Field notes on things that run themselves
It Cannot Breathe Harder
A bird’s egg does nothing. It sits where it was put, has no muscle and no moving part, and for three weeks does not so much as twitch. It is also, that whole time, running a respiratory system — and the thing doing the respiring cannot take a breath.
The shell is not the seal it looks like. It is a crust of calcium carbonate drilled through by thousands of funnel-shaped channels running from the outside surface to the membranes beneath. Oxygen diffuses in. Carbon dioxide and water vapour diffuse out. Nothing pulls: each gas moves down its own gradient, and the gradients exist because the embryo consumes one and makes the others.
Counting the holes is the wrong question — not every pit on a shell’s surface goes all the way through. What physiologists measure instead is conductance: how much gas the shell passes per day per unit of pressure difference, set by open area over the length of the path. Nor are the holes evenly spread. Taking the blunt end over the air cell as a reference, pore density falls to about four-fifths at the equator and three-fifths at the point.
Here is the difficulty. Across those three weeks an embryo’s oxygen consumption climbs about fivefold. The shell’s conductance does not move at all; it was fixed the day the egg was laid and no part of it can be opened wider. Flux is conductance times pressure difference. If flux must rise fivefold and conductance cannot, the difference has to supply every bit of it.
It does. Inside a chicken egg’s air cell, oxygen pressure falls steadily as the embryo grows while carbon dioxide climbs nearly threefold. That deterioration is not a symptom of an egg in trouble. It is the delivery mechanism. The only quantity free to move is the quality of the air inside, so it moves, and each day the embryo pulls harder on the outside air by falling further below it. A chick cannot breathe harder. It gets more oxygen by needing more.
So the leakiness has to be right from the start, which defeats the intuition that more air must be better. Seal part of a shell and the embryo takes up less oxygen. Cut part away instead, raising conductance, and it also takes up less. The peak sits at an ordinary untouched egg. Water is the other half: a shell leaky enough to breathe through is leaky enough to dry out, and across dozens of species eggs lose nearly the same fraction of their mass as vapour — around a sixth, accounts ranging from fifteen to eighteen per cent. That loss is not waste. It hollows out the air cell the chick takes its first real breath from.
The tuning is evolutionary, not on the fly. Across an Andean gradient in nearly two hundred species, shell conductance drops with elevation, by different routes — tanagers thicken the shell, nightjars thin out the pores. Within a single species spread over a kilometre of mountainside, nothing tracked at all: same thickness, same pore density, same pore size, top to bottom.
Inside, the arrangement changes three times while the boundary changes none. First the yolk sac’s vascular sheet does the exchanging. Around day six the chorioallantois reaches the inner shell and takes over, lining the whole interior by day twelve — a lung pressed flat against a wall of stone. Then, near the end, oxygen uptake stops rising. Shell and membrane have both run out of capacity, and the plateau is the cue: the chick punctures the air cell, breathes with its lungs while still walled in, and only then begins to crack.
It has already been undermining the wall. From about day eleven, cells on that same membrane secrete acid onto the shell’s inner face and dissolve it, ferrying calcium inward — a hen’s shell holds a couple of grams, and for days the growing skeleton is most of the calcium in the animal. The knobs underneath erode and the inner shell weakens, while the outer layers the pores run through stay intact until late. The organ using the shell as a lung is quarrying it for bone, and that is how the door gets thin enough to break.
A sealed terrarium (No. 3) trades nothing with the outside and runs on light alone; this is its deliberate opposite, a boundary built to leak. A cell membrane (No. 16) is molecular and never stops adjusting — pumps, channels, gates. An eggshell is mineral and adjusts nothing. And where last week’s star (No. 68) alters its boundary’s resistance every cycle, which is exactly why it oscillates, this one works by never altering at all. Two boundaries doing the work; two ways to do it.
The shell is the fixed point. Everything else in the egg — gas tensions, blood, membranes, animal — arranges itself around a number settled before any of it existed. Then the animal eats the number and walks out.
One loop I’m watching
Next: a tardigrade in its tun. Dried past the point where any chemistry can run, it isn’t dormant in the ordinary sense — its metabolism is not slow but unmeasurable, and by several definitions the animal is not currently alive. Glass-like sugars and its own strange disordered proteins hold the machinery in position, unfolded but not lost, sometimes for years. Add water and it resumes mid-sentence. A loop that can stop completely and still be the same loop.
Tip: the ← and → arrow keys move between issues.
Prefer feeds? Follow by RSS or JSON Feed.
Want something playable between issues? Thought Toys has its own email list and RSS feed.
New to The Standing Wave? Start here →