Paper - William Harvey as an embryologist (1897)
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William Harvey as an Embryologist
By William K. Brooks, LL. D.,
Professor of Zoology, Johns Hopkins University.
The immortal discoverer of the circulation of the blood is held to be also the discoverer of the law of embryology — "that all animals are produced out of ova" (Encyc. Brit., 9th ed., art. Embryology, p. 164); and he is also held to have had some vague premonition, scarcely worth mentioning in history, of the great law that the complex animal arises, from a relatively homogeneous germ, by gradual differentiation or epigenesis.
I hope to show, by quotations from his work on embryology (Exercitationes de Generatione Animalium, Amstelodami,
Read before the Johns Hopkins Historical Club, February, 1897.
1651, translated into English by Robert Willis, M. D., London, 1848), that both these current impressions are erroneous. He not only formulated but demonstrated epigenesis. liis statement of this law is clear, definite and thoroughly modern, and it is based upon actual observations which are fully described.
On the other hand the conception which he sought to express by the dictum "omne vivum ex ovo" is totally foreign to the principles of modern embryology. Harvey was a worker, not a dreamer, and his dictum is no mere guess or happy inspiration. It sums up results reached by laborious research, and as a generalization based on actual study it still has value, although its meaning has nothing in common with tliat which the words have as we now use them. He repudiates in most energetic language the opinions, current at his time, which come nearest to the modern discovery that the physical continuity of living matter is never broken. In fact the chief aim of his treatise is to show that his observations, as he interpreted them, prove that there is no physical or " corporeal " continuity between parent and child.
Embryologists who permit Germans to write the history of their science, and make no protest when the demonstration that the embryo arises from the egg by epigenesis is attributed to Wolff (1759), and to Von Baer (1839), are either ignorant of Harvey's researches (1651) or indifferent to the fame of this great Englishman, who studied the history of the chick as laboriously and faithfully as Von Baer, nearly two hundred years before. While his resources were more limited, his ability to reflect upon the meaning of his observations and to state in clear and energetic words the results of his " Beobachtung und Eeflexion," were inferior in no way to those of the justly famous author of the "Entwickeluugsgeschichte der Thiere."
Harvey means just what more modern writers mean by "epigenesis," but the strangeness of the views he opposed gives us difficulty. The form of words into which an account of a scientific discovery falls is fixed by the view of the matter which is current at the writer's day, and later generations of readers may be puzzled by inability to occupy his standpoint. Thus it is with Harvey, and we thus explain the prevalence of the opinion that he had no more than a dim adumbration of truths the demonstration of which is generally credited to Wolff and Von Baer.
The evolutionary teachings of Bonnet are quite intelligible to us; and as we easily put ourselves in Von Baer's place, his refutation of Bonnet appeals to us with all its native force; but it is much harder for us to stand where Harvey stood.
So far as I can discover, no notion at all equivalent to Bonnet's conception of germs ever entered Harvey's mind or the mind of any one before his time. He presents the evidence for epigenesis as opposed, not to "evolution," but to "metamorphosis," and his way of using the last word is so unfamiliar to us that we cannot grasp what he has in mind without effort.
They who studied embryology before him held one modification or another of the very ancient belief that embryos arise from "excrement"; that they are products of decomposition.
He gives the evidence for epigenesis as opposed to this opinion which finds no pigeon-hole in the modern mind. Fortunately he is a ready writer. Hlustrations and analogies overflow his brain and pen; and patient study enables us to pick out passages which give his views on epigenesis uncomplicated by reference to " metamorphosis." When we have done this we find his reasoning as modern and definite as that of Von Baer, although his resources did not qualify him to sum up the evidence with modern exhaustiveness.
While Harvey does not deny that some " imperfect " animals may be genei-ated "out of a putrescent material, the drying of a moist substance, or the moistening of a dry one," he tells us, clearly and definitely enough, that the generation of all "perfect " animals, such as the lion and the cock, " is the result of epigenesis as the man proceeds from the boy; the edifice of the body, to wit, is raised on the punctum saliens as a foundation ; as a ship is made from the keel, and as a potter makes a vessel . . . For out of the same material from which the first part of the chick or its smallest particle springs, from the very same is the whole chick born ; whence the first little droji of blood, thence also proceeds its whole mass by means of generation in the egg ; nor is there any difference between the elements which constitute and form the limbs or organs of the body, and those out of which all their similar [i. e. homogeneous] parts, to wit, the skin, the flesh, veins, membranes, nerves, cartilages, and bones derive their origin. For the part that was at first soft and fleshy, afterwards, without any change in the matter of nutrition, becomes a nerve, a ligament, a tendon ; what was a simple membrane becomes an investing tunic ; what had been cartilage is afterwards found to be a spinous process of bone, all variously diversified out of the same similar [homogeneous] material." From what "appears to be homogeneous in the beginning and resembles the spermatic jelly" the structure of the body arises; its parts being "at first delineated by an obscure division, and afterwards become separate and distinct organs."
