Talk:Paper - The interrelations of the mesonephros, kidney, and placenta in different classes of animals (1916)
THE INTERRELATIONS OF THE MESONEPHROS, KIDNEY AND PLACENTA IN DIFFERENT CLASSES OF ANIMALS
JOHN LEWIS BREMER
FroiN iJic Drpdrhnciit of Anato))!!/, Harvard Medical School
TWf:LVE FIGURES
111 most aiiuiiiiiia the luesoiicphros 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 jirocesses has without further consicloration l)eeii transferred to the relation between the mesonephros and the metanephros, and the question whether the mesonephros is actually functional in the amniota 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 (^xcretory function, he has assumed that the mesonephros did not function, for if it had Ijeen 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 tulniles 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 tul)ules were capable of functioning; in all the tubulus secretorius had separated from the tubulus collectivus. If one inquires how far the develo]oment of the metanephros has progressed at this time, one finds that embryos of 22 mm. have just reached the anlage 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 couivse, imply that it may not have been active in another manner unknown to us.
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180 JOHN LEWIS BREMER
This paragraph by Fehx in the Keibel-Mall Hiiiiian Embiyology' 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 tul)ules and the collecting tubules in both organs, the facts that develoi)mentally the excreting portions of both organs are derived from the nephrogenic tissue and have practically the same history, and that both Wolffian 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 meson ephros is not after all the 'middle kidney.' The statement is little short of iconoclastic, for ever since the time of Joh. Miiller 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 oi-gan during a part of its existence. If 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 mesone]:)hros, apparently active previously, nuist have had some other, as yet unknown function.
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INTERRELATIONS OF THE MESONEPHROS, ETC. 181
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 ehtodermal 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 mm., 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 miiseten wir uns zu der Annahme einer Sekretstauung in den ableitenden Wegen mit all' ihren Folgen verstehen, oder wir mtissen, und das ist wohl zweifellos das richtigere, auf die Annahme der lebhaften absondernden Funktion verzichten."- 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 bj^ 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
- Weber, loc. cit., p. 67.
THE AMERICAN JOURNAL OF ANATOMY, VOL. 19, NO. 2
182 JOHN LEWIS BREMER
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.
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 developm.ent, to take over the work, leaving no gap when neither is available. But this exception 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.
INTERRELATIONS OF THE MESONEPRHOS, ETC. 183
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 entirelj^ 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 Wolffian 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 tj^e 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
INTERRELATIONS OF THE MESONEPHROS, ETC. 185
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 (Hlille) 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 highlj^ 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
186 JOHN LEWIS BREMER
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 elimi
INTERRELATIONS OF THE MESONEPHROS, ETC. 187
nate 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 ME80NEPHR0S
1 have long been interested in the relative size of the Wolffian 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 Wolffian bodies; sheep, medium size to small; cat, man, guinea pig, and opossum, small; mouse and rat, practically none at all. It 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 body in 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 Wolffian 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
188 JOHN LEWIS BREMER
steadily to 32.0 mm., while in the guinea pig and in man signs of involution soon set in, and bj^ 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, 1 have counted and measured the active glomeruli in different ages 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 Wolffian 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
INTEREELATIONS OF THE MESONEPHROS, ETC. 189
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 sufficiently 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 separated 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.
190 JOHN LEWIS BREMER
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 1 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 1 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 Wolffian body, early developed to its full capacity, but retaining its function, as far as the glomeruli 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
INTERRELATIONS OF THE MESONEPHROS, ETC. 191
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, wdth 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, ^ 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 cat 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 dis ' Bremer, loc. cit., p. 3.
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JOHN LEWIS BREMER
appeared, but several rows of renal glomeruli are apparently active, as is shown in figure 4.
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 mi'cra.
