Book - The Hormones in Human Reproduction (1942) 1

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Corner GW. The Hormones in Human Reproduction. (1942) Princeton University Press.

   Hormones in Human Reproduction (1942): 1 Higher Animals | 2 Human Egg and Organs | 3 Ovary as Timepiece | 4 Hormone of Preparation and Maturity | 5 Hormone for Gestation | 6 Menstrual Cycle | 7 Endocrine Arithmetic | 8 Hormones in Pregnancy | 9 Male Hormone | Appendices
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Chapter I. The Place of the Higher Animals, and of Mankind in Particular, in the General Scheme of Animal

"of the cell, the wondrous seed Becoming plant and animal and mind Unerringly forever after its hind. In its omnipotence, in flower and weed And beast and bird and fish, and many a breed Of man and woman, from all years behind Building its future"

William Ellery Leonard, Two Lives.

Among the life that swarms in our southern waters, /% there is a charming tiny animal called Cothurnia, the ^ %. buskin animalcule. These creatures cling by thousands to the vegetation on wharf piles in our harbors, and can be brought into the laboratory on a bit of seaweed in a drop of water. Because a single Cothurnia is much smaller than the printed period at the end of this sentence, it must be watched through the microscope (Fig. 1). It consists of a graceful transparent cup (formed more like a wineglass than the classical buskin from which it got its name) which is attached by its stem to some larger object. Inside the cup and fixed to its base is a single animal cell, shaped like a trumpet. While the stem sways gently in the water, the cell projects from the cup. Into its open gullet particles of food are swept by a brush of beating lashes or cilia and drift down into the jelly-like cell substance until they are dissolved and digested.

This simple career of food-gathering is interrupted from time to time by a few hours devoted to reproduction. Our pretty little trumpet withdraws itself inside the cup, rounds up a bit, and slowly separates into two cells by dividing lengthwise. For a time, both cells resume the task of feeding, but afterward one of them retires into the cup and begins a struggle to get away. It pulls so strongly, indeed, upon its stalk that its shape changes; from a trumpet it becomes a shoe. The cilia change position so that they can serve for propulsion in swimming. At last the cell breaks from its attachment and slips out into the sea, ultimately to settle

Fig. 1. The one-celled animal Cothurnia, reproducing itself by simple division. The parent animal is seen at a. From 6 to d, successive stages of division. In e the daughter cell has freed itself and is swimming away, to settle in a new location. Greatly magnified.

down upon a near-by strand of seaweed, or perhaps (venturing greatly) as far away as the next timber of the wharf.

All this makes no difference to the first cell; it undergoes no pregnancy, feels no pangs while giving birth and takes no responsibility to nurse, guard, or educate its offspring. The latter in turn asks nothing at all of its parent, and never realizes the disadvantages of birth at so low a level of organization, one of which is that the newborn cell faces immediately and alone all the dangers of its world. The infant mortality of Cothurnia must be enormous, for there are many enemies and risks, but what of that.? The parent can easily split off another cell, and in spite of the wastage it is more economical (if all you want is a one-celled child) to breed by excess production than by the intricate process through which man and the higher animals turn out their limited output of complex and troublesome offspring.

Other unicellular animals have developed variations of the process of reproduction by division. Sometimes they do not divide into two equal cells, but put out their daughter cells as mere buds which break off while small and only later reach

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"adult" size (Fig. 2). Sometimes the parent animal breaks up by multiple fission into a relatively large number of very small daughter cells resembling spores (Fig. 3).

Reproduction by fission is so easy that in the course of evolution the animal kingdom held on to it for a long time, and many animals higher than the unicellular animals made use of it. Some of the worms, for example, split in two transversely by forming an extra head from some of the segments near the middle of the body (Fig. 4). This head, with the rest of the worm that lies behind it, drops off and wriggles away, while the original worm forms a new tail at its truncated posterior end. Sometimes the worm breaks up into a whole chain of segments each of which becomes a new worm.

Reproduction by budding also continued in higher ani


'â– ' \

Fio. 2. A one-celled animal, AcanthocystiSy reproducing itself by budding. 3 buds are seen. Greatly magnified. After Schaudinn.

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Fio. 3. A one-celled animal, Trichospherium, reproducing itself by the formation of spores. In this kind of reproduction the original "parent" ceases to exist as an individual, being completely dispersed into its offspring. Greatly magnified. After Schaudinn.

Fio. 4. The marine worm Autolytus reproducing itself by transverse fission. At a, a new head is forming, and the rear part of the worm will soon drop off to become a separate individual. Magnified. After Alexander Agassiz.