He says the result of the process of development is just as if the chick were created by a command to this effect: " Let there be a similar [homogeneous] colorless mass, and let it be divided into parts and made to increase, and in the meantime, while it is growing, let there be a separation and delineation of parts ; and let this part be harder and denser and more glistening, that be softer and more colored."
" Now it is in this very manner that the structure of the chick in the egg goes on from day to day; all its parts are formed, nourished and augmented out of the same material. . . . For there is a greater and more divine mystery in the generation of animals than the simple collecting together, alteration and composition of the whole out of parts would seem to imply; inasmuch as here the whole has a separate constitution and existence before its parts, the mixture before the elements."
These passages summarize conclusions from observations which have been more fully described in forty-four preceding chapters or "exercises," and it would be difficult, even at the present day, to state in more definite language the truth that the developing embryo passes " from indefinite, incoherent homogeneity to definite, coherent heterogeneity by successive integrations and differentiations."
While we have no desire to ignore the merits of Wolff or to belittle the greatness of Von Baer, we find it hard to understand how any one who knows Harvey's works can, without protest, read this assertion or similar ones in the German works from which it is derived:
" It was reserved for Caspar Frederick Wolff, a German by birth ... to bring forward observations which . . . established the theory of epigenesis upon the secure basis of ascertained facts" (Encyc. Brit., 9th ed.. Embryology, 1(35).
We now know that the germ itself is an organism of wonderful complexity ; that its homogeneity is relative, not absolute ; but there is great mystery to us as well as to Harvey in the manner in which " the whole has a separate constitution and existence before its parts," and while the doctrine of "metamorphosis " as held in Harvey's day has vanished from science, I venture to believe that we shall Knd in his discussion of this doctrine, clear statement of other difficulties which are still as grave as he found them.
It is hard to decide just what his opinion on spontaneous generation was. No less careful a student than Huxley tells us (Encyc. Brit., 9th ed., art. Evolution, p. 746) that "Harvey believed as implicitly as Aristotle did in the equivocal generation of the lower animals." "Harvey shared the belief of Aristotle — whose writings he often quotes, and of whom he speaks as his precursor and model, with the generous respect with which one genuine worker should regard another — that such germs may arise by a process of 'equivocal generation' out of non-living matter."
1 am by no means confident that this assertion does justice to Harvey, or that the quotations from Aristotle prove anything except that Harvey was not yet quite prepared to demonstrate their error. I believe there is ample evidence that he had made many observations which, while he never published them, led him to distrust most of the familiar examples of spontaneous generation, although he may not have been fully armed to attack the teachings of " my leader," Aristotle, "one of nature's most diligent inquirers," "whose authority has such weight with me that I never think of differing from him inconsiderately." It is true that he quotes without comment, and occasionally without credit, many passages in which Aristotle affirms sjjontaneous generation ; but as an offset to this he tells us explicitly (Exercise the forty-first) that he shall show in another place " that many animals, especially insects, arise and are propagated from elements and seeds so small as to be invisible (like atoms flying in the air), scattered and dispersed here and there by the winds ; and yet these animals are supposed to have arisen spontaneously, or from decomposition, because their ova are nowhere to be found." He was far too cautious to have ventured to criticise " the philosopher," even to this extent, without pretty good evidence; and in Exercise the sixty-ninth he tells us why this evidence was never published. "Let gentle minds forgive me," he asks, "if, recalling the irreparable injuries I have suffered, I here give vent to a sigh. This is the cause of my sorrow: — whilst in attendance on his majesty the king, during our late trouble and more than civil wars, not only with the permission but by the command of the Parliament, certain rapacious hands stripped not only my house of all its furniture, but, what is subject of far greater regret with me, my enemies abstracted from my museum the fruits of many years of toil. Whence it has come to pass that many observations, parlictilarly on the generation of insects, have perished, with detriment, I venture to say, to the republic of letters."
