Paper - The interrelations of the mesonephros, kidney and placenta in different classes of animals

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Bremer JL. The interrelations of the mesonephros, kidney and placenta in different classes of animals. (1918) Amer. J Anat. 19(2): 179-209.

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This historic 1918 paper by Bremer describes the mesonephros, kidney and placenta in different animals.



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The Interrelations of the Mesonephros, Kidney and Placenta in Different Classes of Animals

John Lewis Bremer

From the Department of Anatomy, Harvard Medical School

Twelve Figures

In most anainnia the mesonephros begins its activity while the pronephros is still at its functional height. For a time both organs function together; then the pronephros undergoes degeneration and the mesonephros becomes the only functional excretory organ. This entire complex of processes has without further consideration beentrans— ferred to the relation between the mesonephros and the metanephros, and the question whether the mesonephros is actually functional in the aniniota has never been seriously considered. Weber (’97) was the first to take it up and he endeavored to answer it in the following manner. He compared the time of degeneration of the mesonephros with that of the development of the metanephros; when he found that the mesonephros degenerated before the metanephros could exercise an excretory function, he has assumed that the mesonephros did not function, for if it had been active and had then degenerated before the metanephros had begun its activity, there must have been a certain period of development during which there was no excretion. Let us adopt this same method in considering the special case of the function of the human mesonephros. Already in an embryo of 19.4 mm. greatest length the majority of the mesonephric tubules are so far in process of degeneration that they cannot be regarded as having an excretory function. Of the 35 tubules of this embryo only four are actually still. intact. In an embryo of 22 mm. greatest length none of the mesonephric tubules were capable of functioning; in all the tubulus secretorius had separated from the tubulus collectivus. If one inquires how far the development of the metanephros has progressed at this time, one finds that embryos of 22 mm. have just reached the anlagc of the second generation of uriniferous tubules. The first generation, however, has as yet no fully formed Malpighian corpuscles. If, then, the mesonephros had functioned as an excretory organ, there must necessarily have been an interruption of this function on its degeneration. Consequently, I regard the question as to the functioning of the mesonephros as settled; it does not function as an excretory organ. This does not, of course, imply that it may not have been active in another manner unknown to us.


This paragraph by Felix in the Keibel—Mall Human Embryologyl can hardly fail to arrest the attention of any one interested in the histological adaptation of cells and tissues to the physiological function which they perform. The similarity of the mesonephric glomeruli to those of the permanent kidney, of the convoluted tubules and the collecting tubules in both organs, the facts that developmentally the excreting portions of both organs are derived from the nephrogenic tissue and have practically the same history, and that both Wolflian duct and ureter open into the cloaca, all point so strongly to a similarity of the function of the two organs that it would need overwhelming proof to show that the product of the one gland is not at least very like that of the other, in other words to show that the mesonephros is not after all the ‘middle kidney.’ The statement is little short of iconoclastic, for ever since the time of Joh. Muller and von Baer the Wolffian body has been known as an excreting organ, continuing and gradually taking over the work of the pronephros, functional in adult life in lower animals, but replaced in turn in the higher orders by the permanent kidney. Apparent proof of their activity is given by Nicolas, who saw fine droplets of elaborated material in the epithelial cells and in the lumen of the tubules, and more especially by Bakounine, who after injections of sulphate of indigo into the aorta or into the vitelline vessels of chick embryos found in the epithelium of the Wolffian tubules the usual coloration given by this dye in the kidney.


A critical examination of Felix’s argument shows that it is based on a single fact, namely, that the mesonephros in man degenerates and therefore ceases to function, before the kidney is capable of activity, thus leaving the embryo without an excretory organ during a part of its existence. 1f during a part of its existence excretion is not provided for and may therefore be assumed to be unnecessary, why should it have been necessary previously? If it is not necessary previously, then the meso nephros, apparently active previously, must have had some other, as yet unknown function.


Weber, by whose paper Felix was admittedly influenced, brings additional facts to bear on the case. He notes that in rodents the embryos have either a small and short-lasting mesonephros, as in the guinea pig and the mole, or small organs totally lacking in the essential glomeruli, the tubules ending in blind enlargements, as in the rat and the mouse. On looking for the receptacle for the mesonephric urine, presumably the allantois, he finds that it does not exist as an entodermal sac in some rodents, and is always a slender tubular reservoir in man. He reviews to a considerable extent the literature in regard to the opening of the urogenital sinus, seeking a possible channel for the urine into the amniotic cavity, and concludes, in spite of a number of observations by others, which he quotes and which place the date of this opening at various periods from 7.0 mm. to as late as 20.0 m., that this channel is not open in man until the embryo has reached a length of 14.0 mm., whereas the mesonephros is apparently in full activity at 11.5 mm. “Sonach miissten wir uns zu der Annahme einer Sekretstauung in den ableitenden Wegen mit all’ ihren Folgen verstehen, oder wir miissen, und das ist wohl zweifellos das richtigere, auf die Annahme der lebhaften absondernden Funktion verzichten.”2 This last argument is later discounted by Keibel, who says that though the presence of well developed and numerous mesonephric glomeruli does not show absolutely that the mesonephros secretes urine, yet even if a very small amount of secretion were present, it does not necessarily follow that the urogenital sinus must be open at this stage, thus indicating his belief in the ability of the human allantois, narrow as it is, to store a small amount of fluid. Weber also lays stress on the interval between the beginning of the progressive degeneration of the mesonephros and the development of the kidney to a stage where it can be considered functional, and accuses Nagel of disregarding this period in his statement that both organs, the provisional and the permanent kidney, are for a time active side by side. The figures of 22.0 mm. for the beginning of involution of the mesonephros, and of 30.0 mm. for the development of the first renal glomeruli in man are given, and the kidney could not be considered fully active till much later. Other authors place the beginning of the involution much earlier. Finally Weber refers to a report by Ahlfeld of a case of the birth at full term of a child with entire absence of both kidneys, and suggests that not only the mesonephros but even the kidney may be functionless for excretion in intra—uterine life. English also reports several cases of the obliteration or stenosis of the urinary passages in fetuses and in the new-born. Usually death occurred in the sixth to the eighth month, but in other cases, which were very surprising to him, the child was born healthy, and showed uraemic symptoms only after two or three days.