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E55 cell

Fio. 5. Diagram of simple many-celled animal {Hydra) cut lengthwise to show the egg cell and the testis with its sperm cells. Compare with the photographs of the same subject, Plate II. From Attaining Womanhood, by George W. Corner, by courtesy of Harper and Brothers.

mals, notably the sponges and jelly fishes. In the common fresh-water polyp, Hydra, for example (Fig. 5 and Plate I), the buds develop from the side of the tubular parent and ultimately break away. An interesting development of this pattern is well seen in Obelia, a hydra-like animal often studied in biology classes, in which the bud is not exactly like the parent, but becomes a free-swimming medusa (jelly fish) which in turn produces a generation of polyps like the original hydroid.

In some of the sponges the buds or gemmules are formed internally and must await the death and decay of the parent before they can get free to begin their own career.

I have not space here to review all the modifications of this general sort that occur in the more primitive part of the animal kingdom. Some of them are decidedly bizarre. The process of budding can, however, be considered (with certain

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technical reservations) as merely a variation of the fundamental process of multiplication by fission.

H. G. Wells, Julian Huxley, and G. P. Wells, in "The Science of Life," summarize the whole subject of reproduction of living things when they say that "cleared of the complication of sex, reproduction is seen to be simply the detachment of living bits of one generation, which grow up into the next." Detachment of living bits of an animal is not always as easy, however, as in these primitive animals we have been considering. Obviously such processes as fission and budding can be effective only with relatively simple creatures. The more complex the parental animals, the more awkward for them to split in two or to produce buds. When, for example, there is a permanent hard shell outside the body, or a complicated skeleton inside, the animal cannot well divide itself in two. Animals with numerous special organs and tissues cannot readily form buds in which all the special features are represented. The new generation cannot take over the complex structure of its parent but must build its own body anew. When the parent detaches a bit of itself for the purpose of reproduction, that living bit must be a germinal organism, elementary and uncomplicated but able to grow rapidly and evolve itself into an adult like its parent.

I call attention to the fact that in this last sentence we have written the specifications of an Egg.

This idea was adumbrated long ago by an ancient balladist :

How should any cherry

Be without a stone? And how should any wood-dove

Be without a bonef

When the cherry was a flower

Then it had no stone; When the wood-dove was an egg

Then it had no bone.

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The egg or ovum. We have already mentioned the simple fresh-water polyp Hydra as an example of animals that reproduce by budding. In this same animal, however, there is another kind of reproduction that occurs from time to time, in which the living part that is detached from the parent to form the new generation is not a bud, made of many cells and resembling the parent, but a single cell. As shown in the diagram (Fig. 5, left) and the photograph (Plate II, A) from time to time one of the cells in or near the surface of the animal enlarges very much and stores up materials with which it can be nourished for a while after it is cast off from the parent. This is the egg cell or ovum. The few cells that surround it where it grows on the side of the animal could be called an ovary (as we call the organ of similar function in higher animals) if it were worth while to dignify so simple and transitory a structure as the egg hillock of Hydra by considering it an organ. An egg, then, is a simple cell that is set aside by the parent and destined to divide into many cells and thus become an adult animal after the fashion of its kind. Seen in this light, reproduction by means of an egg is merely another case of reproduction by fission, in which the two living products of division are very unequal, the egg on one hand, the maternal animal on the other. If we compare a Hydra and its egg with an animal of a single cell, say a Cothurnia, that is going to divide, we see that the animalcule though an adult has also the function of an egg, for it can give rise, by division, to another animal body. In short, in one-celled animals the same cell must necessarily carry on all the functions of life, including reproduction ; in many-celled animals the function of reproduction can be delegated to special cells.

The Scholastics debated which came first, the hen or the egg. Modern biology has an answer: they were contemporaneous ; among protozoans the hen is the egg. Neither came first ; they merely became distinguishable whenever it was (the

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record of evolution has some torn-out pages at this point) that an animal first became sufficiently complex to set aside a germ cell, specialized for reproduction. Had a scholastic philosopher been present on that prehistoric occasion the only question would have been which he noticed first — probably not the egg, because it was smaller than the rest of the animal.

The sperm cell. Hydras do not, however, always form eggs ; half the time they develop not an ovary, but a testis, in which a few cells of the animal give rise by repeated division to a large number of very small sperm cells (diagram. Fig. 5, right; and photograph, Plate II, C). These cells can swim independently, when they are discharged into the water, by means of a motile tail with which each is provided. The sole function of such a cell is to swim until it meets an egg cell released from another Hydra and to enter it. When the egg is thus "fertilized" by union with a sperm cell, and then only, it begins to divide and ultimately to become a new Hydra. Herein we have the elements of sex, for this new polyp has two parents, which were not exactly alike in spite of their general similarity, because one of them furnished the egg and was temporarily at least a female ; the other, which furnished the sperm cell, was temporarily a male.


