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Getting Eggs into Shape

There's more to an egg than meets the eye.

Peter Meredith

Which end of the egg comes out of the hen first?

When you ask people this, roughly half answer the large end, and half answer the small end, and almost everyone follows up their answer with a lengthy reason why. Only one person has ever given me a short answer and, when I prompted him, he explained that he'd watched hens laying eggs -- he didn't need a reason for his answer.

It won't do you much good to go to the books in the library. They'll tell you lots about the structure of the egg, its composition and its nutritional value. You'll find information about the hen's nutritional needs, about the controlling factors of light and temperature and how one breed differs from another. Shell colour is specific to the variety of hen, not what she eats; yolk colour depends on her diet. But the books avoid the subject of which way round the hen lays her eggs.

First Make Your Egg

A young chicken has two ovaries and two oviducts, just like most higher animals. But as the chicken grows up, one oviduct shrivels, leaving just one functional and highly efficient egg manufacturing production line. She lays something like 275 eggs a year, each weighing up to 2% of her body mass per day, amounting to many times her body weight in a year.

The oviduct is a predominantly muscular tube about 600 mm long, with maybe six distinguishable regions (though the number depends on who is describing it). There are regions specialising in secretion of the component parts, particularly the white and its water component; there's elaboration of the various albumen proteins, the shell membranes and the shell, around the central mass of yolk within its own membranes.

The further down the oviduct we look, the greater the amount of muscle we see in its wall. The egg is passed along the tube by peristalsis, the walls contracting behind it to squeeze it along, while there is some backward squeeze ahead of it to prevent it passing along too rapidly. The assembly mechanisms must have time to work.

The secretion of albumen and its uptake of water is a process known as pumping or plumping. This generates internal pressure so that the egg is blown up like a party balloon. This expansion effect is opposed by the muscular wall of the oviduct. The shell itself forms by a combination of secretion of the required materials from the oviduct and organised deposition by the specialised cellular membrane on the outside of the developing egg, starting from the leading end.

In a simplistic manner, the egg would like to be spherical because it has formed from a near-liquid mass under internal pressure. But it is constrained by the muscular wall of the oviduct squeezing it along and by the formation, from one end first, of a rigid shell. So we could expect the leading end of the egg to be nearly hemispheric in shape. The trailing end we can expect to be more pointed because of the additional constricting pressure needed to force the hardening egg along the duct. Both ends are likely to approximate to halves of circular ellipsoids, though of differing axial ratios.

So now you know which way round an egg was formed: larger end first. I've never watched a hen lay an egg, but I gather that most hens probably lay their eggs the same way round that they make them. However, life is rarely simple and it is certain that some hens turn the egg round in their vagina, the last part of the oviduct, before laying.

I can certainly agree with the many conversationalists who said it must be smaller end first because it would be more comfortable for the hen. Remember that most human babies are born head (the larger end) first, but we all know of the occasional breech presentation that gives the midwife so much more trouble because the arms and legs get in the way. At least the hen doesn't have that problem.

Twisting and Turning

Two other important stages occur along the assembly line that, in factory terms, are analogous to case hardening and centreless grinding. And after those we had better consider candy wrapping and gun rifling.

What I have likened to case hardening is, of course, the deposition of the hard shell. While the shell is forming, and indeed most of the way down the oviduct, the egg is rotating, just like a bullet passing down a rifle barrel. And, just like the rifle bullet, the rotation is driven from the outside. This is a centreless deposition or growth which ensures that the egg in cross section is a perfect circle.

Rifling can be of two sorts, corresponding to left-hand and right-hand threads. I can find no information about the rifling of hens' oviducts: are they left-handed or right-handed, or are they paired in the two original oviducts so that those hens having a left oviduct would impart the opposite spin to those having a right oviduct? Life is full of interesting questions...

The spin of the rifling imparts a lasting handedness to the egg that is somewhat overlooked. The yolk, within its membranes, was formed in the ovary before passing into the oviduct. The several layers of white protein were added before much else happened and the thick outer part of this becomes attached to the two membranes that are next formed around the outside of the whole putative egg.

From here on, the rotation of the egg by the rifling effect ensures that the yolk and its membrane rotate relative to the outer membranes. Something has to twist to accomodate this motion; the yolk is gravicentric stationary and at each end of it the membranes plus thick protein are twisted just like the ends of a candy wrapper. And if you've ever looked critically at a candy wrapper you'll have realised that the ends have both been twisted in the same direction relative to the centre so that when you pull the ends the whole thing twists open.

For that to happen, one end is equivalent to a right-hand thread and the other end to a left-hand thread. These twisted membranes in the egg, called the chalazae, seem always to be portrayed by artists as both left-hand or both right-hand twists. And while we're inside the egg, there's an air space between the two membranes at the leading (larger) end. I have not seen an explanation of how it forms at this end and not at the other.

Getting Eggs into Shape Figure A (19KB)

From Front to Back

We don't need much equipment to measure eggs; a balance, some callipers and a carpenter's square. And, of course, some eggs. I looked at two sets, one of consecutive layings by "the little red hen", and the other of a single day of laying by a small flock of free range hens.

