UNSCRAMBLING THE EGG
IT BEGINS WITH ONE PERFECT SOLAR CELL Put a few adults in a room with a sweet-tempered infant, and you may as well leave a tub of butter sitting out in the midday sun. Within moments of crowding around the crib, their grown-up bones begin to soften and their spines to bend. Their eyes mist over with cataracts of pleasure. They misplace intellect and discover new vocal ranges countertenor, soprano, piglet. And when they happen on the baby's hands, prepare for a variant on the ancient Ode to the Fingernail. Nothing so focuses adult adoration as a newborn's fingernail, its lovely condensed precocity. See the tiny cuticle below, the white eyebrow of keratin on top, the curved buff of the nail body, the irresistible businesslike quality of the whole: it looks like it really works! We love the infant fingernail for its capacity to flatter, its miniature yet faithful recreation of our own form. More than in the thigh or the eye or even the springy nautilus shell of the ear, in the baby's nail sits the homunculus, the adult in preview. And so, we are reminded, the future is assured. Myself, I prefer eggs.
At some point midway through my pregnancy, when I knew I was carrying a daughter, I began to think of myself as standing in a room with two facing mirrors, so that looking into one mirror you see the other mirror reflecting it, and you, off into something approaching an infinity of images. At twenty weeks' gestation, my girl held within her nine-ounce, banana-sized body, in a position spatially equivalent to where she floated in me, the tangled grapevines of my genomic future. Halfway through her fetal tenure, she already had all the eggs she would ever have, packed into ovaries no bigger than the letters ova you just passed. My daughter's eggs are silver points of potential energy, the light at the beginning of the tunnel, a near-life experience. Boys don't make sperm their proud "seed" until they reach puberty. But my daughter's sex cells, our seed, are already settled upon prenatally, the chromosomes sorted, the potsherds of her parents' histories packed into their little phospholipid baggies.
The image of the nested Russian dolls is used too often. I see it everywhere, particularly in descriptions of scientific mysteries (you open one mystery, you encounter another). But if there were ever an appropriate time to dust off the simile, it's here, to describe the nested nature of the matriline. Consider, if you will, the ovoid shape of the doll and the compelling unpredictability and fluidity of dynasty. Open the ovoid mother and find the ovoid girl; open the child and the next egg grins up its invitation to crack it. You can never tell a priori how many iterations await you; you hope they continue forever. My daughter, my matryoshka.
I said a moment ago that my daughter had all her eggs in mid-fetushood. In fact she was goosed up way beyond capacity, a fatly subsidized poultry farm. She had all her eggs and many more, and she will lose the great majority of those glittering germ cells before she begins to menstruate. At twenty weeks' gestation, the peak of a female's oogonial load, the fetus holds 6 to 7 million eggs. In the next twenty weeks of wombing, 4 million of those eggs will die, and by puberty all but 400,000 will have taken to the wing, without a squabble, without a peep.
The attrition continues, though at a more sedate pace, throughout a woman's youth and early middle age. At most, 450 of her eggs will be solicited for ovulation, and far fewer than that if she spends a lot of time being pregnant and thus not ovulating. Yet by menopause, few if any eggs remain in the ovaries. The rest have vanished. The body has reclaimed them.
This is a basic principle of living organisms. Life is profligate; life is a spendthrift; life can persist only by living beyond its means. You make things in extravagant abundance, and then you shave back, throw away, kill off the excess. Through extensive cell death the brain is molded, transformed from a teeming pudding of primitive, overpopulous neurons into an organized structure of convolutions and connections, recognizable lobes and nuclei; by the time the human brain has finished developing, in infancy, 90 percent of its original cell number has died, leaving the privileged few to sustain the hard work of dwelling on mortality. This is also how limbs are built. At some point in embryogenesis, the fingers and toes must be relieved of their interdigital webbing, or we would emerge from our amniotic aquarium with flippers and fins. And this too is how the future is laid down.