No characteristic of man and the other animals is so fundamental, so completely taken for granted, as the existence of two sexes. It is the first fact the Bible mentions about the human race: ". . . male and female created He them." In every nature myth the animals enter two by two. In primitive song and story every Jack that cracks his crown has a Jill that tumbles after. Man that is born of woman finds it impossible to think of a race with only one sex, or to imagine other sexes than two. Nor does the biologist contradict this axiom; everywhere in nature he also sees two sexes.

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Even in the lowest and simplest living things, in which it is fanciful to speak of male and female, there is (as we shall see) sexual mating or at least a process of renewal of life by the mingling of living substances.

In our day, however, science makes bold more than ever to question fundamental assumptions. The concept that space has three dimensions is as obvious as that animals have two sexes, but physicists do not hesitate to calculate in four, five, or n dimensions. We may boldly ask, therefore, why sex is necessary at all ; or why there are not several sexes. If living things must mate in order to reproduce why could not nature have arranged some other system, for example a state of sexual relativity, in which an individual might be (without any change in itself) male with respect to one potential mate, female with respect to another.? or it might take part in reproduction in response to another of its species, neither of them being either male or female. Such conjectures are not more fantastic than the concepts of mathematical relativity, with their notions of a warp in space, and of an expanding universe. Similar questions have indeed long been asked by the poets and philosophers. John Milton vigorously states an unfavorable view of the two-sex system:

why did God, Creator wise, that peopVd highest Heav*n With Spirits Masculine, create at last This noveltie on Earth, this fair defect Of Nature, and not fill the World at once With Men as Angels without Feminine, Or find some other way to generate Mankind? This mischief had not then hefalVn.


and Sir Thomas Browne, the famous physician philosopher, tough-minded as he was on many subjects, was personally

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squeamish about the whole matter of reproduction by physical contact of the sexes :

I could be content that we might procreate like trees, without conjunction, or that there were any way to perpetuate the World without this trivial and vulgar way of union.


In illustration of these roving thoughts of poet and scientist, let us return for a moment to the sea-born Cothurnia. When such an animalcule reproduces itself by division, we find it handy to call the new animal a "daughter cell," but this is only a figure of speech. The offspring is even closer than a child ; it is more truly a twin of the cell that produced it, for their relationship is exactly like that between a pair of human identical twins, which arise by the splitting of one cell, i.e. the human egg. Presently the parent Cothurnia will give off another "daughter cell" ; will that be niece or sister of the first? And if a Cothurnia is sister to its mother (i.e. the cell that produced it) is it not equally sister to its grandmother . . . and so on, as far back as the line was reproducing by simple division? We have here indeed a situation in which the terminology of the human bisexual family tree breaks down, for all the offspring of a single cell that reproduces without mating are related together even more closely than the members of a human family. They all have identically the same heredity, and are all "twins" one of another, though they may number thousands and represent many "generations." For this kind of group the biologists have had to invent a new name; they call it a clone.

Such a clone, being a group of cells that all came from one cell, may be considered something like the body of an individual many-celled animal. That too is a group of cells that all came from one cell. Like an animal body the clone seems to have its phases of youth, maturity, and old age. After

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many divisions it becomes old; the vigor of its members diminishes. Individual animals become abnormal and enfeebled, and the rate of fission slows down. The clone is in danger of extinction. It needs a shake-up, which it cannot get through the process of complete inbreeding (or rather, lack of outbreeding) by which the clone develops.

Conjugation or mating. This seems to be the reason that even in such simple animals a process much like sexual union occurs. Fig. 6 shows the mating of the one-celled animal Scytomonas. Animals of this species are not attached, like Cothurnia, but swim about in the water. As we watch them under the microscope, two individuals that are going to mate swim near each other, come into contact and actually cohere side to side. One of them loses its whiplike flagellum. The protoplasmic substance of which they are made becomes continuous from one animal to the other, and the nuclei move toward each other and unite. In this particular species the conjugated animal then becomes dormant for a time, but ultimately resumes

Fio. (i. Conjugation of the one-celled animal Scytomonas. Proceeding from A, which shows a single individual, through B, C, D, and E, two cells are seen to join, fuse their nuclei (the dark round objects) and unite into one cell. This single individual remains dormant for a time but ultimately becomes active again. Greatly magnified. After Dobell, simplified.

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activity and becomes the parent of a new clone. This process is very much like fertilization of an egg by a sperm cell in higher animals, and we can get from it a better understanding of the meaning of sexual union. For example, the late Professor H. S. Jennings (a brilliant predecessor of mine in the Vanuxem Lectures) with his fellow workers has investigated the mating habits of a very well known one-celled animal, Paramecium. They have discovered the remarkable fact that two Paramecia of any one clone will not conjugate with each other. The animal must find a mate not closely related to it. By studying an immense number of animals of the species Paramecium hursaria the investigators found that the whole population of the species is distributed into several "mating types" such that an individual of one type will mate with one of another type, but not with one of its own.