There are four measures we can calculate to describe the egg:

  • the shape of its equator, to see how truly it rotated or extruded as the shell was hardening
  • how long it is relative to its width at the equator, the "shape index" used by the egg industries for many years
  • the shapes of the two ends, to describe the two half ellipses and to compare them for the degree of taper of the egg
  • the volume of the egg compared to its overall dimensions, to see how truly ellipsoid it is

The egg is an almost perfect circle at its equator. Those eggs I have measured had transverse diameters in several axes (of the one egg) that varied by no more than 0.10 mm, in a diameter about 45 mm. The shell grows outwards from nuclei within or upon the membrane, so it is difficult to be clear whether the process is growth or deposition.

The shape index is the width of the egg as a percentage of its length: an average of 71.9 for the successive eggs from one hen and 75.2 for the batch of eggs from the free-range flock, with coefficients of variation 2.2% and 3.8% respectively. These compare reasonably well with other measurements, so I'm not dealing with abnormal eggs.

The formation of biological structures by internal pressure and mobile external constraints is likely to lead to ellipsoid shapes. The presence of ellipsoidity can be tested in two ways, by measuring the curvature of the surface (particularly at the ends), or by comparing the measured volume with that required of an ellipsoid having the same axial dimensions. I have done both of these tests for the two batches of hens' eggs.

We know that an egg isn't symmetrical about its two ends, and several investigators have struggled to deal with this situation mathematically, describing it as a "prolate spheroid" (that is, an ellipse modified such that the scale of the long axis is pulled out towards one pole and compressed towards the other) or as two half-ellipses of unequal eccentricity. A set of near-ellipsoid equations have been developed to describe the eggs of a wide range of avian species.

In my view, it is sufficient to describe these sorts of shapes (whether eggs, potatoes or coconuts) as two differing ellipsoids joined back to back at the equator. That does not affect the volume check but does mean that we must measure the two ends separately.

The function called "end shape" is calculated such that zero is a sharp point, 100 is square, and a semicircle has a value 41.42 . For the single hen, the average egg ends were 31.7 and 34.7, rather less difference than for the free range flock set of eggs where the average ends were 30.4 and 35.3. The difference between the shape functions of the two ends, expressed as a percentage of their mean, is a convenient measure of the apparent overall taper of the egg; 9.0 for the single hen and 15.0 for the flock, both these values being much less than I had expected.

Pointy Ends

For some of the eggs, careful consideration was required to decide on a visual basis which was the larger end. But the very important finding is that both ends are markedly pointed; the large end is a long way from spherical according to the numbers, and this can be confirmed by giving an egg more than a glancing look. So the idea that the hen should lay the eggs small end first for her own comfort is not a strong argument. Both ends are pointed.

I had expected the large end of an egg to be near spherical but in fact these were notably more pointed, visibly so when I had the numbers in front of me. That is a feature that I have repeatedly observed in my potato work, where the numbers point to observations that are visible when looked for but which otherwise pass unnoticed because of prior beliefs.

The small ends were more pointed, by definition, but not so pointed as I had expected and certainly not as pointed as I recollected from childhood. Maybe eggs are not as pointed as they used to be. Certainly potatoes have changed shapes over the years, as cultivars and growing practices have changed.

How I wish we had measurements like this for eggs of the past! Defined numbers open up a whole world that is denied us by vague descriptors and it is quite conceivable that improved lines of hens and improved feeds and living conditions will have improved the shape of eggs.

The true volume of an egg can be obtained by that method irreverently called "Archie's Bathtime", weighing the egg in air and in water according to the principles enunciated by Archimedes in the third century BC. The difference between the two weighings, the weight of water displaced, is a measure of the volume of the egg.

The cuboid volume is obtained by using a calliper gauge to measure the lengths of the three principal axes. The theoretical volume of an ellipsoid is 52.36% of the cuboid volume (pi over 6), regardless of how fat, skinny or flattened is the ellipsoid, so we can calculate the expected ellipsoid volume.

Fullness is the percentage by which the actual volume exceeds the ellipsoid volume and is, therefore, a measure of departure from the ellipsoid model. For the series of eggs from one hen, the average fullness was -0.2 and for the series from the flock was -0.7 so that on average of the two ends the ellipsoid was a fairly exact model.

Eggs I have measured show a similar response to the ends of potatoes such that when one end is over or under filled, the other end responds to maintain the average end shape constant. This supports the concept of an hydraulic basis for the shape formation, with external constraints that do not completely mould the shape. The developing coconut is extremely hydraulic, having a peak pressure that exceeds 5 atmospheres, and potentially dangerous (like a truck tyre) if punctured while the outer layers are still elastic. Beware of nature.

Discussions of the shapes of eggs generally mention their tapered shape as being suited to fit the nest and minimise the risk of rolling off the edge of the cliff. But I have been looking at egg shapes as part of a comparison with potatoes and coconuts. They're all roughly the same shape and for the similar reason -- they have grown by blowing up like balloons.

So my view is that eggs are tapered just because of the peristaltic way they are transported through the manufacturing process. The advantages to the birds are incidental but useful. Despite our familiarity with the end product, there's still a lot we don't know about the processes involved.

Peter Meredith welcomes correspondence from any practical or theoretical egg experts on any of the questions he has raised. He can be contacted via the NZSM.

Peter Meredith is a research scientist with Ilam Potato Sciences.