2 Weber, loc. cit., p. 67.



One stumbling block was found, however, by Weber in his studies, namely, the conditions existing in the pig. The pig differs absolutely from the other animals he examined by retaining an apparently fully functional Wolffian body up to the time when the kidney should be Well able, from its anatomical development, to take over the work, leaving no gap when neither is available. But thisexception Where a continuous secretion is possible, but not proved, should not, in Weber’s mind, vitiate his conclusions drawn from so many cases Where a continuous excretion is impossible.

Had Weber gone further in his investigations he might have found more of these exceptions. Among mammals the cat, the sheep, the opossum, and in other classes the birds and reptiles all retain a functional mesonephros until the kidney is ready, the lizards using the provisional organ, according to Wiedersheim, sometimes for a year or more after hatching. Of the continuous activity of the urinary organs in the lower forms Felix seems to have had no doubts; his statements refer only to mammals and to the supposed error of assuming the sequence of events to be the same in mammals as in birds and reptiles. But if many mammals, and those of quite different orders, are found to show the possibility of a continuous urinary excretion, the status of the urinary organs is more and more established, and it is increasingly hard to believe that this continuous excretion is not universal.


The possibility that some other fetal organ might assume the excretory function during the interim when neither the mesonephros nor the kidney is apparently capable of activity, or might even in some animals like the rat and the mouse replace the Wolffian body partially or entirely is not considered by either Weber or Felix; yet that this is the case, and that this organ is the placenta seems to me to be strongly suggested by the facts brought out in this paper. Among physiologists the ability of the placenta to assume the role of the kidney is an accepted idea, derived from many experiments on the permeability of the placenta to a variety of substances passing from the fetus to the mother. Wertheimer, in Richet’s Dictionnaire de Physiologic, published in 1904, mentions most of these experiments and concludes that “the activity of the kidney does not appear to be a function absolutely necessary during intra-uterine life; the excretory products which form in the fetal organism could be eliminated by the placental exchanges.” Yet in this statement he was forced to overlook the contradictory evidence of certain physiologists, who had found that the placental permeability might not be the same in the earlier part of pregnancy as in the later days, and that, even in the same stages of intra—uterine life, one class of animals might differ from others in the results obtained. Thus Krukenberg found that materials which passed easily through the placentae of guinea pigs and rabbits were retained in dogs and cats. Since Krukenberg, comparative researches on the different classes of animals have not been undertaken, all experimenters relying with a singular unanimity on the results obtained from guinea pigs, rabbits, and man; but these exceptions to the general rules are very interesting, in the light of this present paper, as their explanation is simple, once the facts of _the different methods of fetal excretion in the different animal classes are set forth.

It is my purpose first to show anatomically the presence in the placentae of certain animals of an excretory organ capable of serving the fetus when neither the Wolflian body nor the kidney is in activity, and second to point out the differences in respect to their excretory activity in the types of animals studied. 184 JOHN LEWIS BREMER

The evidence here adduced is wholly anatomical, based on the very similarity of tissues for similarity of function which Felix and Weber seem to deny. Moreover it is only the glomeruli of the mesonephros and kidney whose counterparts are recognized in the placenta. Further investigations will be necessary to distinguish, if indeed it is found possible to do so at all," the cells of the placenta which correspond to those lining the convoluted tubules of the excretory organs.

The Glomerulus

The essential part of a glomerulus, whether of the mesonephros or of the kidney, is the arrangement of the inner capsule covering the ‘knot of capillary blood vessels. The epithelial cells of this layer, when it is first recognizable, are of a rather tall cylindrical type, as we know from the researches of Stoerk, Huber, and others. With further development the cells become more flattened, but not to the shape of the usual squamous cells, such as those lining serous cavities, with the nucleus sharing in the flattening process, and no part of the cell much thicker than another. The modification consists in the formation of an extremely thin, flange-like process, extending usually from one side of the cell, while the remainder of the cell, including the nucleus, retains its cuboidal shape. At the same time the flanges or plates of adjacent cells seem to fuse so that cell limits can no longer be recognized. In some glomeruli, more commonly those of the mesonephros, the plates at first represent the bases of the cells, each nucleus with its surrounding protoplasm protruding beyond the general level (figs. 2 and 3); in other glomeruli the surface of the epithelial layer is smooth, and the base uneven. But the first type can easily be, ‘and actually is, converted into the second by the pressure from within the glomerulus of the blood in the capillaries. For it is only in contact with or directly overlying the capillaries that the thin plates are found, while the nucleated portions dip deeper between the vessels, leaving a smooth outer surface. The future development of the glomerulus consists of an expansion of the plates, until the nucleated cuboidal portions of the cells are far apart, of the lobulation of the glomerulus to increase its free surface, and of the apparent fusion of the plates with the underlying endothelium of the capillaries, which thus seem to project uncovered into the intercapsular space (fig. 1).


Long ago Drasch investigated the glomeruli of the kidney and was able to distinguish two types, differing in size, lobulation, and position, and also, more interesting in the present study, differing in the kind of sheath (Hiille) which, by gentle shaking, he was able to detach from the knot of blood vessels. In one type of glomerulus this sheath contained nuclei, in the other none, or only a very few, in both the imprint of the capillaries was plainly visible. Von Ebner, in the 6th edition of Kolliker’s Gewebelehre, offers, I think, the correct interpretation of these facts in supposing that in the one type of glomerulus the surface of the cuboidal parts of the epithelium had become transformed into an exoplasm similar in structure to the plates, and that this surface layer with the plates could be detached, leaving the nuclear portion connected with the capillaries. This would account for the non—nucleated sheath; the whole epithelial layer, plates and cuboidal portions detached together, would furnish the other picture. Drasch has established two facts of interest to us, first, that the blood vessels of the glomerulus are actually covered by thin plates, which can be separated from the endothelium by certain artificial means, though in sections it is usually impossible to distinguish more than a single layer, so closely are the two applied; and secondly, that these plates are of a highly differentiated protoplasm, non-granular and rather stiff, in that they hold their shape even after being removed from the capillaries about which they have been molded. The first of these facts is important as explaining a very natural mistake made by Duval in his description of the placenta of the rabbit; the second shows that these modified plates have apparently become inactive membranes, through which a purely physical osmosis may take place, but which themselves may be supposed to be physiologically inert. A physiological activity is presumably present in the glomerulus, but limited to the thicker, still granular portions of the cells.


