The Virgin Egg

by N. J. BERRILL

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MUCH of my time as a biologist has been spent looking through a microscope at the beginnings of individual lives, partly in an effort to understand a little of how animals develop from eggs, partly and perhaps primarily in sheer fascination at the unfolding picture. It is something to be seen, although seen as action, and more and more you realize the truth of the philosopher Whitehead’s statement that the process itself is the actuality. When you sit down in the early morning and watch a myriad of dancing specks of spermatozoa buzzing around an egg, itself no larger to the naked eye than the smallest speck of dust, you are held entranced. When you see that egg divide into two, into four, eight, sixteen, and more cells all closely united, and watch the ball of living substance gradually change its form until by evening, still small, it has a body and a tail, a simple eye and an organ for balance — a miniature tadpole — you believe it has happened only because you have seen it.

You can look at the events scientifically and do things to the egg or the embryo or the water in which it develops, and see what effect your interference has upon the outcome, as though you were winding up a clock or taking out this part or that to see what difference it might make to the way it runs; but in the end what holds your mind and eye is the way the clock keeps time and the steady transformation of an egg into a tadpole, or whatever else it might be, between dawn and dusk. The development of a living active organism from a seemingly inert egg, while you watch and wait, is a miracle that never palls. So is the explosive vigor with which the process starts, when a sperm first, enters an egg and releases the trigger.

The egg is launched on its course the instant the sperm hits its surface. Up to that moment its life has been sluggish, in a metabolic way, and the touch of the sperm is electrifying, like the kiss that wakened the sleeping beauty. And this is no idle metaphor, for in every true sense of the word the egg is revitalized, and develops in consequence. Whether the eggs are minute like the eggs of the sea squirts, or enormous shell-protected gobs like the eggs of ostriches, or small but inaccessible, like those of elephants and human beings, the process is the same. An infinitely small spermatozoon, so small that ten thousand or more can be placed side by side and not cover an inch, must reach the surface of the egg; and it must reach the egg while there is still some of its limited energy left in the little lashing tail.

Such are the sex cells. Any small egg is much like another, no matter what kind of animal it finally becomes — a starfish egg and a human egg are about the same size and look much alike, semitransparent and about one two-hundredth of an inch across, small enough by any measure except that of a spermatozoon. This is our starting point. All of the nature of sex and the meaning of sex stems from it. Sex cells must be brought together so that eggs may be fertilized; and eggs and sperm, for the most part, must come from different parents. And not all of it is for the sake of reproduction.

If nature as a whole can be said to have an obsession it lies in the need to produce fertilized eggs and to launch them upon their career. And it calls attention to the difference between the sex cells — why should they be so different? The perennial human question concerning the equality of the sexes goes far beyond sex as we know it and finds its roots in the sex cells as such. Why are eggs and sperm so unlike, and what are their respective contributions to the organisms-to-be?

Much of it is simply a division of labor. It is difficult to be a jack-of-all-trades and do any particular job well. And the cell that is to develop into a new plant or animal must unite with another cell, and between them they must possess enough material to make something much more complex than they arc to begin with. So one kind stores the substance and the other does the traveling. In this way they get together and have enough to build with.

Even the smallest egg is large for a cell, many times larger than a white corpuscle in your blood, while the egg of an ostrich may well be the largest cell in the universe. We will never know. But storing up material makes eggs round and large, and this alone renders impossible any movement on their part to meet a sperm. So it is all left for the sperms to do. All the responsibility for reaching an egg is theirs and theirs alone. They are simply cells with a delicate tail for swimming and everything else reduced to a minimum.

Numbers are important too. At best the chances arc slight that any particular sperm will ever make contact with an egg, and astronomical numbers are always necessary for successful fertilization. The largest possible number, and therefore the smallest possible size, is the rule. In the case of eggs the problem is different. For the larger the individual egg becomes, the farther it can progress along its course before it has to be an organism fending for itself. Large eggs are better than small eggs. Yet every animal and plant works on a budget and can set aside just so much material for producing sex cells. Large eggs mean fewer eggs and that is not so good. The greater the number of eggs produced the more likely some of them will survive to carry on the race. And so every race and species is torn between producing large eggs and enough eggs. It is a difficult choice and there is no single answer — it all depends on circumstances.

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CELL for cell the sexes are not equal — a sperm is minute compared with even the smallest egg. Yet a male may hope to console himself with the thought that matter is not everything. What else is there? As a rule an egg must be fertilized by a sperm in order to develop, but it is only as a rule and far from being always. Masculine self-esteem gets shakier the more you look into the question.

It all started more than half a century ago when biologists were looking closely for the first time into the nature of the fertilization of an egg and of the processes involved — sheer curiosity of the kind that led to the making of an atomic bomb. Only this merely put the male in his place.

