Meyer - Essays on the History of Embryology 9
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Meyer AW. 1932 - Essays on the History of Embryology: Part I | Part II | Part III | Part IV | Part V | Part VI | Part VII | Part VIII | Part IX | Part X | Part XI | Arthur Meyer | Historic Embryology Papers
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Essays on the History of Embryology IX
By A. W. Meyer, M. D.
- This is the ninth paper of a series of essays on this subject. Previous papers were printed in this journal as follows: Part I in December California and Western Medicine, page 447; Part II, in January number, page 40: Part III, in February number, page 105; Part IV, in March number, page 176; Part V, in April number. page 241; Part VI, in May number, page 341; Part VII, in June number, page 394; Part VIII, in July number, page 41.
It is easy to understand Bonnet’s enthusiasm over Spallanzani’s experiments on cross-fertilization as expressed in the following words from his letter of January 13, 1871:
“You are now in possession of a sure and easy way of ascertaining what species can procreate together; and the experiments you propose attempting next spring, by putting your voluptuous spaniel in the company of cats and rabbits, promise not so fair as those which you will make, by introducing the semen of this spaniel into the uterus of a doe-rabbit and a she cat, and on the other hand, by introducing the semen of the male rabbit and cat into the uterus of a bitch. You hold in your hand a precious clue, which will guide you to the most important and unexpected discoveries. I know not, whether what you have now discovered, may not one day be applied in the human species to purpose we little think of, and of which the consequences will not be trivial. You conceive my meaning: however that may be, I consider the mystery of fecundation as nearly cleared up.”
Bonnet surmised that,
- “From your numerous experiments on artificial fecundation it appears that attempts of this kind upon germs, while yet in the ovarium, or at the upper end of the ducts, will fail. I can, I think, assign the reason. The seed acts on these fetuses as a simple stimulant. Now there is an original relation between the latent power which causes the irritability of the contraction of the muscular fibre. If it has not yet attained the necessary degree of consistence, it will not be in proportion to the mode of action of that power, and its impression will therefore produce no effect. The germ must have arrived at a determinate growth before it can be susceptible of irritation. Such was the reasoning of the great Haller.”
Although Spallanzani was a preformationist and believed that the amnion and umbilical cord exist in an invisible form, before fecundation, his investigations on fertilization rank very high indeed. They are outstanding for their number and ingenuity. The modest title of the two volumes, the one on plants and the other on animals, give little intimation of the many splendid experiments recorded there. In the introduction Spallanzani says that the subject of his second dissertation is “Artificial Fertilization,” the first outlines of which may be seen in his “Prospectus Concerning Animal Reproduction.” By artificial fecundation Spallanzani means insemination, of course. He says, “This was accomplished by means of the seminal liquor of the animals themselves; and I have succeeded as well as if the male himself had performed his proper function.”
It is strange that by his experiments on plants, Spallanzani was convinced that the “fecundating dust,” or farina foecundans as it was called, that is pollen, was “not so absolutely necessary as botanists in general suppose.” However, he had an entirely correct attitude toward the problem of fertilization for he added, “It is said by many that fecundation is among the mysteries of nature; and like many of her operations, an object of admiration, rather than of inquiry. Such an opinion is highly agreeable to the idleness of man.” He repeated the experiment of Swammerdam and Roesel von Rosenhof and found that mating frogs will discharge their sexual products even when confined, thus contradicting the statements of Linnaeus and Vallisneri in this regard. Vallisneri, it seems, held that the female frog will not lay eggs when separated from the male, but Spallanzani showed that she will do so if the separation is effected after the eggs have descended into the cavity of the uterus (oviduct), but that they will be sterile. Since it is now well known that the female frog will not lay eggs if wholly unmated, Vallisneri was right. The frogs Spallanzani used must have mated long enough for ova to descend into the oviducts, when expulsion instead of absorption will take place. Since the duration of mating seems to depend much upon temperature, and since the period is relatively long when the weather is cool, Spallanzani must have obtained females in which mating had been interrupted.