Is there not reason to believe that, if they are ever discovered, those lost observations will be found to cover some of the ground which was so successfully explored by Spalanzanni more than a hundred years later ?
Harvey's reference (Exercise the twenty-seventh) to " the animalculae which are engendered in our bodies . . . lumbrici, ascarides, lice, nits, syrones and acari," and to " the worms which are produced from plants and their fruits, as from gallnuts, the dog-rose, and various others," might be held to imply belief in heterogenesis, if he did not tell us, almost immediately, that: "It certainly cannot be that the living principles of these animals which arise in the gall-nuts existed in the oak, although these animals live attached to the oak and derive their sustenance from its juices."
Notwithstanding Huxley's opinion, Harvey seems to have been nearer than any of his successors for a hundred years to the modern discovery that all living things come from germs, although I shall show soon that he did not intend to imply anything at all like the modern view by his statement that this is true.
In his discussion of epigenesis as contrasted with " metamorphosis," he assumes the reality of " equivocal " generation, as he does in many other places, although, in view of the passages I have quoted, I believe that this is admitted out of courtesy to Aristotle, and for the sake of the argument, as something which he is not yet fully prepared to disprove.
He tells us (Exercise the forty-fifth) that there are two ways in which one thing may be made out of another. When a workman cuts the material already prepared, divides it and rejects what is superfluous, till he leaves it in the desired shape, as in nuiking a statue from a block of stone, the whole material of the future piece of work has already been in existence before it is finished into form or any part of the work is yet begun. AVhen on the other hand a potter educes a form out of clay by the addition of parts, increasing its mass, and giving it a figure, at the same time that he provides the material, which he prepares, adapts and applies to his work, "the form may be said rather to have been made than educed." " So exactly it is with regard to the generation of animals. Some, out of a material previously concocted and that has already attained its bulk, receive their forms and transfigurations ; and all their parts are fashioned simultaneously, each with its distinctive characteristics, by the process called metamorphosis, and in this way a perfect animal is at once born; on the other hand, there are some iu which one part is made before another, and these from the same material afterwards receive at once nutrition, bulk and form ; that is to say, they have some parts made before, some after others, and these are at the same time increased in size and altered in form. The structure of these animals commences from some one part as its nucleus and origin, by the instrumentality of which the rest of the limbs are joined on, and this we say takes place by the method of epigenesis, namely by degrees, part after part; and this is, in preference to the other mode, generation properly so called. In the former of the ways mentioned, the generation of insects is effected when by metamorphosis a worm is born from an egg ; or out of a putrescent material, the drying of a moist substance or the moistening of a dry one, rudiments are created from which, as from a caterpillar grown to its full size, or from an aurelia, springs a butterfly or fly already of a proper size, which never attains to any larger growth after it is first born. But the more perfect animals with red blood are made by epigenesis or the superposition of parts. In the former, chance or hazard seems the principal promoter of generation, aud the form is due to the potency of a pre-existing material ; and the first cause of generation is 'matter ' rather than an ' external efficient,' while the more perfect animals owe their immortality to one constant source — the perpetuation of the same species . . . Bees, wasj^s, butterflies, and whatever is generated from caterpillars by metamorphosis, are said to have sprung from chance, aud therefore to be not preservative of their own race. . . . The lion and the cock owe their existence, as it were, to nature, or an operative faculty of a divine quality, and require for their propagation an identity of species, rather than any supply of fitting material." " In the generation by metamorphosis forms are created as if by the impression of a seal, or as if they were adjusted in a mould . . . but an animal which is created by epigenesis attracts, prepares, elaborates, and makes use of the material all at the same time. The processes of formation aud growth are simultaneous. In generation by metamorphosis the whole is distributed and separated into parts, but in that by epigeuesis the whole is put together out of parts, in a certain order, and constituted /ro?» them. In the one case the result is due to matter ; in the other the animal makes itself.
" Now it appears clear from my history that the generation of the chick from the egg is the result of ejjigenesis rather than of metamorphosis, and that all its parts are not fashioned simultaneously, but emerge in their due succession and order; it appears, too, that its form proceeds simultaueously with its growth, aud its growth with its form; also that the generation of some parts supervenes on others previously existing, from which they become distinct; lastly, that its origin, growth aud consummation are brought about by the method of nutrition.