The experiments have been repeated ever since. Every summer in marine laboratories on the coasts students and others place ripe starfish eggs in seawater that has been tampered with, usually by the addition of a little organic acid, and they watch the eggs develop in consequence without the aid of spermatozoa at all. Apparently all the potentiality for development and the making of a new starfish is there in the egg itself. All it needs is a little push to get it started.

Frog eggs are equally susceptible, although they need a somewhat different kind of push. It is a fascinating process and its practice is a common classroom exercise in most of the colleges throughout the land. It gives you an unwarranted godlike feeling. You need only some female frogs a month or two before the onset of their breeding season. If you in ject one of them with extract of pituitary gland, that small but all-important gland that lies beneath the brain of every backboned animal from fish to man, the frog lays her eggs within twentyfour hours. It works almost every time and it is about the only way you can get fully ripened eggs entirely free from any sperm. For, once they are laid in a pond it is too late, and there would be spermatozoa around to confuse the issue.

The release of the eggs by pressing a physiological button is impressive enough, but making them develop by mechanical means is more than that. The jelly that surrounds each egg is carefully removed by rolling it gently on blotting paper, so that the naked egg is exposed. Then, with extreme care, you prick its surface with the point of a fine glass needle. It is a delicate operation and the steadiest hand is all too clumsy, but out of a hundred so treated a score or two will start to divide and develop into a tadpole. And all such tadpoles will be males.

Actually there is a clue here to sex as distinct from sex cells. The fully ripened egg of almost any kind of animal, if it will develop at all without being fertilized by a sperm, develops into a male. It takes a double dose of something to become a female. In the case of frogs and human beings there are two kinds of sperm, although they all look alike. One kind carries that extra component that added to an egg makes a female, the other kind does not. Since the two kinds are produced in approximately equal numbers, the chances are the same that one or the other kind will fertilize an egg. The result is that males and females are normally produced in equal numbers. Pricked frog eggs lack the spermatozoan contribution and are doomed to develop into males, if they develop at all.

Parthenogenesis — the long name for a short process, and meaning origin from a virgin — has been brought about by methods such as these in a number of different animals, particularly among the so-called lower forms. Simply raising the temperature will often do it in the case of frog eggs. It has been only natural to wonder if it can be accomplished in a mammal.

In spite of ancient notions concerning conception and the general belief that all conceptions in human beings were the result of a spirit of some kind entering the womb of the woman, there is no good evidence that in mammals, whether human or otherwise, eggs ever naturally develop without being fertilized by spermatozoa. Such an occurrence is not implausible for it does happen in bees and other creatures, but not in mammals. And I doubt whether it is ever a natural event in frogs or starfish.

Yet the thought lingers; and sooner or later, as you might expect, someone would attempt to make mammalian eggs develop without benefit of paternity. The victims have been rabbits. Ripe but unfertilized mammalian eggs are notoriously hard to obtain, and they are always few in number. Nearly thirty years ago in France the embryologist Champy discovered among a group of unfertilized rabbit eggs cultivated outside the body, although at the body temperature, that some eggs underwent divisions as though they had been fertilized. Gregory Pincus, in America, followed this up and tried out all of the tricks we know that have made the eggs of lower animals develop spontaneously. But only an occasional egg ever reached a recognizable stage in embryo formation. The possibility of parthenogenesis seemed to be present, yet something was wrong.

Apparently it is one thing to cause a mammalian egg to divide, it is another to keep it going in a normal manner — which is not surprising when you consider that the mammalian egg is adjusted to develop in the tubes and womb of the mother. So Pincus treated the unfertilized eggs as before and then transplanted them into virgin rabbits. Out of a large number of trials, some of these virgins bore normal rabbits. In many others the embryos were well advanced but died prematurely. It is not very promising, though perhaps one day we may breed our cows without utilizing bulls. Perhaps it would be better if we should not be able to, for the human race so far shows no sign of knowing when or where to stop.

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THE immediate interest of these rabbit experiments, to me at least, is that all the born and unborn offspring that have come from these unfertilized eggs have been females. The explanation goes back to t he ripening of t he egg. Starfish eggs, frog eggs, rabbit and human eggs all ripen normally in essentially t he same way. When the fully grown egg leaves the ovary, whether to he shed into the sea if it is a starfish or to enter and lie in the oviduct if it is a mammal, it has to go through a process of division before it is ready to be fertilized. It divides, not into two equal parts as a cell usually does, but into a minute polar cell and one so large it looks like the original egg. And the event is important out of all proportion to its magnitude.

This particular division of the egg cell is possibly the most important in all of nature and throughout earthly time. For it is during this division that the differences arise that make one egg different from the next, that supply the raw material for evolution and adaptation to a changing world. Here is the general importance of it. The egg is different because of the division, and the polar cell is simply a by-product of the process. Yet sometimes the polar division is suppressed and the egg may still develop. That, according to another French investigator, is exactly what happens in those eggs that produce parthenogenet ic female rabbits. The eggs retain a double quantity of the female component and develop into females. The rabbit eggs that do not retain the substance of the polar cell but ripen fully do not develop at all unless they are fertilized by a sperm. I doubt if this indicates the final fate of the human male, but it does suggest that if we ever succeed iu producing a race with eggs that develop spontaneously the result will be a purely female population capable of carrying on more or less indefinitely without the male — in fact, no males could be expected, unless the system became elaborated.