The millions of eggs that we women begin with are cleanly destroyed through an innate cell program called apoptosis. The eggs do not simply die they commit suicide. Their membranes ruffle up like petticoats whipped by the wind and they break into pieces, thence to be absorbed bit by bit into the hearts of neighboring cells. By graciously if melodramatically getting out of the way, the sacrificial eggs leave their sisters plenty of hatching room. I love the word apoptosis, the onomatopoeia of it: a-POP-tosis. The eggs pop apart like poked soap bubbles, a brief flash of taut, refracted light and then, ka-ping! And while my girl grew toward completion inside me, her fresh little eggs popped by the tens of thousands each day. By the time she is born, I thought, her eggs will be the rarest cells in her body.
Scientists have made much of apoptosis in the past few years. They have sought to link every disease known to granting agencies, whether cancer, Alzheimer's, or AIDS, to a breakdown in the body's ability to control when pieces of itself must die. Just as a pregnant woman sees nothing but a sea of swollen bellies all around her, so scientists see apoptosis gone awry in every ill person or sickly white mouse they examine, and they promise grand paybacks in cures and amelioratives if they ever master apoptosis. For our purposes, let us think not of disease or dysfunction; let us instead praise the dying hordes, and lubricate their departure with tears of gratitude. Yes, it's wasteful, yes, it seems stupid to make so much and then immediately destroy nearly all of it, but would nature get anywhere if she were stingy? Would we expect to see her flagrant diversity, her blowsy sequins and feather boas, if she weren't simply and reliably too much? Think of it this way: without the unchosen, there can be no choosing. Unless we break eggs, there can be no soufflé. The eggs that survive the streamlining process could well be the tastiest ones in the nest.
And so, from an eggy perspective, we may not be such random, sorry creatures after all, such products of contingency or freak odds as many of us glumly decided during our days of adolescent sky-punching (Why me, oh Lord? How did that outrageous accident happen?). The chances of any of us being, rather than not being, may not be so outrageous, considering how much was winnowed out before we ever arrived at the possibility of being. I used to wonder why life works as well as it does, why humans and other animals generally emerge from incubation in such beautiful condition why there aren't more developmental horrors. We all know about the high rate of spontaneous miscarriages during the first trimester of pregnancy, and we have all heard that the majority of those miscarriages are blessed expulsions, eliminating embryos with chromosomes too distorted for being. Yet long before that point, when imperfect egg has met bad sperm, came the vast sweeps of the apoptotic broom, the vigorous judgment of no, no, no. Not you, not you, and most definitely not you. Through cell suicide, we at last get to yes a rare word, but beautiful in its rarity.
We are all yeses. We are worthy enough, we passed inspection, we survived the great fetal oocyte extinctions. In that sense, at least call it a mechanospiritual sense we are meant to be. We are good eggs, every one of us.
If you have never had trouble with your eggs, if you have never had to worry about your fecundity, you probably haven't given your eggs much thought, or dwelled on their dimensions, the particular power that egg cells enclose. You think of eggs, you think food: poached, scrambled, or forbidden. Or maybe you were lucky enough as a child to find in your back yard a nest with two or three robin's eggs inside, each looking so tender and pale that you held your breath before venturing to touch one. I was unhappily familiar in my girlhood with another sort of animal egg, that of a cockroach; usually I found the empty egg case after its cargo had safely departed, a sight as disturbing as that of a spent shotgun shell and more evidence of the insect's supremacy.
The symbolic impact of the egg in many cultures is as an oval. The egg of the world, thick toward the bottom to ground us, thinner at the apex as though pointing toward the heavens. In medieval paintings and cathedral tympana, Christus Regnans sits in a heavenly ovoid: he who gave birth to the world was born unto the world to secure it from death. At Easter we paint eggs to celebrate rebirth, resurrection; in the egg is life, as life is cradled in the cupped, ovoid palms of the hands. The Hindu gods Ganesha and Shiva Nataraja sit or dance in egg-shaped, flame-tipped backdrops. In painting her vulval flowers, the petals opening onto other petals like abstract pastel matryoshkas, Georgia O'Keeffe evoked as well the image of the egg, as though female genitalia recapitulate female procreative powers.