" The formative faculty of the chick rather acquires and prepares its own material than only finds it when prepared, aud the chick seems to receive its growth from no other than itself. And as all things receive their growth from the same power by which they were created, so likewise should we believe that the chick is created by the same power by which it is preserved and caused to grow."
The meaning of this rather puzzling passage will be somewhat clearer after we have examined Harvey's views on generation, but when we omit the complications which come from the reference to "metamorphosis," its meaning as interpreted by the rest of the essay is about as follows :
The substance which composes the body of all "perfect" animals does not exist as such before the body itself is formed; but it consists of unorganized or "homogeneous " food which is changed by nutrition into all the diversified parts of the complicated body, so that nutrition, growth aud development go on together. As the organized body is constructed by the assimilation of unorganized food, its structure cannot be the outcome of the ordinary or physical properties of this food. There must be some organizing iufiuence at work making use of these properties to construct out of homogeneous matter a definite organism belonging to the same species with the parents. To the question what this organizing influence is, he answers that this is a " divine mystery," or, in plain English, that he does not know, although he finds clear evidence of its existence. He says in many places that the egg has a " vital principle," but the context shows that he means by this no more than we mean when we say it is " alive," and nothing is farther from his thoughts than recourse to supernatural agencies, for he tells us clearly that while the cause of its development is a " divine mystery," " the egg is a natural body endowed with animal virtues ... it is moreover a body which under favorable circumstances has the capacity to pass into an animal form; heavy bodies, indeed, do not sink more naturally, nor light ones float, when they are unimpeded, than do seeds aud eggs in virtue of their inherent capacity become changed into vegetables aud animals." (Exercise the twenty-sixth, p. 373.)
It would be a gross error to infer, from this passage or from others like it, any further similarity between Harvey's opinions aud the results of moderu microscopic study of ova and male cells. In order to understand the meaning of his celebrated dictum " omne vivum ex ovo " we must undertake more extended analysis of his observations aud reflections on generation, aud of the opinions of his predecessors.
Aristotle saw nothing strange or exceptional in the generation of animals from decomposing organic matter, for he believed that all generation takes place in essentially the same way ; and he regarded the generation of insects from putrescent slime as a simple or typical example, what we should now call a primitive type, of generation in general, in comparison with which more complicated instances are to be interpreted.
As a bloody substance is discharged at intervals from the reproductive organs of woman, during the fertile period of her life, and as its apjiearance marks the beginning and its cessation the end of fertility, he believed that the mammalian embryo is formed out of this substance just as other animals are generated from decomposing matter of other kinds.
" Milk aud the menstrual discharge," he tells us (De Gen. II, i), " are of the same nature." " When the semen masculinum enters the female uterus it coagulates the purest part of the catamenia," aud when this has " set in the uterus " it forms a coagulum like curdled milk. As heat causes milk to curdle, so " the semeu or geniture of the male bears the same affinity to the nature of the catamenia " aud causes it to " set " without itself contributing any part of the substance of the coagulum. " The female always supplies the matter, the male the power of creation, and this it is which constitutes one male and another fenuile." "The male is the efficient agent, aud by the motion of his geniture creates what is intended from the matter contained in the female." " The body and the bulk therefore are necessarily supplied by the female; nothing of the kind is required from the male; for it is not even requisite that the instrument, nor the efficient agent itself, be present in the thing that is produced. The body then proceeds from the female ; the life (anima) from the male."
Harvey points out the inconsistency of Aristotle's admission that hybrids "partake of the species of both parents" (De Gen. Anim. II, 3), and his assertion that '• the conception or egg receives '" "from the feuuile its body solely and its dimensions," and that the mother has no part in the trausmissiou of "form, species and life"; for the studj" of jiybrids shows the error of his opinion that creative force or vital power is derived exclusively from the male, and proves that both parents must be efficient in determining form or species.
The medical men of Harvey's day held a different opinion, as he tells us iu Exercise the thirty-second. Like Aristotle, they held that the embryo arises from "excrement"; but they held, in opposition to his teaching, " that the prime matter of conception is not blood, but the mingled geuitures of both sexes." They also held, iu opposition to Aristotle, the opinion, which Harvey shares, that the male is no more " the efficient cause of generation " than the female.
"Conception, according to the opinion of medical men, takes place in the following way: during intercourse the male and female dissolve in one voluptuous sensation, and eject their seminal fluids (geniturae) into the cavity of the uterus, where that which each contributes is mingled with that which the other supplies, the mixture having both equally the faculty of action and the force of matter; and according to the predominance of this or that geniture does the progeny turn out male or female. It is further imagined that immediately after the intercourse something of the conception is formed iu the uterus."