Spallanzani confirmed the fact that the ova of frogs are fertilized outside of the body of the female and emphasized “the falseness of the strange opinion of Professor Menzius that the seed is emitted from the fleshy prominence of the toe and passing through many windings unknown to us, penetrates into the thorax, and then impregnates the egg.” This idea that the male sperm enters the female through the clasping or nuptial pads of the front feet of the male was probably suggested by the tetanoid clasping of the female by the male and because many observers had failed to see the emission of sperm observed by Swammerdam. Some of the older observers believed that the female sometimes is strangled during mating in species in which the fore limbs of the male clasp her in front of her own fore limbs, instead of behind them.
In connection with these experiments Spallanzani states that the Abbé Nollet, in a letter written to him eight years before, said:
- “What you say of the existence of a tadpole, before we can perceive any act of fecundation, has particularly struck me. About thirty years ago Mr. Réaumur and myself made many researches relative to this subject. We attended for weeks very diligently and patiently, to what passed while the male embraced the female. I remember putting britches of wax taffety on the male and watching a long time, without perceiving any appearance that denoted an act of fecundation.”
In commenting on this quotation from Nollet, Spallanzani wrote:
- “I will not here stop to inquire by what fatality it happened, that two naturalists, so intelligent and attentive, failed in their inquiries. The idea of britches, however whimsical and ridiculous it may appear, did not displease me and I resolved to put it in practice. The males, not withstanding this ineumbrance, seek the females with equal eagerness and perform, as well as they can, the act of generation; but the event is such as may be expected, the eggs are never prolific for want of having been bedewed with semen which sometimes may be seen in the britches in the form of drops. That these drops are real semen, appeared clearly from the artificial fecundation that was obtained by means of them.”
By paying “unremitting attention to every phenomenon” during the mating of frogs and toads and newts, Spallanzani observed the ejaculation of sperm by the male and carefully noted the developing eggs, but unfortunately came to the conclusion reached before him by Swammerdam, that the pigment in the frog egg represents the preformed tadpole. He described and pictured this tadpole as Swammerdam had done, and indicated how the one gradually changed into the other. He was firmly persuaded that the globules of two colors surrounded by mucus, were eggs, as all who had written on frogs, as Jacobaeus, Vallisneri, and Roesel had said, and rightly declared that since “greater deference was due to what nature showed so plainly, than to the authority of the most celebrated writers, it is fit to call these globules tadpoles or fetuses instead of eggs, for it is improper to name any body an egg, which, however closely it may resemble one, takes the shape of an animal without leaving any shell, as is the case with all animals that come from an egg.”
Spallanzani exposed tadpoles to the sun and found that most of them died when the temperature reached 35 degrees centigrade. He tested the intensity of the sexual libido of the male frog by ablating different portions of the body and finding, to his amazement, that even a decapitated male will return to clasp the female if he is placed in contact with her. He found that he will continue to clasp her until death from hemorrhage releases the tonic contraction of his fore limbs. He also boiled and treated fertilized ova with vinegar and ascertained the effect of stagnant and replenished water upon them and tried the experiment suggested by Bonnet “the philosopher of Geneva,” of drying fish eggs for a time to see whether they could subsequently be developed, but he found that this would not happen, thus discrediting the story told by Bonnet to the effect that large numbers of fish suddenly appeared in a dry pond with the onset of rain.