The egg of a chicken or other bird is a triumph in packaging. A female bird makes the bulk of the egg inside her reproductive tract long before mating with a male. She supplies the egg with all the nutrients the chick embryo will require to reach pecking independence. The reason that an egg yolk is so rich in cholesterol, and thus that people see it as gastronomically risqu , is that a growing fetus needs ample cholesterol to build the membranes of the cells of which the body, any body, is constructed. The bird gives the egg protein, sugars, hormones, growth factors. Only after the cupboards are fully stocked will the egg be fertilized by sperm, sealed with a few calciferous layers of eggshell, and finally laid. Bird eggs are usually oval, in part for aerodynamic reasons: the shape makes their odyssey down the cloaca, the bird's equivalent of a birth canal, that much smoother.
We gals have been called chicks, and in Britain we've been birds, but if our eggs are any indication, the comparison is daft. A woman's egg, like that of any other mammal, has nothing avian about it. There is no shell, of course, and there really is no yolk, although the aqueous body of the egg, the cytoplasm, would feel a bit yolky to the touch if it were big enough to stick your finger in. But a human egg has no food with which to feed an embryo. And though one springs to fullness upon ovulation each month, it most certainly is not the pit-faced, frigid moon.
I have another suggestion. Let's reject the notion that men have exclusive rights to the sun. Must Helios, Apollo, Ra, Mithras, and the other golden boys take up every seat in the solar chariot that lights each day and coaxes forth all life? This is a miscarriage of mythology, for a woman's egg resembles nothing so much as the sun at its most electrically alive: the perfect orb, speaking in tongues of fire.
Dr. Maria Bustillo is a short, barrel-bodied woman in her mid-forties who frequently smiles small, private smiles, as though life dependably amuses her. She is a Cuban American. Her features are round but not pudgy, and she wears her dark hair neither short nor long. As an infertility expert, Bustillo is a modern Demeter, a harvester and deft manipulator of human eggs, a magician in a minor key. She helps some couples who are desperate for parenthood get pregnant, and to them she is a goddess. But others she cannot help. For those others, it is no metaphor to say they flush many thousands of dollars down the toilet with each cycle of IVF or GIFT or other prayers by alphabet.* That is the reality of infertility treatment today, as we have read and heard and read again: it is very expensive, and it often fails. Nevertheless, Bustillo smiles her small amused smiles and does not coddle gloom. She manages to seem simultaneously brisk and easygoing. Her staff loves working with her; her patients appreciate her candor and her refusal to condescend. I liked her instantly and almost without qualification. Only once did she say something that reminded me, oh, yes, she is a surgeon, a wisecracking cowgirl in scrubs. As she washed her hands before performing a vaginal procedure, she repeated a smirking remark that she'd heard from one of her instructors years earlier. "He told me, `Washing your hands before doing vaginal surgery is like taking a shower before taking a crap,'" Bustillo said. The vagina is quite dirty, she continued, so there is nothing you could introduce into it with your hands that would be worse than what's already there. (This bit of orificial wisdom, by the way, is an old husbands' tale, a load of crap, as we will discuss in Chapter 4. The vagina is not dirty at all. Really, is it too much for us who mount the gynecologist's unholy stirrups to ask, "Physician, clean thyself"?)