If the uterus contains a " conception " immediately after a fertile union, in the form of a bloody coagulum, as Aristotle supposes, or iu the form of the mingled emissions or geuitures of both sexes, as the medical men taught, this ought to be discoverable, and Harvey, a true scientific investigator, set himself to hunt for it without a microscope.
His facilities for making the search, and its results, are best described in his own words. He was the attending physician of the King of England, and he tells us : " It was customary with his Serene Majesty, King Charles, after he had come to man's estate, to take the diversion of hunting almost every week, both for the sake of finding relaxation from grave cares and for his health; the chase was principally the buck and the doe, and no prince iu the world had greater herds of deer. This gave me an opportunity of dissecting these animals almost every day during the whole of the season when they were rutting, taking the male and falling with young. I had occasion so often as I desired it to examine and study all their parts, particularly those devoted to the offices of generation."
His studies upon the development of the embryo of the deer are fully described at length in the essay on generation, but only those which relate to the question of conception concern us at present. Here his researches had a very definite result, "llepeated dissections performed in the course of the month of October, both before the rutting season was over and after it had passed, never enabled me to discover any blood or semen or a trace of anything else, either in the body of the uterus or its cornua." Neither the bloody coagulum of Aristotle nor the geniture of the medical men has any existence. The "conception" which should be discoverable iu the uterus if their teachings are correct, cannot be found there when a search is made for it, and actual observation shows that their teachings are erroneous and fanciful.
The keepers and huntsmen said " that I was both deceiving myself and had misled the king, and that there must of necessity be something of the conception to be found in the uterus. These men, however, when I got them to bring their own eyes to the inquiry, gave up the point." Harvey tells us that the king fully appreciated the value of the investigation, and in order " that this important question might be the more satisfactorily settled in all time to come," provided means for isolating the does and for proving that there was no error as to the fact of conception ; but the physicians were still unconvinced, and "held it among their impossibilities that any conception should ever be formed without the presence of the semeu masculinum, or some trace remaining of a fertile intercourse within the cavity of the womb." But the man who had proved the error of their teachings regarding the function of the heart and blood-vessels had little tolerance for their belief iu anything which they were unable to demonstrate.
If they had insisted that Harvey's resources were inadequate, that the "conception" for which he sought is too minute to be found by such rough means, we now know they would have been in the right, for even at the present day our knowledge of the essential facts of mammalian conception is, for the most part, a deduction from observations on the eggs of animals which were almost or quite uuknown to Harvey, the sea-urchin and ascaris, for example. But his proof of the non-existence, in the uterus of the doe, of anything corresponding to their teachings is conclusive. He did not stop here, however, for he tells us: " In the dog, rabbit and several other animals, I have found nothing iu the uterus for several days after intercourse ; I therefore regard it as demonstrated that after fertile intercourse among viviparous as well as oviparous animals there are no remains in the uterus either of the semen of the male or of the female emitted in the act; nothing produced by any mixture of these two fluids, as medical writers maintain, nothing of the menstrual blood present as 'matter' iu the way Aristotle will have it; in a word, that there is not necessarily even a trace of the conception to be seen immediately after fruitful union of the sexes. It is not true, consequently, that in a prolific connexion there must be any prepared matter in the uterus, which the semen masculinum, acting as a coagulating agent, should congeal, concoct and fashion or bring into a positive genei'ative act."
His study of the generation of birds leads him to the same result. "As the hen does not emit any seminal fluid, and as the seminal fluid of the cock does not reach the uterus of the heu, and as there is no trace of an egg to be found in the uterus immediately after intercourse, it is obvious that it is not engendered" by the mixture of seminal fluid in the way the medical men teach (Exercise the thirty-second). After quoting Aristotle's opinion that the chick is formed out of menstrual blood coagulated by the influence of the cock, he says (Exercise the twenty-first) : " The business in the generation of an egg is very different from this; for neither does the semen or rather the 'genitura' proceeding from the male in the act of intercourse, enter the uterus in any way, nor has the hen after she conceives any particle of excremeutitious matter, even of the purest kind, or any blood in her uterus which might be fashioned or perfected by the discharge of the male. Neither are the parts of the egg produced by any kind of coagulation ; neither is there anything like curdled milk to be discovered iu the uterus. The cock, I say, contributes neither form normatter to the egg, but that only by which it becomes fertile and fit to engender a chick. And this faculty the cock confers by his semen (genitura) emitted in the act of intercourse, not only on the egg which is already begun, but on the uterus and ovary and even on the body of the fowl herself, in such wise that eggs which have yet to be produced, eggs, none of the matter of which yet exists either in the ovary or in any other part of the body, are thence produced possessed of fecundity." "Inflammable material is not set on fire by the contact of flame more quickly than is the hen made pregnant by intercourse with the cock" (315).
