nature, intermediate between a land and a water animal. He says that it is so ‘warped’ because of some event that occurs very early in its ontogeny. He goes on to explain that the environment in which the animal grows up – land or water – influences some ‘infinitesimally minute but absolutely essential organ’ that, in turn, dictates whether the animal will have the features of a terrestrial or aquatic animal. Much about the kordylos is vague, but the general argument isn’t: early in ontogeny some small organ is responsible for the many features in which aquatic and terrestrial animals differ from each other: A moves B moves Γ.
LXVIII
WHEN THE ANATOMISTS of the Renaissance took to looking inside eggs again, they used The Generation of Animals as their guide. They had, of course, nothing else. Aldrovandi (Ornithologia, 1600), his student Volcher Coiter the Frisian (Externarum et internarum principalium humani corporis partium tabulae et exercitationes, 1573) and Hieronymus Fabricius ab Aquapendente (de Formatione ovi et pulli, 1604) scarcely improved on Aristotle’s descriptions of the development of the chick – though they made some fine figures.
William Harvey, who revered Aristotle, approached him with a more critical eye. In his Exercitationes de generatione animalium (1651) Harvey correctly identified the cicatricula (blastoderm), rather than the punctum saliens (embryonic heart), as the first manifestation of the embryo. He called it ‘The Fountain of All Life’, but he also saw, contra Aristotle, that blood forms before the heart. It was Harvey, too, who searched for the coagulum of sperm and menses that Aristotle’s theory of fertilization predicted. He dissected newly inseminated does, victims of Charles I’s hunting parties in the Royal parks, failed to find the Aristotelian fluids, took the other unifying option and declared (on the frontispiece of his book) Ex ovo omnia – ‘from the egg everything’.* Acute critic though he was, much of Harvey’s embryology remained utterly Aristotelian. ‘There is no part of the future foetus’, Harvey declared, ‘actually in the egg, yet all the parts of it are in it potentially . . .’ Note the contrast between ‘actually’ and ‘potentially’ – Aristotle couldn’t have phrased it better himself.
Harvey called this process of actualization ‘epigenesis’.* It’s here that Aristotle’s argument with the Neo-Ionians gets a replay. Many of Harvey’s successors, enchanted by the structures revealed by their microscopes, argued that the Aristotelian model was simply wrong. The embryo, they claimed, contained, from its very beginnings, all its parts complete. Some said that they could see miniature embryos in spermatozoa; others saw them in eggs. Historians call this doctrine, in all its varieties, ‘preformationism’. Charles Bonnet, a Swiss naturalist, and not a man to shy from logical consequences, proposed that each seed contained within it a fully pre-formed embryo, whose seeds contained within them fully pre-formed embryos, whose seeds . . . and so on to Creation’s very start.
The argument between epigenesis and preformationism ran for about two hundred years. For a while the preformationists seemed to have modernity and mechanism on their side. Better microscopes running on optics by Zeiss showed that they didn’t. Preformationism was an illusion; embryos really do build themselves.
You can see them do it. All you need is a really good microscope with some fancy filters and a healthy culture of nematode worms. You take a single fertilized egg, mount it on a little pad of agar with a drop of buffer to stop it getting crushed and keep it moist, protect the whole thing with a coverslip and then flip to 1000× magnification. Then you watch. Nothing much happens at first, but then the cytoplasm begins to swirl and deform and, quite suddenly, there are two cells where previously there was only one. They divide again and then again and then again – the whole thing happens with remarkable speed and unvarying precision. Cells begin to shuffle about, some duck beneath others; cavities form and bulges extrude; organs – a pharynx, a gut – begin to appear in ghostly outline and then become increasingly defined. The mass of cells contracts, first into something resembling a bean, then a comma, then a pretzel – that is to say, a little worm. Around seven hours after you began to watch, it starts to twitch; by ten it’s rolling around in its egg.
There’s a lot about Aristotle’s developmental biology that seems quite strange. In our biology the parental materials are gametes not fluids; they do not merely come into vague proximity with each other but fuse; the carrier of inherited information is not a pattern of ‘movements’ but a peculiarly stable macromolecule. And the form of the incipient animal comes not just from the father, of course, but from both parents. Still, you have to admire the sheer audacity of his system. It’s all there – a mechanistic account of the most mysterious process in all of biology – how apparently raw matter comes to be a living thing, complete with all its parts. And, if you contemplate the invisible gradients of molecular signals, the cascades of transcription factors and the networks of signal transduction proteins driving the cells to their destinations and differentiated forms, it seems that Aristotle’s automaton logic – A moves B moves Γ – echoing down in ramifying causal chains, captures something very fundamental about how it all works. It is ‘exactly as if nature were a painter producing a work of art’. If there’s a lovelier and truer metaphor for the act of self-creation that made you and me and Aristotle and every other living thing, then I do not know it.
THE VALLEY
OF SHEEP
PROBATON – SYRIAN FAT-TAILED SHEEP – OVO ARIES
LXIX
THE POTAMIá, WHICH means simply ‘river’, drops from the Ordimnos massif to the alluvial plain on the Lagoon’s northwestern shore. One spring day I walked from Anemótia, following its course down. I saw no one else. The hills are barely populated, though they are used, for periodically the path was barred by small dogs that emerged from boxes or barrels to bark at me, s
training at the end of ropes. I wondered what they were doing there in this solitude, for there was nothing for them to defend, but later I learnt that their function is to regulate the movements of the sheep that wander through the valley’s olive groves. Indeed, turning a corner, I came across a flock that was wandering about apparently untended. The sheep of Lesbos are lean and intelligent. In the olive groves they graze on boughs that farmers have cut down for them, but in the island’s arid interior they live on the aromatic plants of the phrygana that grow in the thin volcanic soil. They carry bronze bells around their necks and in the stillness of the hills you can often hear their soft chimes long before they appear.
Aristotle, who has much to say about sheep husbandry, describes how it is one sheep in particular, a castrated ram, who carries the bell and who has been trained to lead the flock and respond to his name. On Lesbos nearly all the sheep have bells, of varying size and timbre, so that as you approach them and they skitter nervously away, a carillon is set in motion that ripples through the flock. One sheep, obviously the leader, planted itself boldly in my path and stared at me with unblinking yellow eyes, and though I was curious to know whether it still had its testicles, I should have had to look under its shaggy coat, and its stance made me doubt that my interest was shared. In Corinth I met a highland shepherd, suitably weathered and taciturn, who confirmed what Aristotle says. At the age of three months, a male lamb, large, disciplined and beautiful, is selected as a future leader. At six months it is castrated, named and apprenticed to a mature ram to learn the command of a platoon of twenty-five sheep. The shepherd added the curious fact that a mature ewe will sometimes usurp control by instinct or force of personality and that, once she does so, she never again bears lambs. He also said that once his commander ram saved him from some mortal danger, but he would not say what it was.
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