I am visiting Bustillo at the Mount Sinai School of Medicine in New York to look at eggs. I have seen the eggs of many species, but I have never seen the eggs of my own kind, except in pictures. Seeing a human egg is not easy. It is the largest cell in the body, but it is nonetheless very small, a tenth of a millimeter across. If you could poke a hole in a piece of paper with a baby's hair, you'd get something the size of an egg. Moreover, an egg isn't meant to be seen. The human egg, like any mammalian egg, is built for darkness, for spinning stories in visceral privacy and you can thank that trait, in part, for your smart, fat, amply convoluted brain. An internally conceived and gestated fetus is a protected fetus, and a protected fetus is a fetus freed to loll about long enough to bloom a giant brain. So we lend new meaning to the term egghead: from the cloistered egg is born the bulging frontal lobe.
How different is the status of the sperm. A sperm cell may be tinier than an egg, measuring only a small fraction of the volume, so it is not exactly a form of billboard art either. Nevertheless, because it is designed to be externalized, publicly consumed, sperm lends itself to easy technovoyeurism. One of the first things Anton van Leeuwenhoek did after inventing a prototype of the microscope three hundred years ago was to smear a sample of human ejaculate onto a glass slide and slip it under his magic lens. And men, I will set aside my zygotic bias here to say that your sperm are indeed magnificent when magnified, vigorous, slaphappy, whip-tailed tears, darting, whirling, waggling, heading nowhere and everywhere at once, living proof of our primordial flagellar past. For mesmerizing adventures in microscopy, a dribble of semen will far outperform the more scholastically familiar drop of pond scum.
A woman's body may taketh eggs away by apoptosis, but it giveth not without a fight. How then to see an egg? One way is to find an egg donor: a woman who is part saint, part lunatic, part romantic, part mercenary, and all parts about to be put under the anesthesia that Bustillo calls the "milk of amnesia," so she will not feel her body crying bloody hell on the battlefield.
Beth Derochea pats her belly and booms, "Bloated! I'm full of hormones! I tell my husband, Stay away!" She is twenty-eight but looks a good five years younger. She is an administrative assistant at a publishing company who hopes to work her way up to an editing position. Her hair is long, dark, parted on the side, casual, and her smile is slightly gappy and toothy. "I hope nobody inherits my teeth!" she says. "Anything but that I've got really weak teeth." Derochea is a woman of gleeful, elaborated extroversion; even being in a flimsy hospital gown doesn't make her act shy or tentative. She bounces; she laughs; she gestures. "She's so good!" a nurse in the room exclaims. "I'm so broke," Derochea says. "I'm a little ashamed to admit it, but I'm in debt." That's one of the reasons she's here, at Mount Sinai, to donate eggs, her pelvis tender, her ovaries swollen to the size of walnuts when normally they would be almonds, tubing about to be slipped into her nostrils to bathe her in milky amnesia.
If somebody were to design a line of fertility fetishes, Beth Derochea could be the model. Clips of her hair or fingernails could be incorporated into the amulets as saints' parts are encased in reliquaries. This is her third time at playing egg donor. She gave eggs twice during graduate school, and each time she yielded up a bumper crop of twenty-nine or so. Now she is back, in part for the fee of $2,500. But only in part. There are other reasons that she doesn't mind, even enjoys, donating eggs. She and her husband don't yet have children of their own, but she told me she likes playing mama. She mothers her friends; she urges them to dress warmly in the winter and to eat their fruits and vegetables. She likes changing diapers on other people's babies and rocking the infants to sleep. She likes the idea of her seed seeding other people's joy. She doesn't feel proprietary about her gametes. A fan of science fiction of the eggheaded variety, she tells me about something that Robert A. Heinlein once wrote. "`Your genes don't belong to you,' he said. `They belong to all humanity.' I really believe that. My eggs, my genes, they're not even something that's me, they're something I'm sharing. It's like donating blood."