Careful observation on the fowl, the deer, the dog, the rabbit, and on many other animals, jiroves that none of them are generated out of excrement or decomposing matter. There is no basis in nature for Aristotle's opinion or that of the medical men, and all these teachings break down when brought to the test of actual observation. It is no small thing to prove the error of the belief, which had been current for two thousand years, and is even now embodied, through a quotation from St. Paul, in our burial service, that all forms of reproduction find their type in generation from dead putrescent matter. This Harvey accomplished by methods which are rigorously scientific ; and with this accurate but very imperfect knowledge he boldly faced and tried to answer the question, what is it which the cock contributes in virtue of which the egg "becomes fertile and fit to engender a chick '"?
He undertakes "to seek the truth regarding the following difficult questions : Which and what principle is it whence motion and generation proceed':' By what virtue does the semeuact":' What is it that renders the semen itself fruitful? Whether is that which in the egg is canse, artificer and principle of generation and of all the vital and vegetative operations — conservation, nutrition, growth — innate or superadded ? and whether does it inhere primarily, of itself, and as a kind of nature, or intervene by accident, as the physician in curing disease? Whether is that which transfers the egg into a jiullet inherent or acquii-ed, or is it already conceived in the ovary, and does it nourish, augment, and perfect the egg there ? What is it besides that pireserves the egg sweet after it is laid ? What is it that renders an egg fruitful ?" (374).
" In truth, there is no proposition more magnificent to investigate or more useful to ascertain than this : How are all things formed by an ' univocal agent ' ? How does the like ever generate the like? . . . Why may not the thoughts, opinions, and manners now prevalent, many years hence return again, after an intermediate period of neglect? " (583).
As we find the embryologists of the present day vexing themselves over the question, " AVhether is that which transfers the egg into a pullet inherent or acquired?" we need not wonder if Harvey's success in the investigation of this magnificent proposition seems small to us. At least we must follow him in order to understand his dictum.
As a starting-point this much seems to be certain. "The egg, even when contained in the ovary, does not live by the vitality of the mother, but is like the youth who comes of age, made independent even from its first apj)earauce; as the acorn taken from the oak, and the seeds of jjlants in general are no longer to be considered parts of the tree or herb that supported them, but things made in their own right, and which already enjoy life in virtue of a proper and inherent vegetative power" (375).
Furthermore, "although some animals . . . are produced from females alone" (386); "it is manifest that a fruitful [hen's] egg cannot be jjroduced without the concurrence of a cock and a hen ; without the hen no egg can be formed ; without the cock it cannot become fruitful. But this view is opposed to the opinion of those who derive the origin of animals from the slime of the ground " (384).
" The egg is the terminus from which all fowls, male and female, have sprung, and to which all their lives tend — it is the result which nature has proposed to herself in their being" (371).
"And this is the round that makes the race of the common fowl eternal; now pullet, now egg, the series is continued in perpetuity; from frail and perishing individuals an immortal species is engendered" (385).
" We cannot conceive an egg without the concurrence of a male and female fowl any more than we can conceive fruit to be produced without a tree. We therefore see individuals, males as well as females, existingr for the sake of preparing eggs, that the species may be perennial though their authors pass away. And it is indeed obvious that the parents are no longer youthful or beautiful, or lusty, and fitted to enjoy life, than while they jDOSsess the power of jiroducing and fecundating eggs, and by the medium of these, of engendering their like. But when they have accomplished this grand purpose of nature, they have already attained to the height, the dxnrj of their being ; the final end of their existence has been accouij)lished ; after this, effete and useless, they begin to wither, and, as if cast off and forsaken of nature and the Deity, they grow old, aud, a-weary of their lives, they hasten to the end. How different the males when they make themselves up for intercourse, aud swelling with desire are excited by venereal impulse! It is surprising to see with what passion they are inflamed, and then how trimly they are feathered, how vainglorious they show themselves, how proud of their strength, aud how pugnacious they prove. But the grand business of life accomplished, how suddenly, and with failing strength and pristine fervor quenched, do they take iu their swelling sails, and from late pugnacity, grow timid and desponding. Even during the season of jocund masking in Yeuus's domains, male animals in general are dejiressed by intercourse, aud become submissive and pusillanimous, as if reminded that in imparting life to others they were contributing to their own destruction. The cock alone, replete with spirit and fecundity, still shows himself alert aud gay, clapping his wings and ci'owiug triumphantly he sings the nuptial song at each of his espousals; yet even he after some length of timeiu Veuus's service, begins to fail; like the veteran soldier, he by and by craves discharge from active duty, and the hen, too, like the tree that is past bearing, becomes effete, aud is finally exhausted."