I have considered the glomerulus thus minutely, and offer a drawing of a portion of one in normal activity (fig. 1) because of the great lack of accurate descriptions and figures in the text-books. This is the more remarkable in contrast with the elaborate care with which the tubules almost uniformly are described.


The presence of thin plates of epithelium covering the capillary blood vessels is, then, the anatomical indication of that part of the excretory function which takes place in the glomeruli, either of the mesonephros or of the kidney. But it is not a sure sign that excretion is actually taking place at the" spot where it is found, and for two reasons. The first is that diffusion or osmosis is dependent on the proper pressure on either side of the membrane, and on the proper chemical constitution of the fluid on the two sides; failing these two requisites the plates may be present but inactive. The second reason is of a different kind, namely that another physiological process, the exchange of oxygen for carbon dioxide and water, also takes place mainly through the medium of thin plates overlying capillaries. In the ‘breathing epithelium’ of the lungs and of the gills of the different types of vertebrates there is again found the modification of the epithelial layer, originally columnar, to a succession of thin plates,_ covering capillaries, and nucleated masses of protoplasm either projecting or imbedded in the meshes of the capillary network. Here again is a provision for both physical diffusion and physiological active secretion or excretion. As far as the simple diffusion is concerned, there is no physical law to prevent the passage of certain urinary constituents, oxygen, and carbon dioxide all at the same time, even in opposite directions, through the same .membrane, if the conditions on the two sides of the membrane are favorable. In ordinary breathing this exchange in two directions is manifest. In other words, in seeking to establish the fact of a urinary excretion from the placenta by showing there the thin plates in their proper relation to the capillaries, it is necessary as far as possible to eliminate the probability of their use for fetal. respiration; but even if this cannot be done definitely, the possibility should be borne in mind that both processes, if each is considered a purely physical diffusion, might go on simultaneously through the same thin plates.

The Mesonephros

I have long been interested in the relative size of the Wolflian body in different types of embryos, because of its influence on their future development; as 1 have pointed out in earlier papers it seems to govern the time of the connection of the rete cords and the testis cords, the position of the spermatic or ovarian arteries, and the development of the renal artery with the consequent differences in the range of anomalies to be expected in these vessels. In this regard the commoner embryos may be grouped as follows; pig and rabbit, large Wolfiian bodies; sheep, medium size to small; cat, man, guinea pig, and opossum, small; mouse and rat, practically none at all. 1t was perhaps not an unnatural mistake to suppose that the larger the Wolffian body, the longer it would remain active, yet this is not at all the case. The pig, as pointed out by MacCallum, has an increasingly large mesonephros up to the 40.0 mm. stage, and no reduction in its size until 95.0 mm. The rabbit, on the other hand, while possessing at first almost as large an organ as the pig, begins to show mesonephric degeneration at about 20.0 mm. In the sheep, though the gland is never large, it is retained as in the pig, increasing up to about 25.0 mm., and with no reduction at 48.0 mm. It is for this reason that the sheep was classed among those animals with large Wolffian bodyin my study of the testis and rete cords; the presence in late embryonic life of this organ was erroneously thought to prove its great size in earlier periods. The oposssum retains an active Wolflian body after birth, and it is not replaced by the kidney for a considerable time. This in itself would prove that for this class of mammals, as in the lizards, the mesonephros is certainly functional as a urinary organ. The cat, the guinea pig, and man all have small Wolffian bodies, yet in the cat they increase steadily to 32.0 mm., while in the guinea pig and in man signs of involution soon set in, and by 15.0 mm. in the guinea pig the organ can no longer be considered active. In man the statements of many investigators are so conflicting as to the time of this involution that it is impossible to draw any very definite conclusions, but from my own observations it seems that the Wolffian body may be functional later than has been usually supposed, though many of its glomeruli certainly degenerate early.


In order to have some definite ideas as to the relative length of time during which the Wolffian body, or at least that part of it represented by the glomeruli, may be considered functional in the different kinds of mammals studied, I have counted and measured the active glomeruli indifferentages of embryos. This can give at best but crude, inaccurate results, since the diameter of a glomerulus may have no definite relation to the area of its surface and to the area of the epithelial plates on that surface; the diameter tells us nothing of the amount of lobulation, each new lobule increasing the surface area, nor of the ratio of epithelial plates to the protoplasmic bodies of the epithelial cells. Still the inaccurate results are sufficiently striking for the purpose of showing the great variation which exists between the different types of animals.


At one end of the scale are the embryos of the mouse and rat, which never develop mesonephric glomeruli. Next to these come the embryos of the guinea pig, in which the glomeruli are immature at 8.0 mm., are never large or numerous, and have undergone marked degeneration at 15.0 mm., when there are only fourteen in one Wolflian body, with an average diameter of about 52 micra. Even these few small glomeruli could hardly have been properly functional, as they lack almost entirely the epithelial plates. The mole, according to Weber and others, is in the same class as the guinea pig, with few, shortlasting glomeruli. . In the rabbit there is early a large organ; an embryo of 9.6 mm. has about forty active glomeruli on each side, with an average diameter of 110 micra. This increases only to forty-two glomeruli in an embryo of 14.5 mm., as many of the anterior ones are already lost; but there is a marked increase in lobulation and in the size of the individual glomeruli, whose average diameter is now 185 micra. By 21.0 mm. the organ has begun to diminish, in that there are only thirtyfour glomeruli on a side, with an average diameter of 200 micra; and soon after this all traces of the mesonephric glomeruli have vanished.


The mesonephros of man and its degeneration have been especially studied by Felix in the article in the Keibel-Mall Human Embryology before referred to. He shows a table of the number of mesonephric tubules found by him in many human embryos up to 21.0 mm. in length, and calls attention to the constant degeneration of the anterior tubules from 7.0 mm. on, and the addition of new tubules caudally. The number of tubules is not an absolute measure of the number of glomeruli, since some of the tubules may branch, or two tubules may lead from a single glomerulus; but it is sufliciently accurate for the present purposes, as the irregular tubules are always few. He finds an early degeneration of the organ, and then a period of rest. “From the stage of 21.0 mm. greatest length onwards, all embryos show a rather constant number of mesonephric tubules in the lumbar segments, but these tubules are almost all broken in one or several places.” In the quotation heading this article he again states definitely that none of the mesonephric tubules in an embryo of 22.0 mm. greatest length were capable of functioning, as in all the tubulus secretorius had sepa rated from the tubulus collectivus. In man, according to Felix, the rete tubules connect only with the tubuli collectivi, so that the secretory portion of all the tubules may disappear before this union takes place.