This may seem to be a fantastic speculation; but it has happened to other creatures, and it goes on in spring or summer in almost any pond or pool where you can find the water fleas, Daphnia in particular. They swim with a jerky movement — hence their name — but they are as crustacean as a crayfish, and are not insects at all. Like all living things they belong to their environment, and what they have to do, in an almost literal sense, is to make hay while the sun shines. For they are specialists in living in small pools and even barnyard puddles, feeding on everything that is microscopic, whether it be plant cells, debris, or bacteria. As long as there is food of this sort and a certain amount of water, Daphnia and its kind can flourish. Sometimes when you come across a pond or a meadow pool the water is cloudy with them, the swarms are so dense. If you look closely you can see each small individual beating its way through the water with a pair of oarlike appendages, directing its movement with a solitary eye.

Daphnias are creatures of a transient and intermittent world — they flourish when the water comes, like flowers in springtime, and when the water has gone there is no more to be seen than the seeds of an annual plant.

The eggs hatch out as miniatures of the parent, a statement that means more than it usually does, for not only does a young Daphnia look very much like an older one, but also it is a female like its mother. Every member of the brood which hatches out after rains or melting snow returns water to the pools in spring is a female. There are no exceptions.

As the water warms and food becomes more plentiful the first broods of females mature and they in turn produce successive broods, each brood being carried in a sheltered pouch on the upper side of the parent. And each such brood in turn is entirely female. No males are present and none is required — the eggs develop without them and parthenogenesis is the rule. How it is that the eggs of Daphnia are able to develop spontaneously in the absence of spermatozoa I believe is beyond our present understanding, although it concerns the metabolic peculiarities of the eggs growing in the ovary. The reason that the Daphnia broods are all female is the same as for the rabbits of virgin birth — the polar cell of the ripening egg is retained, the double dose of the sex determinant is present, and the outcome is female.

Yet this is far from being the whole story, and when you put it all together it seems that the Daphnia is exquisitely adapted to the little world in which it lives. Given the ideal conditions, the Daphnia community remains exclusively female; but this is an ever-changing universe, whatever its scale may be, and Daphnia, no more than we, can keep things perfect. The better the conditions the faster they breed, and the more they breed the more crowded the community becomes and the less food there is to go around — the time of trouble arrives. The response is almost miraculous.

The signal may be the lack of space that comes from overcrowding, perhaps the irritation that even the lowliest forms of life exhibit when the course of travel is continually being interrupted; it may be the sudden shortage of food that follows a population peak, or a sudden drop in temperature. A cold night and a chilly day may indicate the end of summer; overcrowding may forecast either lack of food or lack of water to live in. Whatever it is, and all may be effective, the response is the same and it is a striking one. There are two steps in it.

The first change is that broods of males appear in addition to the females. The males when grown are smaller, redder, and more active, for their role is to catch onto and mate with a female. Their redder blood may or may not be significant. What brings them into being is another matter. All that is necessary is that the eggs complete their ripening before development starts. The polar cell is given off, only a single unit of the sex determinant remains within the egg — and a male develops instead of a female. The environmental change affects the maternal organism so that the polar division of the grown egg is no longer suppressed, and the rest follows. It leaves us with the original problem — how do the eggs manage to develop without the stimulus of a sperm ?

Only some females in the community react like this and produce male offspring. Others go right ahead producing female broods, which shows that even in a purely amazonian community of parthenogenetic females, individuals react differently to a given set of circumstances. They are not all alike no matter what they seem to be.

A scattering of males through the virgin paradise is the first step only. If the signal was a false alarm no harm is done: the females go on producing female broods and the males simply live out their lives without any propagative purpose — as thwarted as in the Greek play Lysistrata, where all the women went on connubial strike for the political welfare of the state.

Yet in Daphnia society if conditions go from bad to worse the second step is called into play. A cell in each female ovary grows to form an egg larger than any produced before — and it completes its final process of polar division. If it could develop spontaneously it would be a male. But this time it does not and fertilization by one of the males already present is essential. These are the winter eggs that become enclosed in cases so tough that they are impermeable to water and are able to survive from one season to the next, like the wellprotected seeds of plants. Females emerge — never a male — because the sperm contributed the additional unit of sex determinant that usually came from the polar cell. And they emerge only in the spring or after such time that the shells have been cracked by alternate freezings and thawings or drying out. Only when water gets in do they complete their development and start the cycle over again. This is sex and reproduction exploited to take advantage of t he peculiar little universe of t heir choice — all for the sake of racial survival under difficult conditions.