Having come to the end of his means of observation, Harvey turns to reflection, the second resource of the man of science, to see how this may help him to discover how "from frail and perishing individuals an immortal species is engendered." As liis studies seemed to prove that the contagion which remains in the female "after intercourse, as the efficient of the future offspring, is not of the nature of any corporeal substance," he was unable to escape the admission that it is "incorporeal." Thus driven to the wall, if he had taken refuge in "soul " or "spirit," no one could greatly blame him, for spiritual agents had been the resource of philosophers for many ages before his time. He was a true soldier of science, however, seeing as clearly as we do that this venerable formula can do notliing to help us, and preferring outspoken ignorance to this antiquated and threadbare cloak for intellectual poverty.
" If on further inquiry it should appear that it [the efficient] is neither spirit nor demon, nor soul, nor any part of a soul, as I believe can be proved by various arguments and experiments, what remains, since I am unable myself to conjecture anything beside . . . but to confess myself at a standstill ? "
What does the modern man of science in such a case? Does he not search through the whole province of knowledge to see if perchance he may find some other natural phenomenon which bears some resemblance to the subject of his studies ? Harvey says he knows well " that some censorious persons will laugh at this . . . Yet this that I do is the practice of philosophers, who when they cannot clearly comprehend how a thing really is brought to pass, devise some mode for it in accordance with the other works of nature, and as near as possible to what is true."
" Since, tlien, nothing can be apprehended by the senses in the uterus after coition, and since it is necessary that there be something to render the female fruitful, and as this is probably not material, it remains for us to take refuge in a mere conception."
Men of science in all ages, from Aristotle to Tyndall, have believed in the virtues of the provisional hypothesis; and, armed by eminent authority, Harvey undertakes, by comparing a "mere conception" with other things in nature, to frame a provisional hypothesis of generation; but natural science seems to be an uncongenial soil for the nurture of such attempts, and if time has shown that Harvey's hypothesis has little value, he errs in good company, and he also takes pains to say he does not wish it "to be taken as if I thought it a voice from an oracle," although he does hope it may "stir up the intellects of the studious to search more deeply into so obscure a subject."
Starting with the belief that "the semen of the male does not so much as reach the cavity of the uterus . . . and that it carries with it a fecundating power by a kind of contagious property " from which the female "seems to receive influence and to become fecundated without the co-operation of any sensible corporeal agent, in the same way as iron touched by the magnet is endowed with its powers and can attract other iron to itself," he holds that "when this virtue is once received the woman exercises a plastic power and produces a being after her own image."
" Yet it is a matter of wonder where this faculty abides after intercourse is completed. ... To what is the active power of the male committed ? . . . Does the woman conceive in the womb as we see by the eye and think by the brain?" " Since there are no manifest signs of conception before the uterus begins to relax . . . and since the substance of the uterus, when ready to conceive, is very like the structure of the brain, why should we not suppose that the function of both is similar, and that there is excited by coitus within the uterus a something identical with or at least analogous to an imagination or a desire in the brain, whence comes the generation or procreation of the ovum ?" "For the functions of both are termed conceptions, and both, although the primary source of every action throughout the body, are immaterial, the one of natural or organic, the other of animal action . . . Just as a desire arises as a conception of the brain, and this conception springs from some external object of desire, so also from the male, as being the more perfect animal, and as it were the most natural object of desire, does the natural (organic) conception arise in the icterus, even as the animal concej)tiou does in the brain. From this desire, or conception, it results that the female produces au offspring like the father. For just as we, from the conception of the 'form' or 'idea' in the brain, fashion in our works a form resembling it, so in like manner the 'idea' or 'form ' of the father, existing in the uterus, generates au offspring like himself with the help of the formative faculty.