Conjugation is therefore a kind of outbreeding. To use a figure of speech taken from higher animals, it introduces "new blood" into the family, which causes an internal rearrangement of the cell materials and gives the race a new start. The "mating types," it will be noted, are not in any strict sense different sexes. As Jennings points out, in one of his examples, types A and B will mate together and type C will mate with either of them. Such an observation shows that type C is not of a fixed "sex." The situation indeed is one of relative sexuality, such as we cited above when we were trying to imagine other ways of reproduction Nature might have tried rather than the bisexual method that universally characterizes higher animals.

In some other one-celled animals, however, there seem to be only two mating types, and in many species the two conjugating types are actually somewhat different in appearance. This is getting closer and closer to bisexuality in the strict sense.

It is probable that conjugation or something like it goes on in every kind of animal, although the details are not

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always the same, and there are puzzling and obscure cases awaiting solution. At any rate the situation is clear in the higher animals, which always reproduce by the union of an egg cell with a sperm cell. Just why the whole animal kingdom, except a few of the lowest and simplest creatures, settled down so completely to the egg-and-sperm system, we can only guess. Perhaps in some early ancestral animal, not originally bisexual, it happened that some of the reproductive cells were unusually well stored with nutritive substances. This would be an advantage, for it would help to tide the embryos over the earliest stages of their development before they could feed themselves. Such cells would, however, be sluggish, and the chance of two of that kind meeting would therefore be reduced. A germ cell that happened to be lighter and more mobile would be more likely to meet the relatively sluggish cell. Once started, such a trend toward two types would progress and become fixed, by the familiar Darwinian process of survival of the fittest, and ultimately we should arrive at the characteristic arrangement, namely large eggs laden with food (yolk) and small active sperm cells.

Fio. 7. Sperm cells of various animals and man. Greatly magnified.

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Fertilization of the Egg

The fertilization of an egg by a sperm cell is one of the greatest wonders of nature, an event in which magnificently small fragments of animal life are driven by cosmic forces toward their appointed end, the growth of a living being. As a spectacle it can be compared only with an eclipse of the sun, or the eruption of a volcano. If this were a rare event, or if it occurred only in some distant land, our museums and universities would doubtless organize expeditions to witness it, and the newspapers would record its outcome with enthusiasm. It is, in fact, the most common and the nearest to us of Nature's cataclysms, and yet it is very seldom observed, because it occurs in a realm most people never see, the region of microscopic things. It is, moreover, in most animals we are likely to see, a recondite event, occurring in ponds or the sea, in the forest or the earth, wherever the creatures lay their eggs. In mammals and birds the fertilization is hidden in the depths of the body. Nor indeed are all eggs suitable for study ; they may, for example, be opaquely loaded with pigment like those of the frog. Such eggs may, of course, be killed and cut into thin slices for microscopic study, and the process of fertilization has thus been observed step by step in the prepared eggs of many species, but only a few biologists ever see the whole continuous process of union of a living egg with a sperm cell.

It need not be so rare a sight, however, for anyone who will go to a seaside laboratory in summer can witness it. The sea urchins, starfish and sand dollars which inhabit our coasts almost seem especially created to reveal the process of fertilization with utmost clearness. While writing this chapter I have before me a sketchbook made while a college student, working at the U.S. Fisheries laboratory at Beaufort, North Carolina, where I studied with amazement the finest of all these marine eggs, those of the white sea urchin, Toxopneus { 16 }


tes variegatus, first described by Louis Agassiz, and introduced to experimental biology by the cytoiogist Edmund B. Wilson. A related species is shown in the beautiful and instructive photographs here presented (Plates III and IV), the work of Dr. Ethel Browne Harvey of Princeton, to whom I am deeply grateful for the opportunity to use them. They were made at Woods Hole from a common northern sea urchin, Arbacia punctidata.

The male and female sea urchin deposit their sperm cells and eggs, respectively, directly into the sea. For purpose of study, however, it is quite readily possible to remove the germ cells from the animals before they are spawned and to bring them together in a dish under the microscope. The observer cuts open the spiny shell of a female urchin and pulls out the ovaries, slits them and catches in a dish of water the hundreds of beautiful glass-clear spheres, 0.9 mm. (0.037 inch) in diameter. A male sea urchin yields its testes, from which exudes a fluid milky with microscopic particles, each of which is a wriggling, dancing sperm cell — a tadpole-shaped object with a lance-shaped head about 0.07 mm. (0.003 inch) in length and a long tail. When the sperm cells are mixed with the eggs, they swim about rapidly until they touch the eggs, to which they adhere, several sperm cells about each egg, trying to push into its substance. When one sperm cell has actually penetrated the egg (Plate III, A, B) it causes the surface of the egg cell to be rapidly congealed into a thin membrane, something like the scum on a cup of cocoa. By this means, the other competing sperm cells are effectively prevented from entering.