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 different ages, in one
Wolffian body
Rat
Guinea pig
Man
Rabbit
Cat
Sheep
Pig
6.6 TO 10 MM.
none
none
34, of 125 micra
40, of 110 micra
20, of 150 micra
20+ 6, of 150 micra
54, of 200 micra
11 TO 16 MM.
none 14, of 52 micra 34, of 125 micra 42, of 185 micra 26, of 165 micra 20+50, of 230 micra
60, of 325 micra
21 TO 40 MM.
none none 12, of 125 micra 34, of 200 micra 30, of 200 micra 20+50, of 285 micra + kidney ' 80, of 325 micia +kidney
none
none
none
none kidney kidney
many of 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.
INTERRELATIONS OF THE MESONEPHROS, ETC. 193
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
194 JOHN LEWIS BREMER
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
If 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 cat 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 limited to the area of the placenta."* In 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, "^ the time when the Wolffian body, as we have seen, ceases to be functional. In 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.
INTERRELATIONS OF THE MESONEPHROS, ETC. 195
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 cat, 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 pregnancy without 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 ^iterine milk,' the excre
196 JOHN LEWIS BREMER
tion of the uterine glands and of the surface epithehum, 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 onl}^ 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 ^^illi, 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
INTERRELATIONS OF THE MESONEPHROS, ETC. 197
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 urinaiy 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 trophodermic 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 Duval- 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 alcove. 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 pro^'ided 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 periode d'achevement de I'ecto-placenta," according to Duval, comes near the middle of pregnancy, at 25-to 30 days.
« Duval, loc. cit., p. 119.
THE AMEIUC-VN' JOl.liVM, OF AX.VTOMV, VOI,. 1!), NO. 2
198 JOHN LEWIS BREMER
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 attained 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 Wolffian 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 \'ery small excreting surface exhibited by the glomeruli of the Wolffian body and 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, merel}' an extravasation, and that no sign of a plate-like chorionic epithelium exists amid the peculiarly high columnar cells. In the
IXTEHKELATIONR OF THE MESONEPHROS, ETC. 199
conjugate placenta of the cat, then, there is the same hick o^ provision for an osmotic interchanoe })et\veen motlier and fetus as in the apposed phicentae of the pig and the sheep. Tlius 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 co\ered 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 protoi:)lasm and many nuclei. Originally very irregular, this syncytium 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 syncj^tium, 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 inacti\e areas of canalized fibrin.
It is probably on account of the great surface area presented in the himian placenta and the consequent utilization of only small portions of it for active functions that the membranous plates formed by the syncytial layer in conjunction with the fetal capillaries have not been heretofore mentioned or figured, to nw knowledge. A little search is necessary to find the scattered plates, but thej^ 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
200 JOHN LEWIS BREMER
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 chorion 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 villus from the placenta at term, for which 1 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 chorion 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
INTERRELATIONS OF THE MESONEPHROS, ETC. 201
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 i)ig, 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 manv davs.
202 JOHN LEWIS BREMER
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 niehr Harn und dadurch mehr Fruchtwasser."'^ Whether the fetal blood pressure of the pig, sheep, and cat, 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 AVertheimei', on rabbits and guinea pigs near term or in the latter part of pregnancy prove
^ Preyer, loc. cit., p. .333.
INTERRELATIONS OF THE MESONEPHROS, ETC. 203
that the kidney of these animals also is capable of activity, but, as Wertheinier 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.
CONCLUSIONS
The AVolfhan jjody 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 sufficiently 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 Wolffian l;)odies, 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 Wolffian 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 i)ossibility 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 Wolffian body or the kidney, thin plates of epithelium overlying the fetal capillaries. These appear in the placenta at about the time when the Wolffian bod}^ commences to degenerate, or in the case 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
204 JOHN LEWIS BREMER
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.
INTERRELATIONS OF THE MESONEPHROS, ETC. 205
LITi:i{.V'ri'RE CITED
Ahlfeld, F. 1897 Archiv f. Gynaek., lid. 14.
Bakouxixk, S. 1897 A. i. B., vol. 23.
liEjEMEK, .1. L. 1911 Am. Jour. Anat.. vol. 11, no. 4.