By this generous, almost communistic imagery, we are all aswim in the same great gene pool, or fishers from the river of human perpetuity. If my line comes up empty, perhaps you will share your catch with me. For such reasons of heart and rightness, Derochea said she would donate eggs even if she weren't paid. "I might not have done it three times, but I definitely would have done it at least once," she says. Her sentiment is rare. In many European countries, where it is illegal to pay a woman for donating eggs, almost nobody does it. Bustillo said that when she attended a conference on bioethics recently, the audience of doctors, scientists, lawmakers, and professional ponderers was asked, just out of curiosity, whether anybody there would donate eggs. "Nobody raised her hand," Bustillo said. "Though two people later said they'd consider doing it for a relative or good friend." Derochea is not donating eggs for relatives or friends. She never meets the couples who receive her eggs, she will never see any progeny that might come of them, and she doesn't care. She doesn't moon over sequelae, she doesn't fantasize about her mystery children. "I've managed to disengage myself from any sense of investment," she says, as calm as a Renaissance madonna.
I say to Bustillo that it's a good thing the best egg donors women at the peak of their fertility, in their early thirties or younger are at a point in life when they are likeliest to need the cash. An egg donor earns every dime of her blood money. Three weeks before I met her, Derochea had begun injecting herself with Lupron, a synthetic version of gonadotropin-releasing hormone, a potent chemical bred in the brain that begins the entire cycle of egg-dropping. For a week she injected herself nightly in the thigh with a narrow needle of the type diabetics use. No big deal, she said. Barely noticeable. Uh-huh, I said, thinking, Oh, sure, sure, anybody could do it, anybody except me, who's always thought the worst thing about heroin addiction is not the way it ruins your life or may give you AIDS but that you have to inject yourself with a needle.
After the Lupron came the hard stuff. She had to switch to a double-barreled shot of Pergonal and Metrodin, a mix of ovulatory hormones designed to spur the ovaries into a state of hyperactivity. (Pergonal, incidentally, is isolated from the urine of postmenopausal women, whose bodies have become so accustomed to the menstrual cycle that they generate ovulatory hormones in extremely high concentrations because of a lack of feedback from the ovaries.) Preparing this sweet brew demanded concentration, to assure that as she pulled the fluid into the hypodermic syringe, no potentially embolizing bubbles were pulled up with it. She also needed to use a much heavier-gauge needle than she did for the Lupron, which means a bigger and more painful shot. This time Derochea had to aim for the rear part of her hip, every night for about two weeks. Not terrible, not an ordeal, but something she admitted she wouldn't want to do each month. Toward the end of this non-ordeal, to stimulate the final stage of ovulation, Derochea gave herself a single shot of human chorionic gonadotropin, again through an ominously large hypodermic.
All the while, between nightly inoculations, she had to return repeatedly to the hospital for sonograms, to check on the expansion of her ovaries. She thickened with excess fluid and jested about her snappishness. When I talked to her, she was more than ready to give up her grams of flesh. Her two ovaries were like overstuffed sacks of oranges, each orange an egg ripened with unnatural haste by three weeks of hormone treatments. In a normal cycle, only one egg would be pushing its way from its ovarian pocket. But at the moment Derochea was an Olympic cycler, and two or three years' worth of oocytic offerings had been condensed into a single month. There's no evidence that she has lost those years that her childbearing potential has in any way been compromised or truncated. We are, after all, overbudgeted with eggs, and think of what management does at the end of the fiscal period to budgets that don't get used: ka-ping! So the medical Demeters of the world simply cannibalize what otherwise would apoptose into the void.
In any event, fertility fetishism runs in Derochea's family: all of her siblings have already reproduced repeatedly. "Having babies is just something we do," she says. Derochea also doesn't worry about the risk of ovarian cancer, which some experts have proposed is heightened by the use of fertility drugs. The data on this question remain inconclusive, and in any case are more associated with the drug Clomid than with any of the follicular stimulants that Derochea has received. "If my family had a history of ovarian cancer, I'd be more concerned about it," she says. "But at this point, I'm not worried. Maybe that's stupid, but I'm not worried."