Meanwhile the observer will have noticed the nucleus of the egg (Plate III, A), a, rounded body about one-sixth the diameter of the whole egg cell, eccentrically placed near one edge and enclosed by a delicate nuclear membrane. This nucleus is the goal toward which the sperm cell is moving, and

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the object of the whole process is to secure fusion of the sperm cell with the egg nucleus. The tail of the sperm cell is broken oif and left behind. The head now advances through the eggf swelling slightly as it goes. In the egg substance a star-shaped aster or region of stiffened egg substance appears and travels with the sperm nucleus. The egg nucleus advances to meet the sperm and within ten minutes from the time the sperm cell enters the egg, the two nuclei have united and blended their substance (Plate III, C). The egg is now fertilized; it inmiediately prepares to divide, and while the observer watches, entranced with the smooth inevitability of these events, the divisions follow one another every twenty-five minutes, so that one cell becomes two, the two become four, eight, sixteen, thirty-two, and so on until the embryo is a mass of small cells looking like a mulberry. All these events are shown in Plates III and IV. We need not follow the fertilized egg through the subsequent complicated changes and metamorphoses by which it becomes an adult sea urchin.

The meaning of fertilization. If the eggs are left alone in the dish, they do not go ahead by themselves and turn into sea urchins. Development cannot begin until after the entrance of the sperm cell and the fusion of the nuclei. Evidently the sperm cell in some way is necessary to set off or stimulate division of the egg. A great step toward understanding what happens was made by Jacques Loeb in 1899 and 1900. Loeb had been studying the effect upon life processes of changing the amounts of certain minerals which are present in living tissue. By increasing or decreasing the concentration of magnesium or calcium in sea water, for example, he could speed up or slow down the rate of division of fertilized sea urchin or starfish eggs. This led him to try dilute solutions of magnesium chloride on the unfertilized eggs, completely free from sperm cells. Eggs so treated began to divide and when carefully handled often went on to form complete larvae. In later

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Plate III. Fertilization and segmentation of the egg of the sea urchin Arbacia, as seen in eggs sectioned and stained for microscopic examination. A, sperm cell about to enter egg. B, sperm nucleus (small black-stained object) approaching egg nucleus. C, sperm nucleus (black object) fuses with egg nucleus, 10 minutes after entry of sperm cell into egg. D, nucleus of fertilized egg begins to divide. E, division in progress. The black-stained objects in a row at center are the chromosomes. F, nuclei of the two daughter-cells re-forming. G, stage of two cells. H, the first two cells now divide again. I, stage of four cells. All magnified 375 diameters. Prepared and photographed by Ethel Browne Harvey.



Plate IV. Segmentation of the egg of the sea urchin Arbacia, photographed while living by Ethel Browne Harvey. A, fertilized egg. B, beginning of segmentation. C, two-cell stage. D, four cells. E, eight cells. F, sixteen cells. G, "mulberry" (morula) stage. H, hollow embryo (blastula) . Ij young adult sea urchin. A to H, magnified 290 diameters; /, almost natural size.


experiments by others, such fatherless young have actually been raised to adult life, and to all appearances were normal specimens of their kind.

Lest this experiment should seem to disparage the importance of the father, we should mention that the contrary experiment also succeeds. If an egg is cut into two pieces, one of which has no nucleus, and the latter is then entered by a sperm cell, it too will divide and become an embryo, though admittedly not as often as in the other, less drastic experiment. In this case the embryo is motherless, from the standpoint of heredity, for it has no egg nucleus in it. This shows that egg stuff, to develop, must have a nucleus and requires to be stimulated, but either an egg nucleus or a sperm nucleus will do. We shall see later, however, that for reasons that concern heredity it is decidedly better for the offspring to get its nuclear material from both parents, as normally happens.

This remarkable experiment of artificial parthenogenesis ("virgin generation") as it is called, has been repeated on many kinds of animals, and it has been found that not only magnesium solutions, but quite a number of different stimuli will start division of the eggs. Exposure to sperm cells of other species, extracts made from dead sperm cells, various dilute acids and alkalies, sudden cooling, heating, shaking, or pricking the eggs all can be used to initiate development in one species or another. Mere staleness will cause the eggs of some animals to divide. In all probability these diverse stimuli produce some sort of common effect on the cell substances, setting up internal changes (not as yet well understood) that start the processes of division and growth of the egg. The point of interest for us is that when the sperm cell acts upon the egg in this way, it is exerting a merely physical or chemical effect. The fact that it is itself a living cell is more or less incidental. The egg contains all the essential elements for

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the production of a perfect animal, and needs only to be given a start/

The eggs of mammals, including the human species, probably do not differ in this respect from those of sea urchins and starfish. Because they are far harder to get at and less resistant to handling and exposure, experimental study has not progressed very far. In very recent years, Gregory Pincus and his associates at Clark University have worked with rabbits, using an experimental method in which the unfertilized egg was subjected to drastic cooling while passing through the oviduct (Fallopian tube). A few such eggs, subsequently replanted into other rabbits, are said to have formed embryos, to have been born, and to have grown to normal adult life. H. Shapiro of Philadelphia has still more recently reported starting development of the rabbit's egg by drastic refrigeration of the whole body of the female rabbit, but up to the present none of these eggs has developed beyond the earliest embryonic stages.