191.3 Anat. Rec, vol. 10, no. 1. ■ ■
Dkasch, O. 1877 Sitzber. d. k. Akad. W'ien, Bd. 79. Duval, M. 1S92 Placenta des Rongeurs, Paris. English, J. 18S1 Archiv f. Kinderheilk. Stuttgart, Bd. 2. Felix, W. 1912 In Keibel-Mall Human Eml)ryology, vol. 2. Grosser, (). 1909 Vergl. Anat. d. P^ihaute u. d. Placenta, Wicn. Hertwig, O. 190G Handl)uch d. vergl. Entw. d. Wirbelth. Jena. HuBER, G. C. 190.5 Am. Jour. Anat., vol. 4, supplement. Keibel, F. 1897 Archiv f. Anat. u. I-hitw. KoLLiKER, A. 1902 Handhuch d. Geweliel. des Menschen. Leipzig. Gth ed.,
vol. 3. Krukenberg 188.5 Archiv f. (iyiiaek., Bd. 26. MacCallum, J. B. 1902 Am. Jour. Anat., vol. 1, no. 3. MixoT, C. S. 1901 Human Embryology. New York.
1889 Jour, of Morph., vol. 2, no. 3. Nagel, \V. 1889 Archiv f. Gynaek., Bd. 3G. Nicolas, A. 1891 Jour. Intern. d'Anat., vol. 8. Preyer, W. 1883 Specielle Physiol, d. Embryos. Leipzig. Robinson, A. 1904 Jour. Anat. and Phys., vol. 38. Stoerk, O. 1904 Anat. Hefte. Bd. 72. Weber, S. 1892 Dissert, med., Freiburg i. Br.
VVertheimer, E. 1904 Article on Fetus, Richet's Dictionnaire de Physiologie. WiEDERSHEiM, R. 1886 Lehrlnich d. vergl. Anat., Jena. 2d ed. von Winiwarter, II. 1910 Archiv f. Biol., Tome 25.
ABBREVIATIONS
b.l., basal layer of fetal ectoderm f-cap., fetal capillary
end., endothelium m.h.s., maternal blood sinus
ep., thicker, {ri-auular portion of (>pi- pi., e])ithelial plate
thelium ^T/"-, fetal syncytium, trophoderm
PLATE 1
EXPLANATION OF FIGURES
1 Portion of adult human renal glomerulus. Bowman's cajisule and walls of convoluted tubules to the right of figure. Note the lobulation of the glomerulus, the epithelial plates covering the capillaries at the border of the capsular cavity, and the cell l)odies, 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 a human embryo of about 10 mm. (Keibel, no. 149.5.) To show development of epithelial plates and cell bodies. X circa 640 diameters.
4 Portion of renal glomerulus of cat fetus of 8. .5 cm. Orientation as in figure 1. Epithelial plates already present. X 640 diameters.
6 Part of labyrinth of placenta of a ral)bit of 27 days, showing the endothelium of the fetal capillaries and the succession of thin jilates and thicker nucleated portions of the ectoderm, between capillary and maternal blood stream. Copied from Duval's Atlas, Placenta des Rongeurs, figure 62. X 470 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.
206
/NTERRELATIOXS OF THE .MESONEPIIUOS, ETC.
JOllX LEWIS B REM EH
PLATE 1
■J
eij m^:^^^^^^ep.
^n§l
m.D.s.
FioS
^^^•'^
207
PLATE 2
EXPLANATION OF FIGURES
5 Port ion of placental chorion of human embryo of 29.0 mm (H. E. C. No. 389). Above, the chorionic mesoderm; the basal layer of the ectoderm and the syncytial layer are both interrupted by a fetal capillary, separated from the maternal blood stream only by an ectodermal plate, pi., 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. E. C. no. 1930, sect. 143). The same production of epithelial plates separating the endothelium of the fetal capillaries from the maternal l)lood 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 ectoderm has partially disappeared. X 250 diameters.