She lies down on the operating table. They pump her first with oxygen, then with anesthesia. They ask her if she's sleepy yet. "Mrrph!" she mumbles. A moment later she's as limp as a Dali clock. The surgical assistants stick her legs in stirrups and douse her genitals with iodine, which looks like menstrual blood as it dribbles along the inner folds of her thighs and onto the table. Bustillo barrels into the room, washes her hands, and jokes about crap and vaginas but no matter, she scrubs. She sits down at the end of the table, at the gynecologist's stirrup-side post, ready for one of the easier breaches of the body's barrier. Her assistants wheel a portable ultrasound machine over to the table and hand her the ultrasound probe, an instrument shaped like a dildo. She slips a stretchy latex casing over the probe "the condom!" she says and threads a needle through the device that will suck the readied eggs from their pockets.
Bustillo inserts the wand into Derochea's vagina and up into one of the two fornices, the culs-de-sac of the vaginal canal that pouch up around either side of the cervix. The needle pierces the fornix wall, moves across the pelvic peritoneum the oily membrane that surrounds most of the abdominal viscera and finally perforates the ovary. Bustillo does the entire extraction procedure by watching the ultrasound screen, where the image of the ovary looms in black and white, made visible by bouncing high-frequency sound waves. Coming in on the top lefthand side of the screen is the needle. The ovary looks like a giant beehive honeycombed with dark bloated egg pockets, or follicles, each measuring two millimeters across. These are all the follicles that were matured by Derochea's diligent nocturnal injections. The sonogram screen is full of them. Manipulating the needle-headed probe with her eyes fixed on the sonogram, Bustillo punctures every dark honeycomb and sucks all the fluid out of the follicle. The fluid travels down the tube of the probe and into a catchment beaker. You can't see the egg suspended in that fluid, but it's there. Immediately after the fluid has been extracted from the follicle, the pocket collapses in on itself and disappears from the screen. A few moments later it slightly distends again, this time with blood.
Prick! Prick! Prick! Bustillo pierces and vacuums out every follicle so quickly that the honeycomb seems alive with accordian motion: pockets fall in, reengorge with blood. Prick! Prick! Prick! It hurts vicariously to watch; I want to cross my legs in discomfort except that I'm standing up. One of the surgical assistants tells me that sometimes the women who have this procedure done demand that it be performed without anesthesia. They regret their choice. At some point they start screaming.
When the left ovary is picked clean of ripe eggs, Bustillo moves the probe over to the other vaginal fornix and repeats the maneuver on the right ovary. The entire bilateral pricking and sucking takes ten minutes or so. "Okay, that's it," Bustillo says as she withdraws the probe. A stream of blood flows from Derochea's vagina, like a fire set by a departing army. The nurses clean her up and start calling her name and shaking her arm to wake her. Beth! Beth! You're done, we're done, we've plucked you clean. Your genes are now floating in the communal pool in which another woman soon will immerse herself, seeking baptism with baby.
Back in the lab, Carol-Ann Cook, an embryologist, separates and counts the day's plunder: twenty-nine eggs, the same number harvested from Beth Derochea twice before. This woman's vineyards are fruitful! Cook prepares the eggs, these grapes of Beth, for fertilization with the sperm of another woman's husband, a woman who lacks viable eggs of her own.
The use of donor eggs for in vitro fertilization is one of the few promising things that have happened to the technique since its introduction in the 1970s. Most women who attempt IVF are nearing the end of their patience and fecundity. They are in their late thirties, early forties. For reasons that remain entirely opaque, the eggs of an "older" woman and it annoys me to use that term for anybody under eighty, let alone my peers have lost some of their plasticity and robustness. They don't ripen as readily, they don't fertilize as well, and once fertilized, they don't implant in the womb as firmly as the eggs of a younger woman do. Older women usually start by trying IVF with their own eggs. They are partial to their particular genomes, their molecular ancestry, and why not? There's little difference between a baby and a book, and it's usually best to write about what you know. So they go through what Beth Derochea went through, weeks of preparatory hormonal injections. At the other end, though, they give forth not dozens of eggs but perhaps three or four, and some of those may be barely breathing. The fertility gods do their best. They join the healthiest-looking eggs and a partner's sperm in a petri dish to form embryos. After two days or so, they deliver the embryos back to the woman by squirting the clusters of cells, afloat in liquid, through a thin tube inserted into the vagina, across the cervix, and into the uterus. No big deal: blink and you miss it. Alas, for the women too it's a case of blink and you lose it. In the vast majority of patients, the technique fails. The chance of an older woman giving birth to a baby conceived from her eggs through IVF is maybe 12 to 18 percent. If you heard that these were your odds of surviving cancer, you'd feel very, very depressed.