Spallanzani experimented also with tree-frogs and tried to determine whether “the gross visible part of the seed be necessary to the fecundation of man and animals, or, whether the invisible attenuated part usually called the seminal vapor or aura, be destined to this purpose.” He says this is a very ancient question which still was debated in his day and calls attention to the fact that those philosophers and physicians who favor this idea “are obliged to maintain it from a sort of necessity rather than by any direct reason of experiment.” He then considers the various arguments and says that he thinks they are insuflicient to decide the matter and hence outlines a crucial experiment. Before telling of the results of this experiment he refers to the fact that “a drop of water, one~fiftieth of a line in diameter, taken from eighteen ounces of water, with which three grains of seed are mixed, is capable of impregnating a tadpole,” and adds:
- “This experiment is apparently favorable to the seminal aura, which, in the general opinion, is nothing but the vapor of the seed exceedingly rarefied. The facts, however, which I shall adduce, clearly prove the contrary. That I might moisten tadpoles abundantly with this exhalation, I put a quantity of seed, amounting to eleven grains, taken from several fetid toads, into a watch-glass. In another somewhat smaller, I placed twenty-six tadpoles, which were fixed very firmly to the bottom by the tenacity of the gluten. I then inverted it over the former, and in this situation both glasses were left five hours in my apartment, where the liquor of the thermometer stood at 18°. The semen lay exactly under the tadpoles, and they could not but be involved in the rising vapours, for the distance was little more than a line. Upon inspecting the tadpoles at the expiration of the fifth hour, I found them so much covered with moisture, as to wet my finger when I touched them; the moisture was the evaporated part of the seed, which had lost one grain and half. The tadpoles, therefore, had been bedewed with one grain and half of the seminal aura, for it cannot be supposed to have escaped out of the glasses, they fitted so closely. The tadpoles, notwithstanding they were placed immediately in water, and left there several days, all perished.
- “Though this experiment is unfavorable to the aura, yet it stood alone, and I could not avoid further enquiry. One grain and a half ought indeed to fecundate many thousands, much ‘more twenty-six. I however chose to increase the quantity of vapour, which could be done by only increasing the heat. Placing everything exactly as in the former experiment, I set the glasses in the window, where the heat of the sun, being moderated by the glass through which it shone, amounted to 25°, and could not therefore be prejudicial to fecundation. In four hours the spheres were so covered by the vapor, that drops were seen hanging from them. But the effect produced was the same as before.
- “I repeated this experiment once more, not so much with a view of confirming the former results, as to see whether the seed, after part had been resolved into vapour, retained its efficacy. After the tadpoles were moistened with the exhalation, half of them were put, as before, into water, all which came to nothing. The other half were bedewed with a little of the residuum, and then put into water; they almost all came to perfection. These experiments shew, that the vapour of the seed of the fetid toad is incapable of impregnating the young, and that the seed, after a, considerable evaporation has taken place, is still efficacious.
- “Both these consequences were confirmed by subsequent experiments. The space between the tadpoles and the sperm was about a line: that the vapour might be more efficacious, I reduced it to one-third of a line, but still to no purpose. I have already observed that by the aura seminalis the vapour of _the seed is generally understood. Some physiologists think that this exhalation consists of the odoriferous particles of that fluid; others that it is the most attenuated part, and others again that it is a very subtil spirit. Whatever it be, it is certainly incapable of producing fecundation. Yet as so subtil a spirit might be thought to escape at the _meeting ‘of the glasses, I determined to obviate this suspicion by cementing the edge of the upper‘ glass to the inside of the other. I moreover substituted a little glass funnel in the room of the upper watchglass, and cemented it as before; the small orifice was hermetically sealed. The tadpoles were fastened to the neck, and the surface of the seed was enlarged that evaporation might be the quicker. I thought that the conical form of the funnel would collect the aura into the point where the tadpoles lay. This new apparatus was kept six hours in a heat of 26°, and the fetuses were surrounded continually by the vapour, but the event was still the same and the residuum was also efficacious.
- “When I tried the effects of the aura in open vessels, where the air had free access, I found it just as ineffectual as ever.
- “My last experiment made with this view was to collect several grains of the evaporated fluid. I then immersed twelve tadpoles in it, and left them several minutes. Twelve more were touched with the residuum, which did not exceed half a grain, but eleven of these grew to perfection, but not one of the others.
- “These various facts concur to prove, that fecundation in the fetid toad is not the effect of the seminalis, but of the sensible part of the seed. But my enquiries have been extended further; the abundance of the toad with red eyes and dorsal tubercles, and of the green aquatic frog, has afforded me ample opportunities of repeating upon them the experiments described in paragraphs CLXII, CLXIII, CLXIV, CLXV. But the aura appeared on all occasions incapable of producing fecundation. My few trials upon the treefrog agree exactly with the others.”