My own observations do not entirely agree with this account. In a former paper I have shown rete tubules connecting with the remains of the mesonephric corpuscles in certain cases, remains which are recognizable as late as the seventh month. This assures us that at least a few of the mesonephric tubules have remained intact (though of course not necessarily functional) from glomerulus to duct up to the time of the urogenital union.


In human embryos of 37.0 mm. to 40.0 mm. there are usually about a dozen mesonephric glomeruli on each side, some undergoing degeneration, others showing every sign of normal activity, with bulging epithelial plates over fully distended capillaries, and no change of the flat epithelial cells of Bowman’s capsule to a cuboidal layer, which is one of the characteristic signs of degeneration, according to von Winiwarter. By reconstruction I have been able to follow the tubules from certain of these apparently normal glomeruli to the duct. In each case, at the junction of the tubulus collectivus with the tubulus secretorius, the lumen suddenly narrows and the epithelium becomes indistinct, stains lightly, and is obviously changed from the normal. In other tubules actual breaks occur at this spot, as Felix noted, but I think he has been misled by the fact of these actual breaks at degenerated portions of some tubules to the conclusion that in all the tubules there is a loss of continuous lumen. The lumen is present in some tubules, and these are probably the ones found acting as ductuli efferentes in the few cases where the rete joins the corpuscle, instead of the tubulus collectivus.


In man, then, there is a small Wolflian body, early developed to its full capacity, but retaining its function, as far as the glom— eruli are concerned, only until the second or third month of intrauterine life, when the embryo has reached a length of 20.0 mm. to 30.0 mm. In a 10.0 mm. embryo there are about thirty-four active glomeruli in one organ with an average diameter of 125 micra; at 13.6 mm. there are about the same number, each of about the same diameter, but greater efficiency is probable as each glomerulus is more deeply lobed. At 30.0 mm. there is still the same number. At 40.0 mm. the number of glomeruli is reduced to about a dozen, and some of these show signs of degeneration.


In sharp contrast, in this respect, to the embryos of mouse, rat, guinea pig, rabbit, and man are those of the pig, sheep, and cat. In the pig, at 8.0 mm., there are fifty—one glomeruli on one side, forty-five on the other, according to MacCallum; my figures are slightly higher, fifty-four active glomeruli on each side. The average diameter, 200 micra, is very large when compared with those of the early embryos noted above. At 11.0 mm. there are sixty active glomeruli, at 24.0 mm. eighty on each side, and in each case the average diameter has reached 325 micra, six times that of the glomeruli of the guinea pig. At 95.0 mm. many active mesonephric glomeruli with the same large diameter are present, and in the same specimen the kidney also contains two or three rows of apparently fully active glomeruli, with bulging epithelial plates. It seems certain that in the pig, as pointed out by Weber, the activity of the mesonephros overlaps that of the metanephros, as is the case in birds, reptiles, and the opossum.


In the sheep the comparison of the size and number of the mesonephric glomeruli is an even less accurate guide to their relative activity than in the other mammals studied, because of the peculiar type of structure which the anterior corpuscles present. As 1 have recently shown,3 the first twenty or thirty corpuscles are without true glomeruli; the lower or caudal ones are of the usual type. But as the number of irregular, atypical corpuscles is approximately the same at all ages, their presence does not vitiate the count to any great extent. In a sheep embryo of 6.6 mm. there are six active glomeruli on each side in addition to the twenty or thirty atypical corpuscles, and their average diameter is 150 micra; at 15.8 mm. there are fifty normal glomeruli, of 230 micra; and at 40.4 mm. again fifty on each side, but with an increased diameter, 285 micra. In the 40.4 mm. embryo the kidney contains several active glomeruli with an average diameter of 100 micra.


The eat of 7.6 mm. has about twenty active glomeruli in each organ, diameter 150 micra; at 15.0 mm. the number has increased to twenty-six, and the diameter to 165 micra, but each glomerulus is much more lobed. At 39.0 mm. there are thirty glomeruli with an average diameter of 200 micra, and in this same specimen the innermost renal glomeruli are developing epithelial plates. At 85.0 mm. the Wolffian body has disappeared, but several rows of renal glomeruli are apparently active, as is shown in figure 4.

3 Bremer, loc. cit., p. 3.


Similar figures for the chick may be interesting. At 7.5 mm. twenty active glomeruli are found in each Wolffian body, with many more in the earlier stages of development; the average diameter is 95 micra. At 15.0 mm. the number has increased to one hundred and eight glomeruli, and the diameter to 105 micra. i

From ‘these figures, grouped below in tabular form, two facts stand out as evident; first, that the different embryos can be classed as those which retain a functional Wolffian body until the kidney is ready to take up the work of excretion, and those

TABLE I

Number and diameter of active mesonephric glomeruli at difierent ages. in one Wolflian body


6.6 T0 10 MM. 11 To 16 MM. 21 TO 40 MM. onmvm Rat . , . . . . . . . . . . . . none none none none Guinea pig . . . . . .. none 14, of 52 micra none none 34. of 125 micra 34. of 125 micra 12, of 125 micra none .. 40, of 110 micra 42, of 185 micra 34, of 200 micra none Cat , . . . . . . . . . . . .. 20, of 150 micra 26, of 165 micra 30, of 200 xnicra kidney Sheep . . . . . . . . . . . . 20+ 6, of 150 micra 20+50, of 230 micra 20+50, of 285 micra kidney —{—_ kidney Pig . . . . . . . . . . . . .. 54. of 200 micra 60, of 325 micra. 80, of 325 micia many of +kidney 325 micra +kidney

in which the Wolffian body disappears early, before the kidney has developed active glomeruli; and second, that -within each of these classes, individual animals are provided with a very Varying amount of excreting surface, showing presumably varying types of metabolism. In the cat, for instance, the paucity and ‘small size of the mesonephric glomeruli up to the time when the kidney becomes active, indicate a greatly reduced urinary excretion as compared with that probable in the pig, with its enormous and numerous glomeruli. Another example of the difference in glomerular number and size in two animals which early lose their Wolffian bodies is found by comparing rabbit and man.