I hasten to add that under ordinary circumstances, when there is no meddling by an experimenter, mammalian eggs live in a perfectly conditioned environment. The temperature and all other conditions to which they are subjected in the

1 In a brief chapter like this, in which I am deliberately selecting those features of the natural history of reproduction which best lead up to the higher animals, it is not possible to follow out all the ramifications of the subject. Life processes are so richly varied that every general statement calls for a bill of exceptions. There are, for example, many animals that can produce parthenogenetic eggs, i.e. eggs that develop spontaneously without fertilization. This is the case in a great many insects. In some of these instances, no doubt, a stimulus akin to that of fertilization is furnished by natural conditions, such as high temperatures, desiccation, or chemical changes within the egg, but in others there is no known special stimulus. Indeed, when we reflect that a tendency to propagate by division is innate in almost all animal cells, the wonder is that in most species the eggs do have to be stimulated in order to develop.

Incidentally, even in the insects and other animals with parthenogenetic generations, sexual reproduction always occurs from time to time to rejuvenate the line and start new clones, just as in one-celled animals. In all vertebrate animals sexual reproduction is obligatory.

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body are so closely regulated that parthenogenesis of the sort described by Pincus would not be possible.

Heredity. We have not yet told the whole story of fertilization. Mere stimulation of an egg to develop, necessary as it is, is not all the sperm cell does. It has another vastly important task, which is to carry into the egg the male parent's contribution to the heredity of the offspring. Packed in the nucleus of the egg and in the nuclear head of the sperm cell are the submicroscopic chemical particles that control the inherited characteristics of the species; when the sperm cell unites with the egg, the nucleus of the fertilized egg acquires an equal share of this controlling material from each parent. When the egg divides, these determiners are distributed to the daughter cells at each division and thus are carried into all the cells of the embryo. This is the way the sperm brings in "new blood" and rejuvenates the cell lineage, as happens in the one-celled animals by means of conjugation. In this way, moreover, two family lines are blended, and special traits of bodily build and behavior are exchanged and distributed, so that the young are never quite identical with either parent.

This blending and assortment of hereditary characters is the object and goal of sexual reproduction. Whatever else follows in this book is merely the story of the arrangements and devices of nature to assure the meeting of egg and sperm cell and to protect the embryo that they produce.

How the determiners of heredity that can shape the whole body of a man or woman, and then bequeath themselves to another generation, are packed into the small compass of egg and sperm cell, how they are distributed to the cells of the body by processes of almost geometrical precision, how they can be traced and how they guide the building of the body — this is the subject matter of the science of genetics, one of the grand divisions of modern biology. It is not to be a theme of the present book. Those of my readers who have

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studied biology have mastered at least the rudiments of genetics and the lore of the chromosomes. Unfortunately, this important and beautiful science is almost impossible of explanation to those who have not seen animal and plant cells through the microscope. Several writers have made brave attempts to do so, and the reader is referred to their books.* For us let it suffice that egg and sperm cell join ; we shall not attempt here to see what goes on within them.

Meeting of the germ cells. The eggs and sperm cells of such simple and small animals as Hydra are discharged directly into the water, and the sperm cell swims to the egg. As animals become larger and more complicated, the gonads (ovaries and testes) are built more deeply into the body. Some sort of opening or channel to the surface is then provided. The ovaries and testes of sea urchins, for example, open through the shell by small pores.

When fertilization depends upon the chance meeting of eggs and sperm cells, or upon such uncertain aids as tides and currents, there is obviously a great risk of failure to make contact. To compensate for this, and also for the subsequent high loss of embryos, due to enemies and unfavorable conditions, an enormous excess of germ cells is usually produced. There would obviously be greater economy and safety if some arrangement were made to bring the germ cells together or to put them near each other in the first place. A fantastic variety of such arrangements is seen in nature. Animals that are sluggish like the sea urchins and starfish, or actually fixed in position, like most shellfish as well as the sponges, corals and many ascidians (of which the "sea squirts" are examples) are often aided, as we said above, by tides or other currents. In higher plants, which are not only rooted to the

2 H. G. Wells, Julian S. Huxley and G. P. Wells, The Science of Life, London, 1929; Charles R. Stockard, The Physical Basis of Personality, New York, 1931; Alan F. Guttmacher, Life in the Making, New York, 1933; A. M. Scheinfeld and M. D. Schweitzer, You and Heredity, New York, 1939.