An older woman may try IVF once or twice, even a third time, but if by then she hasn't conceived with her own harvest of DNA, she probably never will. At that point a doctor may recommend donor eggs, combining the seeds of a younger woman with the sperm of the older woman's husband (or lover or male donor) and then implanting the resulting embryo in said senior's uterus. Using donor eggs can make a woman of forty act like a twenty-five-year-old, reproductively speaking. Who knows why? But it works, oh girl does it work, so well that suddenly you're no longer in the teens of probability but instead have about a 40 percent chance of giving birth in a single cycle of in vitro maneuvers. That number starts to sound like a real baby bawling. If the wine is young enough, it seems, the bottle and its label be damned.
And so the egg rules the roost. It, not the womb, sets the terms of tomorrow. Carol-Ann Cook takes one of Derochea's eggs and puts it under a high-powered microscope, which transmits the image to a video monitor. "This is a beautiful egg," Bustillo says. "All her eggs are beautiful," Cook adds. They are eggs from a healthy young woman. They have no choice but to shine.
To think of the egg, think of the heavens, and of weather. The body of the egg is the sun; it is as round and as magisterial as the sun. It is the only spherical cell in the body. Other cells may be shaped like cinched-in boxes or drops of ink or doughnuts that don't quite form a hole in the middle, but the egg is a geometer's dream. The form makes sense: a sphere is among the most stable shapes in nature. If you want to protect your most sacred heirlooms your genes bury them in spherical treasure chests. Like pearls, eggs last for decades and they're hard to crush, and when they're solicited for fertilization, they travel jauntily down the fallopian tube.
Carol-Ann Cook points out the details of the egg. Surrounding the great globe that glows silver-white on the screen is a smear of what looks like whipped cream, or the fluffy white clouds found in every child's sketch of a sky. This is in fact called the cumulus, for its resemblance to a cloud. The cumulus is a matting of sticky extracellular material that serves to bind the egg to the next celestial feature, the corona radiata. Like the corona of the sun, the corona of the egg is a luminous halo that extends out a considerable distance from the central orb. It is a crown fit for a queen, its spikes and phalanges emphasizing the unerring sphericity of the egg. The corona radiata is a dense network of interlocking cells called nurse cells, because they nurse and protect the egg, and it may also act as a kind of flight path or platform for sperm, steering the rather bumbling little flagellates toward the outer coat of the egg. That thick, extracellular coat is the famed zona pellucida the translucent zone the closest thing a mammalian egg has to a shell. The zona pellucida is a thick matrix of sugar and protein that is as cunning as a magnetic field. It invites sperm to explore its contours, but then it repels what doesn't suit it. It decides who is friend and who is alien. The zona pellucida can be considered the mother lode of biodiversity, the place where speciation in nature often begins, for it takes only a minor change in the structure of its sugars to make incompatible what before was connubial. The genes of a chimpanzee, for example, are more than 99 percent identical to ours, and it is possible that if the DNA of a chimpanzee sperm cell were injected directly int o the heart of a human egg, the artificial hybridization would produce a viable, if ethically repulsive, embryo. But under the natural constraints of sexual reproduction, a chimpanzee sperm could not breach the forbidding zona pellucida of a human egg.