The total glomerular surface of each of the animals here studied seems to have a definite relation to the rapidity of its intra-uterine growth and the consequent length of the period of its gestation. The size of the animal at birth must naturally be taken into consideration, as no one would expect a large full term pig fetus, for instance, to have developed in as short a time as the new-born guinea pig. But in comparing the guinea pig and the rabbit, one may be surprised that the former, smaller animal should have a gestation period twice as long as the latter. The number of embryos can have no influence in this case, as the rabbit normally has a much larger litter than the guinea pig. There is obviously an actual difference in the growth rate, and this is correlated with a difference in the excretory surface in the embryo; the slower the growth rate, the less active the cell changes, and also the less rapid the formation of waste products to be eliminated. Other factors undoubtedly underlie the causes of the differences, and it is not intended to suggest that a small Wolffian body can cause slow growth, or vice versa, but a comparison of the periods of gestation, as given by Grosser, of the animals here studied With the table showing the size and number of their mesonephric glomeruli shows that, if the size of the animal be considered, the growth rate is evidently correlated with the- glomerular surface. Grosser gives as the period of gestation for the rabbit, 28 days; for the rat, 35 days; for the guinea pig, 63 days; for the cat, 65 days; the pig, four months; the sheep, five months; man, nine months. The pig has a larger glomerular surface than the sheep, and a shorter period of gestation; man has less glomerular surface than either pig or sheep, and a longer intra-uterine life. If We consider the relative sizes of the cat and the sheep, the cat’s period of gestation might be regarded as proportionately the longer of the two, perhaps comparable to that of man; and the total glomerular surface of the cat embryo is ‘very similar to that of the human embryo. A systematic study of the comparative metabolism of different animals has not as yet been attempted, to my knowledge, though scattered facts such as the analyses of many types of urines are available. From the differences found in the amount of excreting surface developed in these embryos and the correlated differences in their intra-uterine growth rates it seems probable that great variations exist, Which, if persistent in the adult, should certainly be carefully considered in animal experimentation.

The Allantois

lf certain embryos excrete urine from the Wolffian body or kidney during their entire intra-uterine life, and others do not, there should be easily recognizable differences in the size of the receptacle for this urine in the two classes. In spite of the interest formerly taken in the time of the opening of the cloacal membrane, as a possible passage for the urine into the amniotic cavity, repeated chemical analyses of the amniotic fluid show that it may contain only traces of urinary matter, and thus make it extremely doubtful if this passage is ever used normally for any length of time. Failing this external’outlet, the urine from both the Wolffian body and the kidney must pass to the allantois. In birds and reptiles the allantois is a large sac capable of containing a considerable quantity of fluid. The same is true of certain mammals. According to Grosser, Hertwig, and others, the allantois is an enormous vesicle in the pig and sheep; in the eat it is again of large size, though smaller than in the former two animals. In the rabbit, on the other hand, it “reaches no special expansion; it is here lim.ited to the area of the placenta.”4 ln man and in the guinea pig, “that is, in those embryos which form no hollow allantois,“ only the slender allantoic stalk is found; yet, as was mentioned above, this narrow cavity seems to Keibel capable of storing a small amount of fluid. Minot states that “the allantois in man increases very little in diameter after the second month/’5 the time when the Wolfiian body, as we have seen, ceases to be functional. ln the rat and mouse, even this small receptacle is absent, as an entodermal allantois never develops.

‘ Hertwig, loc. cit., vol. 2, p. 250. ‘ Minot, loc. cit., p. 355.

It will be seen at once that in the animals selected for study there is a close relationship between the size and duration of the Wolffian body and the size of the allantois. The cat has a smaller allantois than the pig or sheep, because its Wolffian body is less effective, but a larger one than the rabbit since the urine is accumulated throughout intra-uterine life in the eat, but only for a short period in the rabbit.

The Placenta

The placentae of various mammals have been studied anatomically and physiologically by many of the best investigators, and the different types of placenta belonging to the different groups of mammals are well known. Grosser, one of the most recent writers on the subject, arranges the types as follows:

A. Semiplacentae (placentae appositae) a) semiplacenta diffusa. Type; pig. b) semiplacenta multiplex. The ruminants. B. Placentae verae (conjugatae) a) placenta zonaria. Type; most carnivora, cat. b) placenta discoidalis. 1. Rodents, rabbit, mouse, guinea pig. 2. Insectivora. 3. Cheiroptera. 4. Primates.

In the apposed placenta,‘ as its name implies, there is no definite union between the fetal and maternal tissues, no destruction of the maternal epithelium by the trophoderm of the chorion. The chorionic epithelium is apposed to the epithelium of the uterus, perhaps sending cell processes between the maternal cells, as maintained by Robinson, but separating from the maternal cells at the end of pregnancyhwithout destruction of the uterine surface. There is no intimate relation between the maternal blood and the fetal Vessels; nutrition and oxygenation of the embryo go on by means of the active absorption by the chorionic epithelial cells of products directly given to them by the maternal epithelium, or of the ‘uterine milk,’ the excretion of the uterine glands and of the surface epithelium, with many maternal leucocytes added. Respiration and nutrition must be active secretory processes, as nowhere in these placentae is found any thin osmotic membrane in relation with the fetal capillaries. Granules of absorbed material have often been found in the fetal epithelium.


The conjugate placenta, on the other hand, is formed by a destruction of the maternal tissue by the trophoderm, a loss of the uterine surface epithelium and of more or less of the deeper layers, and the consequent pouring out of the maternal blood into the intervillous spaces. The chorion is bathed in slowly circulating maternal blood, and it is from this, instead of from the uterine milk, or by the transference from one epithelial cell to another, that the embryo obtains food material and oxygen. The maternal epithelium of the uterus plays no part; the seat of the transfer is the epithelial covering of the chorion and its prolongations, the chorionic villi.