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ground, but have male germ cells that cannot move of their own accord, the winds or the insects transport the pollen.

Most of those animals that are free to move mate by propinquity. They can at the very least deposit their eggs and sperm cells at the same place. This is the case in many fishes, in which the male and female place themselves close together when they spawn, so that the sperm cells are deposited upon the eggs. Frogs and toads provide an even better chance of contact between the germ cells, for in the mating season the male instinctively clasps the female with his fore limbs and the two animals remain in close contact for days, until the eggs are discharged, whereupon the sperm cells are deposited directly upon the eggs. In the tailed amphibians (such as newts and salamanders) sperm cells are not discharged externally at all. Like all other vertebrates below the mammals (i.e. fish, amphibians, reptiles, and birds) these tailed amphibians have a combined cloacal passage into which both the intestinal and the genital canals open. The openings of the cloacas of the male and female are placed so close together in mating that the sperm cells pass directly from one to the other. They then pass up into the oviduct and fertilize the eggs there. Much the same process occurs in many birds, for example the common fowl and their kin.

An obvious advance is the development of an organ for direct transmission of the sperm cells. The elasmobranch fishes (sharks and rays) possess specially modified anal fins, called claspers, which are grooved so that the seminal fluid containing the sperm cells is guided along them from the cloaca of the male to that of the female. This particular method of solving the problem is, of course, not available to land animals, since they have no fins. In snakes and lizards there are saclike branches of the cloaca that can be turned inside out and protruded into the cloaca of the female, carrying with them the sperm cells. In other reptiles, namely turtles and crocodiles, and in many birds the final solution was

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attained and has become standard in mammals. This is a special male organ, the penis, adapted to insertion into the female genital tract. In female mammals the lower end of the genital canal is expanded into a special canal, the vagina, which receives the penis. The sperm cells can thus be safely placed well within the reproductive system of the female. In both sexes, in mammals, the intestinal outlet is separate, leaving the genital outlets (penis and vagina respectively) associated only with those of the urinary system.

In the higher vertebrates, then, the eggs leave the ovary and pass down the egg ducts. If mating occurs, sperm cells are put into the vagina (or into the cloaca in reptiles and birds) and travel upward to meet the eggs in the oviduct and fertilize them there. What is to be done next with the eggs? Turtles coat them with a parchment-like shell (secreted by the lower end of the oviduct), lay them, and bury them in the sand. Birds provide a hard shell and put them in a nest. Mammals do much more for their fertilized eggs — they keep them in the mother's body and develop them there. We shall have to study in later chapters the elaborate arrangements necessary for this process of gestation.


Although the development of the young within the mother's body is characteristic of the mammals, and is most highly perfected in that order of animals, it is by no means unknown in lower orders. In some of the mollusks, for example, the embryos are kept within the shell of the mother until their development is well advanced. The European oyster thus long retains its embryos in the gill chamber. The viviparous fish which have become popular in home aquariums in recent years raise their young in the oviduct. Some fish actually retain them in the ovary itself. The young fish, which are very small, live on the yolk that was in the egg from which each one sprang, and in some species probably absorb nour { u }


ishment from the tissues of the mother, but they are not actually attached to her. In some species of dogfish, the lower part of the egg duct is expanded into a special chamber. In this the embryos are retained. The lining of the chamber is thrown into a mass of finger-like projections, between which grow similar projections from the belly wall of each embryo. Nutriment brought to this zone of interlacement by the blood vessels of the mother filters through the coverings of the two sets of projections into the blood vessels of the embryo, as moisture filters into the roots of a tree. Much the same arrangement prevails in the viviparous snakes.

In mammals the brood chamber is more than a mere dilatation of the oviduct. It becomes a special organ, the uterus, which has thick muscular walls, to enable it to withstand the distention produced by large embryos during weeks or months of development, and afterward to expel them when the time comes for their birth. The attachment between mother and child (the placenta^ to be described more fully later) becomes very intimate and very effective in transmitting nutritive substances to the embryo and carrying waste products away. Instead of being thrust into the outside world as an unprotected egg, the mammalian infant is sheltered and nourished in the uterus for a long time — three weeks in the mouse, four months in the pig, nine months in man, two years in the elephant. Even after so long a period of gestation, when it enters the world it is still dependent upon the body and secretions of the mother, for it cannot do without the milk she provides for its nourishment.