These experiments finally ended the fable of the existence of a seminal aura, and of impregnation at a distance and of the old simile that “animals conceive young as the brain conceives a thought.” Cole quotes Gautier d’Gouty in connection with this idea of Aristotle, Harvey, and others to the eflect that Harvey apparently was writing for readers who already knew how a thought is formed, but what applies to Harvey applies also to others.
According to a letter to Spallanzani from Bonnet, “the famous Malpighi first imagined" the possibility of artificial insemination; that is, fecundation or fertilization, as they called it; and "the second Malpighi first succeeded in it.” The animal concerned was a dog, and Ma1pighi’s success in this instance stimulated Spallanzani to repeat this experiment. He also exhorted others to do so and to try hybridization as well, pertinently adding, “Reasoning will not illustrate this subject; experiment alone can supply the information we desire.”
Although Spallanzani thought that sperm entered the egg through preéxisting pores, and energetically championed preformation, he nevertheless was a great experimentalist who did crucial investigations on the question of spontaneous and sexual generation. In spite of his errors he was cautious and declared:
- “From these conjectures we may be willing to believe, that it is the same in the species that have not been examined. But we can not be certain without experiments. . . . On many other occasions, when the law was supposed to admit of no exception . . . logical arguments have been found to disagree with experiment. . . . Daily experience shows, that in an immense number of animals, fecundation takes place within the body of the female. It might perhaps be thought, that we are warranted to conclude, from analogy, that this is an universal law of Nature. And it has accordingly been admitted as such by vulgar reasoners. But as on many other occasions, when the law was supposed to admit of no exception, so on this, analogical arguments have been found to disagree with experiment. Swammerdam first shewed, that impregnation is effected without the body of the female in one species of frog; and Roesel extended this discovery to another amphibious animal of a similar kind. I have had the satisfaction to discover this external fecundation in other species of frogs and toads, and have, moreover, clearly beheld the fecundating liquid issuing from the male, and falling on the fetuses (ova), after they were expelled from the uterus of the female.” , When he refers to the idea of Linnaeus that the female fish is fertilized by following the male and devouring the sperm as the former discharges it; to the opinion of Buffon who asserted that they copulated, and to Haller who gave reasons to the same effect, Spallanzani adds that he cannot “think them [the statements] decisive, as they are destitute of facts, by which alone the problem can be solved.” He continues, saying:
- “The mode of fecundation in fishes is very extraordinary, according to Linneaus; he supposes, that the female pursues the male while he is emitting the semen, and devours it, and thus is impregnated. In the time of Vallisneri, there lived at Rome a physician, who taught that pigeons, sparrows, and many other animals, were fecundated by the mouth. Both these opinions are palpably false. Female fishes have, indeed, been observed to swallow the semen, not because it then serves to impregnate them, but simply for food. The male devours it with equal greediness for the same reason. The same observations are applicable to the eggs.”
Spallanzani’s many experiments on the effect of dilution of sperm until, as he said, it was “almost infinitely divided,” were well conceived. They demonstrated that the fertilizing power of sperm was not lost through dilution, and some of his filtration experiments brought him similar proof. However, he was not deceived by these because he found that the fertilizing power of sperm was destroyed after filtration through several thicknesses of cloth. From this it followed that the animalcules were necessary for fertilization and that the fluid alone was insufficient. It would be unjustifiable to conclude, however, that Spallanzani even remotely surmised the mechanism of conjugation of sperm and ovum, still commonly spoken of as fertilization. His experimental work with sperm definitely established the fact that an aura spermatica does not exist and that the spermatozoa, as von Baer later called them, were indispensable agents in fertilization. These were important advances.