An exception seems to exist in those rodents with the so-called ‘inversion of the germ layers,’ for in these the yolk-sac entoderm, after the loss of its distal layers and of the chorion originally covering it, is spread out over a portion of the inner surface of the uterus, and may receive nutriment from the secretion of the uterine glands. But this is at best an accessory source, as the placenta proper is in these cases also composed of chorionic prolongations, bathed in circulating maternal blood. Another exception is found in the ‘green column’ or border zone of the zonate placenta of the cat and other carnivora, a large reservoir of extravasated blood, between the uterus and the chorion, from which the chorionic epithelial cells, here of a tall cylindrical type, can be seen to ingest certain red blood corpuscles. This again is an accessory source of food supply, as it develops only in the second half of pregnancy, and has no noticeable effect on the size or activity of the true placenta.


It is the characteristic of conjugate placentae, then, that the fetal chorion and villi, of whatever shape they may be, are bathed in circulating maternal blood, and that the transference of material between mother and embryo takes place through the fetal ectodermal epithelium. The character of this epithelium and its relations to the fetal capillaries, by which the material can be carried to or from the fetus itself, is of importance in this study, as it is through thin plates in this epithelium, closely covering the fetal capillaries, that osmosis of urinary matter would necessarily take place, if the present thesis is correct. Once passed from the fetal blood to the maternal circulation, these materials could be finally excreted by the maternal kidneys.


In certain placentae the extreme thinness of this epithelial layer and its close relation to the endothelium of the fetal capillaries are features of such prominence that they have been noted and figured by every writer on the subject. This is the case in the rodents. Duval, Minot, Grosser, all agree that in the placental labyrinth of these animals the earlier thick tropho— dermic layer becomes reduced to a very thin syncytium, with thicker areas containing nuclei, on one side of which is the maternal blood, on the other side the fetal capillaries (figs. 6, 7, and 8). In fact Duvalfi goes further and reports the ultimate disappearance of the syncytial plates. The capillaries thus appeared to him naked in the maternal blood-stream. The other authors do not agree with this, and, I think, rightly; it is much more likely that the plates and the endothelium are so intimately associated as to appear a single layer, as in the renal glomeruli, Where they have been-proved by Drasch to retain their independence, as already stated above. The conception of the nucleated portions of the syncytium remaining in situ but isolated from each other is rather hard to grasp.


The rodent placenta is thus provided with a membrane presumably suitable for osmosis as well as thicker nucleated portions for active secretion or absorption. But the different types of rodents differ in the length of time necessary to attain this arrangement. In the rabbit this modification of the trophoderm, “la période d’achévement de l’ecto-placenta,” according to Duval, comes near the middle of pregnancy,_ at 25 to 30 days.

“Duval, loc. cit., p. 119.


In the guinea pig, whose gestation is twice as long as that of the rabbit, it appears at about the same time, that is relatively early, the embryo in the same period having attainec little more than half the size of the rabbit embryo. The modification of the trophoderm is still more precocious in the rat, for at 13 days in the rat the succession of plates and cell bodies is easily recognizable (fig. 6).


By referring to the figures given in former paragraphs (page 187), one can readily see that the dates of the development of the plates in the placenta correspond accurately with those of the involution of the VVolffian body, seen in the rabbit at about 21.0 mm. and in the guinea pig quite advanced at 15.0 mm. In the rat, since no glomeruli ever develop, the placental plates are found at about the period in which the mesonephric glomeruli become active in other mammals. The development of possible osmotic membranes in the placentae of various rodents at the precise time when such membranes are lacking in the corresponding embryos can hardly, it seems to me be a mere coincidence.


Another type of conjoined placenta is that found in most carnivora, of which the cat and the dog have usually been taken as types. A placental labyrinth is present, with what corresponds to the chorionic villi; the separate villi are not free, however, but are bound together by their trophoderm, through which the maternal blood has made a network “of channels. Everywhere between the maternal blood stream and the fetal capillaries in the mesoderm of the villi the thick syncytial layer of the trophoderm intervenes. Nowhere does this layer become changed into membranous plates. At each edge of the zonate placenta there develops at about the middle of pregnancy the ‘green column’ of which mention has been made. Surprised at first by the Very small excreting surface exhibited by the glomeruli of the Wolffian body an.d kidney of the cat, I sought in this ‘green column’ for a possible osmosis between fetal and maternal blood, but only to find that there is here no maternal circulation, merely an extravasation, and that no sign of a plate-like chorionic epithelium exists amid the peculiarly high columnar cells. In the conjugate placenta of the cat, then, there is the same lack of provision for an osmotic interchange between mother and fetus as in the apposed placentae of the pig and the sheep. Thus in the placentae Whether apposed or conjoined, of all the animals here studied which retain functional mesonephric glomeruli until the renal glomeruli are active, the thin plates of fetal epithelium are absent at all periods of gestation.


The placenta of man is of the conjoined type with free floating villi of vascular fetal mesoderm covered by an ectodermal epithelium. The nature of this epithelium is well known from the researches of many authors. At first it is of two layers, the inner cellular, the outer syncytial, with more deeply staining protoplasm and many nuclei. Originally very irregular, this syney— tium assumes during the first and second months a more epithelial character, so that each villus shows clearly the two separate layers. Later degenerative processes set in; portions of the cellular layer disappear in places, leaving the syncytium as the only covering; the syncytium itself becomes changed in part into the ‘cell knots’ and ‘canalized fibrin’ of Minot. Degenerative vacuoles may appear in the syncytium later in pregnancy. The fetal capillaries approach nearer to the epithelium, running close to the bases of the still healthy portions of the syncytium, as they do in the active part of any gland. The surface of the chorion and its villi is so enormous in the human placenta that apparently much of it may become degenerated without destroying its functional ability ‘to too great an extent, and the areas of functional epithelium may be widely separated by the inactive areas of canalized fibrin.


It is probably on account of the great surface area presented in the human placenta and the consequent utilization of only small portions of it for active functions that the membranous plates formed by the syneytial layer in conjunction with the fetal capillaries have not been heretofore mentioned or figured, to my knowledge. A little search is necessary to find the scattered plates, but they may be encountered in all parts of the labyrinth. Perhaps their relative infrequency, as compared with those found in the rodent placenta, is another expression of the small amount of excretion in man, already noted in the paucity and small size of the mesonephric glomeruli. That they are actually present is shown in figures 5, 9, 10, and 11, drawings of the c.horion and villi of the human placenta at different ages. Their extent in relation to the blood—vessels is also shown in figure 12, the drawing of a model of a placental vi1lus'from the placenta at term, for which I am indebted to Mr. Alan Gregg, a student of this School. The relation of fetal capillary, plate, and maternal blood stream is the same as in the rodent placenta, and that of capillary and plate the same as in any glomerulus. As in the glomerulus, here also the plates are sometimes obviously separate from the endothelium, sometimes so closely adherent as to appear fused with it into a single layer. The stretches of the thicker, granular protoplasm intervening between adjacent plates are longer in the placenta than in the glomerulus, because the cells, presumably of granular type, which supply nutrition, etc., to the fetus are present in the placental epithelium, but absent in the glomerulus.