A modest word about the father may be in order at this point. It will perhaps seem from our sketch of his biological function that in all the various races of animals his duty and usefulness are done when he has put his sperm cells where they can reach an ovum, serving thus to set the mechanism of the egg into action and to contribute his equal share to the heredity of his offspring. The rest takes care of itself in lower

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animals, and in the higher orders seems a task for the female alone. There is indeed one species in which the male animal plays no other part at all in life than this — the marine worm Bonellia, famous in biological lore, in which the male is nothing but a very small parasite on the gills of the female. She carries this petty creature about with her for the sole purpose of getting the eggs fertilized. Yet he cannot wholly be dispensed with, however brief his moment. In human history a not dissimilar career has been that of certain prince consorts of masterful queens.

Such is not altogether the case in mammals. The very fact that gestation is a heavy burden, putting the female at a disadvantage in the struggle of life, while she is carrying her young and afterward mothering them, gives the male parent another task — that of protector and leader of the family. This finds biological expression in the fact that in almost all mammals the male is larger, stronger, and fiercer than the female (Rudyard Kipling to the contrary notwithstanding). In the human race the mother's burden is heaviest of all, and by that very fact the father becomes again biologically useful to his offspring during the long period of gestation and infancy, as guardian and provider of food and shelter.

Meaning of Sex for Human Beings

The gist of our preface to human reproduction is that our own species and most of the others, high and low, reproduce themselves by the production and union of eggs and sperm cells. To get this essentially simple task done in the highest animals requires the functioning of an elaborate set of organs. In order to tell the whole story of reproduction in man and the higher animals we shall have to discuss :

The anatomy of the ovary and testis and the formation of eggs and sperm cells,

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Transportation of the egg from ovary to uterus, and of the sperm cell from testis to the egg.

The union of the germ cells,

Development of the fertilized egg.

The attachment of the embryo to the mother, and its nutrition,

Birth and the nursing of the young.

These are complicated matters, which must be timed so that each stage fits into the next. What goes on in one organ must be coordinated with events in another. The chemical environment of the egg and the sperm cells must be kept in adjustment to their needs ; muscle cells in the oviduct, uterus and the male reproductive system must be ready to act when required; the lining of the uterus must be prepared for the embryo; in short, a whole complex system of organs and tissues must work as a unit.

The body has two important ways of linking the action of its separate organs. One of these is the nervous system, through which run innumerable signals connecting the organs of sensation and motion and regulating many functions of the internal organs. The other, with which we are much more concerned in this book, is the system of the chemical messengers or hormones. A hormone is a chemical substance made in one of the special glands called ductless glands or glands of internal secretion, among which are the pituitary, thyroid, parathyroid, adrenal, and parts of the pancreas, which do not discharge their product through a duct to the outside of the body or into another organ, as for example do the sweat glands, the salivary glands, the liver and the kidney. Instead, these endocrine glands (for such they are also called) put their respective secretions into the blood as it courses through the blood vessels which pervade their substance. The hormones are thus carried all over the body and

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reach the various organs and tissues which each of them is respectively destined to affect. The hormone of the thyroid, for instance, influences the utilization of oxygen by the tissues ; the pancreatic hormone (insulin) regulates the combustion of sugar ; adrenin affects the blood pressure by causing the arteries to contract. Just how each ductless gland produces its own special secretion, and why certain particular tissues, and these only, respond to a given hormone, are questions which must be solved by the physiologists and the chemists of the future.

The ovaries and the testes are also glands of internal secretion, or to put the case more precisely, they include such glandular tissue among their complicated make-up. The hormones made in the sex glands perform the function of linking the action of the various organs of the genital system, timing and regulating their activities. What these hormones are, where they are produced, and how they act to accomplish the bodily tasks of human reproduction, is the theme of this book.

One more item should be added to the list of reproductive activities cited above. This is the sexual urge, the totality of impulses that serve to bring the sexes together for mating. It is the most important coordination of all, for without the union of the sexes all the other intricate processes are useless. It, too, is partly regulated by the hormones, but we know too little about this as yet to discuss it profitably here. The way of an eagle in the air, the way of a man with a maid . . . are still in part beyond the reach of science. In view of the fact that we are still ignorant of the means by which the simplest one-celled animal is impelled to conjugate with another of its kind, we can only wonder at the complexity of sex psychology in the higher animals, and at all the lures that nature has provided to insure the union of the sexes. What marvels of color and fragrance, bird song and firefly radiance, have been lavished to this end! and for mankind what emotions are bound up with it, of young romance and mature devotion, hope and fear, selfishness, slyness and cruelty. To the fanatic, sex is a snare of the devil, to a Casanova a heartless game ; to Stephen Dedalus as a young man it was torment ; to some happier lad, it is a rosy dream. All these have much to gain by seeing it also, with the biologist, as part of the inevitable process of animal life. To understand is not to demean ourselves, nor to rob the human heart of virtue and the love of beauty.

Cite this page: Hill, M.A. (2021, October 17) Embryology Book - The Hormones in Human Reproduction (1942) 1. Retrieved from

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