The zona also thwarts the entry of more than one sperm of its own kind. Before fertilization, its sugars are open and genial and seeking similar sugars on the head of a sperm. Once the zona has attached to the head of a sperm, it imbibes the sperm, and then it stiffens, almost literally. Its sugars turn inward. The egg is sated; it wants no more DNA. Any sperm that remain at its threshold soon will die. Still, the zona's task is not through. It is thick and strong, an anorak, and it protects the tentative new embryo during the slow descent down the fallopian tube and into the uterus. Only when the embryo is capable of attaching to the uterine wall, a week or so after fertilization, does the zona pellucida burst apart and allow the embryo to join its blood with mother blood.
The corona, cumulus, and zona all are extracellular, auxiliaries to the egg but not the egg. The egg proper is the true sun, the light of life, and I say this without exaggeration. The egg is rare in the body and rare in its power. No other cell has the capacity to create the new, to begin with a complement of genes and build an entire being from it. I said earlier that the mammalian egg is not like a bird's egg, insofar as it lacks the nutrients to sustain embryonic development. A mammalian embryo must tether itself to the mother's circulatory system and be fed through the placenta. But from a genetic perspective, the cytoplasm of a mammalian egg is complete, a self-contained universe. Somewhere in its custardy cytoplasm are factors proteins, or bits of nucleic acid that allow a genome to stir itself to purpose, to speak every word its species has ever spoken. These maternal factors have not yet been identified, but their skills have been showcased in sensational ways. When Scottish scientists announced in 1997 that they had cloned an adult sheep and named her Dolly, the world erupted with babble about human clones and human drones and God deposed. The endless exercises in handwringing resolved very little of the ethical dilemma that surrounds the prospect of human cloning, if dilemma there be. But what the sweet ovine face of Dolly demonstrates without equivocation is the wonder of the egg. The egg made the clone. In the experiments, the scientists extracted a cell from the udder of an adult sheep, and they removed the nucleus from the cell, the nucleus being the storehouse of the cell's genes. They wanted those adult genes, and they could have taken them from any organ. Every cell in an animal's body has the same set of genes in it. What distinguishes an udder cell from a pancreatic cell from a skin cell is which of those tens of thousands of genes are active and which are silenced.
The egg is democratic. It gives all genes a voice. And so the scientists harvested a sheep egg cell and enucleated it, taking the egg's genes away and leaving behind only the egg body, the cytoplasm, the nonyolk yolk. In place of the egg nucleus they installed the nucleus of the udder cell, and then they implanted the odd chimera, the manufactured minotaur, into the womb of another sheep. The egg body resurrected the entire adult genome. It wiped the slate clean, washed the milky stains from the dedicated udder cell, and made its old genes new again. Maternal factors in the egg body allowed the genome to recapitulate the mad glory of gestation to recreate all organs, all tissue types, the sum of sheep.
The egg alone of the body's cells can effect the whole. If you put a liver cell or a pancreatic cell into a uterus, no infant would grow of it. It has the genes to make a new being, but it has not the wit. Small wonder, then, that the egg is such a large cell. It must hold the secrets of genesis. And perhaps the molecular complexity of the egg explains why we can't produce new eggs in adulthood, why we are born with all the eggs we will ever have, when men can sprout new sperm throughout their lives. Scientists often make much of the contrast between egg and sperm, the prolificacy and renewability of the man's gametes compared to the limitations and degradative quality of a woman's eggs. They speak in breathless terms of sperm production. "Every time a man's heart beats, he makes a thousand sperm!" Ralph Brinster burbled to the Washington Post in May of 1996. But a woman is born with all the eggs she'll ever have, he continued, and they only senesce from there. Yet the mere ability to replicate is hardly cause for a standing ovation. Bacteria will double their number every twenty minutes. Many cancer cells can divide in a dish for years after their founder tumors have killed the patient. Perhaps eggs are like neurons, which also are not replenished in adulthood: they know too much. Eggs must plan the party. Sperm only need to show up wearing top hat and tails, of course.