The time of the appearance of these plates in the human placenta agrees sufficiently accurately with the time of the progressive degeneration of the mesonephric glomeruli in the embryo. They are found, as is shown in figure 5, in the chorion of the placenta of a fetus of 29.0 mm. Felix, as we have seen, places the end of mesonephric involution at 22.0 mm., which should be advanced slightly according to my own observations. The plates in the placenta may well be present even earlier; I have made no systematic search for them in closely graded stages. The plates increase progressively in number as pregnancy advances, and are quite striking at term. Their location seems to be gradually changed from the surface of the chori.on in younger placentae to the smaller villi in older ones, which means, of course, that the early plates degenerate as new ones are constantly being formed.


We have seen that the placentae of all of that class of embryos in which we have found an early involution of the mesonephros exhibit membranous plates in the proper relation to the fetal vessels, and that the time of the disappearance of the one is approximately the same as that of the appearance of the other. Moreover, the placentae of those animals studied which are able to utilize the Wolffian body until the kidney is ready for action show no such modification of the ectoderm. There are two objections, it seems to me, which might stand in the way of an immediate acceptance of these facts as proof that the placental plates surely represent the lost glomerular ones. The first has already been mentioned; since the ‘breathing epithelium’ of the lungs and of the gills is also characterized by membranous plates, may not the placental plates represent the apparatus for fetal oxygenation instead of for fetal excretion? As was pointed out previously, it is possible that the same apparatus may serve for both purposes. The strongest argument for considering the plates excretory instead of respiratory is the fact that all embryos from. very early periods require oxygen, yet many classes of mammals never develop the placental plates, and in many others they appear relatively late, long after the need for oxygen must have been felt. In the rat alone, of those animals studied, would the development of the apparatus keep pace with the needs of the embryo. On the other hand, the fact that in the pig, sheep, and cat placental respiration must in all probability be an active secretory process does not eliminate the possibility of an osmotic respiration for other embryos at certain periods. The second objection is perhaps a little more puzzling. If in certain cases the kidney becomes functional during fetal life, as We must suppose in animals provided with a large allantoic reservoir, and no placental osmotic apparatus, why should it not also become active in all mammals? In other words, Why should excretion in certain animals take place through the placenta instead of through the kidney in the later periods of pregnancy, when the kidney is apparently capable of activity? Of the readiness of the older fetal kidneys of various classes of animals to perform the excretory function there is hardly a doubt. The kidney glomerulus of a new-born rabbit is quite similar to that of a new-born cat, though the one has not yet presumably been functional and the other has_been active for many days.


In the case of man the question of the activity of the fetal kidney has been much considered by anatomists and physiologists, and many conflicting reports are published pro and con. The question is discussed by all those interested in the origin and constitution of the amniotic fluid. That the fetal kidney in man may be active just before birth is abundantly proved by the numerous observations of immediate micturition in the newborn, yet the small quantity passed in these cases and the absence of any great amount of the urinary constituents in the accompanying amniotic fluid show that the renal activity has not been of long duration. Ahlfeld and English demonstrate, by their cases of total absence of the fetal kidneys and of the blocking of the fetal urinary passages, already referred to, that the kidney is not a necessity before birth; on the other hand, cases of fetal hydronephrosis, of fetal calculi, and the occasional presence of large amounts of urinary matter in the amniotic fluid, all point to a prolonged renal activity. Preyer reviews the various statements and comes to the conclusion that the real cause of the inactivity of the fetal kidney is probably the low blood pressure of the fetus, and in proof of this contention quotes from Schatz a case of twins with separate amniotic cavities. One, which had an enormous amount of amniotic fluid, urinated a great quantity and almost hourly during the six hours of its life; the other, which had very little amniotic fluid, passed no urine in twelve hours. The kidneys and heart of the first weighed one and a half times as much as those of the second. The first had a much higher blood pressure, “liefert mehr I-Iarn und dadurch mehr Fruchtwasser.”7 Whether the fetal blood pressure of the pig, sheep, and eat, animals in which the fetal kidney is active, is relatively higher than that of rodents and man, in which it is not, I do not know; nor is it easy to understand the rise of fetal blood pressure within the placenta of rodents and man, to cause excretion there, which this explanation seems to call for.

Various experiments, quoted by Wertheimer, on rabbits and guinea pigs near term or in the latter part of pregnancy prove that the kidney of these animals also is capable of activity, but, as Wertheimer points out, this does not prove that excretion does normally take place, as in all of these experiments some disturbance of the fetal circulation is inevitable. The question, it seems to me, may be left open to further investigation, undertaken with a knowledge of the possibility of a placental excretion in certain animals.

7 Preyer, loc. cit., p. 333.

Conclusions

The Wolffian body or mesonephros is a gland of urinary excretion.

Mammalian embryos may be divided into two classes; those which retain functional Wolffian bodies until the kidneys are sufiiciently developed to excrete urine, as is the case in birds and reptiles, and those in which the Wolffian bodies degenerate before the kidneys reach functional ability. The first class includes the pig, sheep, and cat; the second, the rabbit, guinea pig, man, and rat.


Within each of these classes individual animals show great differences in the size and presumable excretory ability of the Wolflfian bodies, without regard to the length of their duration.


The allantois is the receptacle of the urine formed within the body of the embryo; it is present as a reservoir only in those animals with an embryonic excretion, and its size varies with the size of the VVolffian bodies and with their duration. The urethral opening, though present, is not normally used for the passage of fetal urine. ,


In those animals without the possibility of a continuous urinary excretion Within the embryo, i.e., with an early degeneration of the Wolffian body, the placenta is provided with an apparatus similar to that found in the glomeruli of the Wolfiian body or the kidney, thin plates of epithelium overlying the fetal capillaries. These appear in the placenta at about the time when the Wolffian body commences to degenerate, or in the ease of the rat, which never develops mesonephric glomeruli, at about the time of the normal development of the glomeruli in other embryos. These plates continue and increase in number till term. They are apparently of greater extent in animals Whose embryos are provided With large Wolffian bodies.