Since Spallanzani was a preformationist and ovist, he regarded the spermatozoa as “animating the heart of the embryo,” as his friend and admirer Bonnet put it. A letter of Bonnet’s indicates that Spallanzani mixed urine, vinegar, saliva, extracts from the liver and lungs, kidneys, and so forth, with sperm to observe the effects upon its fertilizing power. He successfully repeated artificial insemination of the dog and tried artificial cross-insemination of cats and rabbits with sperm from a spaniel. Spallanzani’s own ideas regarding this matter were expressed in the following words:
- “What remains principally to be discovered, is the formation of the mule, and what occasions the different marks of resemblance between children and their parents. Meanwhile, I pass on to the artificial fecundation of quadrupeds, concerning which I will observe to you, that I feel the greatest satisfaction at finding that my sentiments do not differ from those of your sovereign. When I succeeded in the artificial fecundation of a bitch, meditating with surprize upon my discovery, I conceived that it might be an excellent way to procure, if the thing be possible, different sorts of strange mules, an idea in which my illustrious friend Mr. Bonnet, to whom I usually first communicate my experiments, concurred. Hence I resolved to provide myself, at my convenience, with a number of female quadrupeds, as cats, bitches, rabbits, and to try to fecundate them with the seed of some different species, at the ,season of their amours. I likewise communicated this idea to Mr. Rossi, a celebrated Professor in the University of Pisa, that he might put it in practice: Dr. Rossi, as you perhaps know, is the naturalist, who last year repeated with success my experiment on the artificial fecundation of a bitch.
Towards the middle of last November, when I returned to the university of Pavia, I procured two cats, one two years old, which had once brought young, the other eleven months old, and which had never produced. Both had the liberty of my chamber, but they could not get out, nor was any male ever permitted to enter: there was also in the same apartment a little spaniel three years and a half old, the same which had furnished the prolific liquor that fecundated the bitch. The older cat was first in season; this fell out on the 3d of December; incessant loud cries, sufficiently expressive of her wants, afforded a clear proof, that she now began to seek and to invite the male. Being unable to satisfy her desires by means of a male of her own species, and being by nature, like all other female cats, exceedingly salacious, she for the present forgot her antipathy to the dog-kind, and did not hesitate to approach the spaniel, and to invite him, by stroking his belly, and reiterated caresses; but he, without either hurting the cat or flying, never consented to her wishes, though he was of a very voluptuous disposition; he would smell her, and then turn with indifference another way. The third day after the appearance of these signs, I tried .to fecundate my cat artificially with twenty-two grains of seed, furnished by the same dog. The same means and precautions were employed as in the experiment on the bitch. But having observed that the females of this species receive the male many times, I was not satisfied with a single injection, but repeated it thrice more before the cat went off her heat, which happened on the 11th of December; I kept her confined along with the other, as in a former experiment.
“You may conceive my anxious expectation of the result of this unattempted experiment. Should any one of my injections prove prolific, and should the young partake, both in form and manners, of the female which conceived them, and the male that furnished the seed, I fancied, that the most singular mules, and such as had never been before seen, would now be produced. With respect to manners, two most opposite natures would be kneaded together and be confounded; the one, that of an animal susceptible of education, full of courage, abilities, and sentiment, all ardour, all affection, all obedience to his master; the other, that of animal in internal qualities, far inferior, by instinct intractable, abhorring all subjection, faithless to its owner, affectionate only through interest, and born with an irreconcilable enmity to the former. Nor would the nature of these two animals engrafted together, be less different in a physical point of view, whether we consider the external configuration, the proportion of the limbs, or the internal organization. But unhappily this was an occurrence not easy to be brought about by the experimenter, and in which his labour is not crowned with success. The cat, notwithstanding all my care, was not fecundated. I was not, however, discouraged by this failure from repeating my attempt with the seed of the same dog and the same precautions, upon the other cat, which began to be in heat on the 18th of January, and instead of four, I injected seed seven times; that is to say, once every day as long as the season of her amours lasted. At each injection I did not introduce less than eighteen grains of seed; but impregnation did not take place; for from the last injection to the date of this letter, thirty-two days have elapsed, and there does not appear the least intumescence of the cat's belly. This is also the case with the other, though the experiment was made so long before. We know that these animals bring forth in about fifty-five days, and bitches in sixty-three at farthest.