"Whoever has pondered with himself how the brain of the artist, or rather the artist by means of his brain, pictures to the life things which are not present in him, but which he has once seen ; also in what manner birds immured in cages recall to mind the spring, and chant exactly the songs they had learned the preceding summer, although meanwhile they had never practiced them; again, and this is more strange, how the bird artistically builds its nest, the copy of which it had never seen, and this not from memory or habit, but by means of an imaginative faculty, and how the spider weaves its web, without either copy or brain, solely by the help of this imaginative power ; whoever, I say, ponders these things, will not, I think, regard it as absurd or monstrous, that the woman should be impregnated by the conception of a general immaterial ' idea ' and become the artificer of generation."
"For my own part then, when I see nothing left in the uterus after intercourse, to which I can ascribe the principle of generation, any more than there is in the brain anything discoverable after sensation and experience, which are the prime sources of art, and when I find the structure of both alike, I have devised this fable."
Whatever the value of this hypothesis may be, it serves well to emphasize the fact that Harvey's opinions on generation have nothing in common with the modern discovery of the physical continuity of living matter, and it shows that his teaching that all animals come from eggs cannot possibly mean what the words now mean.
He believes the uterus conceives an animal in the same way that the brain conceives an idea; and he also tells us that he agrees with Fabricius that " the egg [of the hen] is in a certain sense an exposed uterus " (290).
Furthermore, "the hen is not the efficient cause of a perfect egg, but that she is made so in virtue of an authority, if I may use the word, or power required of the cock. For the egg, unless prolific, can with no kind of propriety be accounted perfect; it only obtains perfection from the male, or rather from the female, as it were upon precept from the male, as if the hen received the art and reason, the form and laws of the future embryo from his addresses" (290). Ho much for the generation of the fowl. In Exercise the sixty-ninth he describes the embryo of the doe at about seven weeks, and the human embryo about the second month after conception, but, following Aristotle, he regards these embryos in their membranes, not as embryos, but as eggs without shells. " In the way above indicated do the hind and doe, affected by a kind of contagion, finally conceive and produce primordia, of the nature of eggs, or the seeds of plants, or the fruit of trees, although for a whole month and more they had exhibited nothing in the uterus."
In this sense, then, he holds that viviparous animals are generated from eggs. He therefore maintains (as contrasted with Fabricius, who held that the greater number of animals are produced from ova) "that all animals, even the vivipara, and man himself not excepted, are produced from ova; that the first conception, from which the foetus proceeds in all, is an ovum of one description or another, as well as the seed of all kinds of plants. Empedocles therefore spoke not improperly of the ' egg-bearing race of trees.' The history of the egg is therefore of the widest scope, as it illustrates generation of every description. . . . Fabricius has these additional words: "The foetus of animals is engendered in one case from an ovum, in another from the seminal fluid, in a third from putrefaction ; whence some creatures are oviparous, some viviparous, and yet others born of putrefaction or by the spontaneous act of nature, automatically."
" Such a division as this, however, does not satisfy me, inasmuch as all animals whatsoever may be said in a certain sense to spring from ova, and in another sense from seminal fluid, and they are entitled oviparous, viviparous or vermiparous rather in resjoect of their mode of bringing forth than of their first formation."
We see then that, unfamiliar as his words often seem, and while he holds that the organizing influence which produces the chick from the egg is a "divine mystery," we owe to Harvey the demonstration and clear formulation of the following truths :
There is no basis for the venerable doctrine that the higher animals are generated from excrement.
The hen's egg, even before it leaves the ovary, is an independent orgauism, which enjoys life by its own right, and perfects itself by nutrition.
The embryo assimilates homogeneous food, and by means of an inherent organizing power converts it into the structure of the living animal Nutrition, growth and development go on together, and the embryo arises by epigenesis or differentiation.
Many animals which have been held to arise from putrescent slime actually come from microscopic eggs.
"Animals are entitled viviparous or oviparous or vermiparous rather in respect of their mode of bringing forth than of their first formation."
Cite this page: Hill, M.A. (2020, April 4) Embryology Paper - William Harvey as an embryologist (1897). Retrieved from https://embryology.med.unsw.edu.au/embryology/index.php/Paper_-_William_Harvey_as_an_embryologist_(1897)
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