In the placentae of those animals with a continuous; embryonic urinary excretion, similar plates are not found, Whether the placentae be of the apposed or conjoined type.

From these facts it appears that embryonic and fetal urinary excretion takes place Wholly through the placenta in the rat, at first through the Wolffian body and later through the placenta in the rabbit, guinea pig, and man, but never through the placenta in the pig, sheep, or cat. A knowledge of these differences should lead to more intelligent experiment on the permeability of the placenta.

Literature Cited

AHLFELD, F. 1897 Archiv f. Gyn-.1ek., Bd. 14.

BAKOUNINE, S. 1897 A. i. B., vol. 23.

BRLMER, J. L. 1911 Am. Jour. Anat., vol. 11, no. 4.

1915 Anat. Roc., vol. 10, no. 1.

Inmson, O. 1877 Sitzber. d. k. Akad. Wien, Bd. 79.

DUVAL, M. 1892 Placenta. des Rongeurs, Paris.

ENGLISH, J. 1881 Archiv f. Kindcrhcilk. Stuttgart, Bd.

FELIX, W. 1912 In Kcibel-Mall Human Embryology, vol. 2. Gnossmn, O. 1909 Vergl. Anat. d. Eihéiute u. (1. Placeiita, Wien. HERTWIG, O. 1906 Handbuoh d. vergl. Entw. d. Wirbelth. Jena. HUBER, G. C. 1905 Am. Jour. Anat., vol. 4, supplement. KEIBEL, F. 1897 Arohiv f. Anat. u. Entw.

'Ki3LLIK1aR, A. 1902 I-Iandbuch d. Gewebcl. dos Menschcn.

vol. 3. KRUKENBERG 1885 Archiv f. Gyn:Lok., B171. 26. MAcCALLmr., J. B. 1902 Am. Jour. Anat., vol. 1, no. 3. .\l1NoT, C. 1901 Human Embryology. New York. 1889 Jour. of Morph., vol. 2, no. 3.

NAGEL, W. 1889 Archiv f. Gynaeln, Bd. 36. NICOLAS, A. 1891 Jour. Intern. d’An:-Lt., vol. 8. Pnmvmc, W. 1883 Specielle Physiol. d. Embryos. ROBINSON, A. 1904 Jour. Anat. and l’hys., vol. 38. STOERK, 0. 1904 Anat. I-Iefte. lid. 72.

\'VEl5ER, S. 1892 Dissert. med., I"i'oibu1'g i. Br.

Leipzig.

Leipzig.

6th ed.,

\Vl1}lt'1‘l1EI.VIER, F}. 1904 Article on Fetus, Riohet’s Dictionnaire de Physiologic.

VVIEDERSHEIM, R. 1886 Lehrbuch d. vergl. Anat., Jena. 2d ed. vow VVIl\'IWARTElR. H. 1910 Archiv f. Bio1.. Tome 25.

Plates

Abbreviations

b.l., basal layer of fetal eetoderm f.cap., fetal capillary 6nd,, endotheliuni m.b.s., maternal blood snus ep., thicker, granular portion of epi- ’pl., epithelial plate thelium syn., fetal syncytiurn, trophoderm

Plate 1

1 Portion of adult human renal glomcrulus. B0wman’s capsule and Walls of convoluted tubules to the right of fi<g;ure. Note the lobuation of the glomerulus, the epithelial plates covering the capillaries at the border of the capsular cavity, and the cell bodies, as at “ep.” on this border between the plates. At “end.” the plate and the underlying; endothelium are each distinct. X 640 diameters.

2 and 3 Portions of immature mesonephric glomeruli from {L human embryo of about 10 mm. (Keibel, no. 1495.) To show development; of epithelial plates and cell bodies. >< circa 640 diameters.

4 Portion of renal glomerulus of cat fetus of 8.5 cm. Orientation as in figure l. Epithelial plates already present. X 640 diameters.

6 Part of labyrinth of placenta of a rabbit of 27 clays, showing the endothelium of the fetal capillaries and the succession of thin plates and thicker nucleated portions of the ectodcrm, between capillary and maternal blood stream. Copied from Duval’s Atlas, Placenta cles l{ongeurs,figure 62. X -170 diameters.

7 Portion of placenta of a guinea pig of the second month, similar to figure 6. Copied from DuVal’s Atlas, figure 262. X 300 diameters.

Plate 2

5 l’ortion of placental ehorion of human embryo of 29.0 mm. (H. E. C, No. 389). Above, the ehorionic Inesoderm; the basal layer of the ectoderm and the syneytial layer are both interrupted by a fetal capillary, separated from tl1e maternal blood stream only by an eetodermal plate, pl., which is closely adherent to the endothelium of the capillary. X 640 diameters.

8 Portion of chorion and labyrinth of placenta of a rat of 13 days (H. 1‘). C. no. 1930, sect. 143). The same production of epithelial plates separating the endothelium of the fetal capillaries from the maternal blood stream. The two streams are recognizable by their blood corpuscles. It will be noticed that the plates occur against both fetal arteries and veins. The basal layer of fetal eetoderm has partially disappeared. X 250 diameters. .

9 Villus of human placenta of 3 months. Note the complete syncytial layer of the fetal eetoelerm, and the basal layer interrupted by a fetal capillary, over which the Syncytium has developed :1. plate. X 480 diameters.

10 and ll Villi of human placenta at term. The basal layer of eetoderm is no longer present. The syncytial layer shows a succession of yhick granular mieleated portions and thin epithelial plates in direct. Contact with the fetal capillaries. The maternal blood stream surrounds the villi. X 480 diameters.

12 ‘.\*loclel of the blorxl-vessels and the ectodermal syneytiltm of a villus of the human placenta at term. It will be noticed that two small villi have fused, makiiig a ring formation, around which capillaries pass. One artery and two Veins pass into the villus. In addition to the areas seen in profile where the eetorlermal covering is of plate-like thinness, the blood-vessels are also covered by plates between at and :3, and at 1/, and 2. This, with figure 11, shows the relative extent ef the plates and the thicl<or syneytillin. X 250 diameters.



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