“I would not, notwithstanding, pronounce the attempt impracticable, for I think that, to warrant such an assertion, a greater number of trials is necessary. These two experiments, however, may justly render us mistrustful of any that shall be hereafter attempted; I shall not be surprized if they should prove unsuccessful, considering the widely different nature of these animals. But should this really happen, we ought not to be discontented, since Nature has thus replied to our interrogatories; and her responses, whatever they may be, should be held precious by us, as they serve to increase the stores of useful knowledge. Further, the failure of these attempts, ought not to prevent us from making others upon animals differing in their nature. It is true, that every kind of seed will not fecundate every species of animal. This liquor, on which depends the perpetuity of the species, must have a certain relation with the embryos to be fecundated; and it is natural to suppose that such a relation does not belong to all kinds of seed. But it is also true, that we can only learn from the effects, that is from experiment, when this relation does subsist. The very experiments which at first seemed contrary to the production of such and such mules, when repeated in a better manner, proved favourable to it. Button destroyed our hopes of procuring mules, by keeping rabbits and hares together, seeing that in the experiment he adduces, the one species never copulated with the other. But this conjunction has been effected by other hands, and hence hare-rabbits have been procured. A dog and a she wolf, kept together for a long time by the same author, never shewed any sign of mutual attention. But the experiment, when repeated by others, had a very different degree of success. The mules that were by these means produced propagated their kind; Button failed in the same manner with respect to dogs and foxes: this experiment has not, as far as I know, been repeated by others. I should think, that in more expert hands, it would have been attended with a more fortunate result. But in the race of mules, there is nothing perhaps so curious and surprizing as the famous Jumart. Three varieties, you know, are enumerated, the offspring of the bull and mare, the ass and cow, and the bull and she—ass. Leger and Shaw admit the existence of all without hesitation; but Buifon, in his history of animals, reckons them all imaginary. Yet in his supplement, he does not absolutely deny the possibility of their existence, though he doubts it much. But in truth, the French Pliny was mistaken. Mr. Bourgelat, formerly inspector general of the Ecole Veterinaire at Lyons, in a letter to the illustrious Bonnet, expressly says, that he had been in possession of several of these Jumarts, and that one was dissected under his inspection in the school at Lyons; and he communicates the result in his letter to the philosopher of Geneva. The authority of this celebrated and ingenious person, merits the utmost deference. Assuming then the existence of this singular sort of mule, I could wish that they should be multiplied much more than they have hitherto been, both because they are well adapted to throw fuller light upon the great function of generation, and because they may possibly prove highly advantageous to mankind, as they are said to have been possessed of extraordinary strength. Natural fecundation, though studiously attempted, would not very fully accomplish this object, on account of the indifference, or rather the aversion of quadrupeds of various sorts to copulate together, especially when they happen to be placed at a great distance from each other.- These illegitimate marriages take place only, when the ass or the bull cannot find the means of satisfying their desires upon their own species, and are moreover uncommonly ardent. Artificial fecundation, properly performed, would be most convenient in this case. I will add, most respectable Marquis, that I am well disposed to put it in practice; but my public and private engagements have hitherto prevented me, as also the expense, for I will openly acknowledge it, of keeping these animals for several months, to which the narrow income of a philosopher is not very adequate. Hence I made application to a rich person in Lombardy, to assist me in these experiments; but he was insensible to the proposal, a circumstance which did not surprize me, as it certainly will not surprize you, who know, that the nobility in many cities of Italy are not very friendly to science and literature. In order to satisfy my wishes I see no better expedient, you will excuse my philosophical freedom, than that of applying to you. The high honour you enjoy, in possessing a station in the court of one of the greatest princes upon earth, suitable to your eminent virtues and valuable endowments, of a prince, who is at once the delight of his happy kingdom, and the great protector of letters and learned men: your ardour for natural philosophy, and for whatever has a tendency to ennoble and advance it; your close connection with our famous royal academy of sciences and polite literature at Berlin, which is so desirous of enlarging the limits of this noble branch of knowledge, by the sure guidance of accurate experiments: these favourable circumstances afford me hopes, that you will not refuse to second my wishes. I am willing to believe, that you will choose the most advantageous moment to speak to your sovereign of this curious project; nor do I despair of his encouragement, since it may, in some sort, be termed a thought of his own. But enough of this. I proceed to another subject.”
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