9 Villus of human placenta of 3 months. Note the complete syncytial layei of the fetal ectoderm, and the basal layer interrupted by a fetal capillary, ovei- which the syncytium has developed a plate. X 480 diameters.
10 and 11 Villi of human placenta at term. The basal layer of ectoderm is no longer present. The syncytial laj-er shows a succession of thick granular nucleated portions and thin epithelial plates in direct contact with the fetal capillaries. The maternal blood stream surrounds the villi. X 480 diameters.
12 Model of the blood-vessels and tlic ectodermal syncytium of a villus of the human placenta at term. It will l)e noticed that two small villi have fused, making 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 ectodermal covering is of plate-like thinness, the blood-vessels are also covered b}- plates between x and x, and at y, and z. This, with figure 11, shows the relative extent of the plates and the thicker syncytium. X 250 diameters.
208
IXTERKELATIONS OF THE MESOXEPHROS, ETC.
JOHN LEWIS BREMEK
PLATE 2
%-5
pi.
ayo
r^.V
">i3, \/-^
Jcap. fig-^ h^
end. pi. sun
'iqlZ
icap.gr ~ / ^,
_...x
Fio.9
209
THE DE^ ELOPMENT OF THE LIVER AND PANCREAS IN AMBLYSTOMA PUNCTATUM
E. A. BAUMGARTNER
Institute (if Axatumy, University of M innesota
FORTY-SIX FIGURES (fOUR PLATES)
CONTENTS
I. Introduction 211
IL The development of the liver, heiiatic ducts and gall-bladder 213
1. Literature 213
2. Early development of the liveV 218
3. Position of the organ during development 223
4. Development of the biliary apparatus 226
a. Description of hepatic ducts in the adult 226
b. Development of the ductus choledochus 230
c. Development of the major hepatic ducts 232
d. Development of the minor hepatic ducts 236
6. Development of the gall-bladder and cystic duct 238
f. Summary of the development of the biliary apparatus 242
III. The development of the pancreas and jiancreatic ducts 247
1. Literature 247
2. Early development of the pancreas and pancreatic ducts 250
3. Description of the adult ])ancreas 260
4. Discussion 261
IV. General Summary 264
V. Bibliography 266
I. INTRODUCTION
Comparatively little work has been done on the morphology of the biliary and pancreatic duct-systems in vertebrates. The arrangement of these structures has been worked out in the adult forms of a few species but no attempt has been made to correlate these scattered observations or to determine what may be considered the typical arrangement in vertebrates and the major variations which may occur in the various groups of the phylum. The development of these systems is also
211
212 E. A. BAUMGARTNER
almost unknown. Although the formation of the anlagen of the liver and pancreas has been investigated in almost ever}' group of vertebrates, the later histor^^ of duct systems of these structures has been quite neglected. The two exceptions to this statement are furnished by the work of Corner ('13) who investigated the development of the pancreatic ducts of the pig by means of injection methods and Scammon's study of the biliary system of selachians.^
The following study is an attempt to follow in detail the development of these duct-systems in the tailed amphibia, and to point out the embryologic significance of the principal variations which are encountered in the adult and the mechanical influences which are, in part at least, responsible for them. Although we are not as yet in possession of sufficient data to formulate a statement of the typical vertebrate plan of biliary and pancreatic duct-systems, it is hoped that this description of these structures in a representative amphibian may add to the material upon which such a schema must eventually be based.
The material used for this work consisted of embryos of Amblystoma punctatum froin 4 nmi. to 20 cm. in length. These were sectioned serially in transverse and sagittal planes. Graphic and wax reconstructions were made of the hepatic ducts, gallbladder, liver and pancreas of different embryos and adults.
It is a pleasure to express my thanks to Dr. Richard E. Scammon for his constant interest and helpful criticisms throughout this study.
A correlation of the embryos employed in this study with those described in the Normal Plates of Necturus maculosus by Eycleshymer and Wilson may be desirable. This correlation is based on a comparison of the digestive system including liver and pancreas, as well as partially on the external form. Probably the greatest difference in the de\'elopment of the digestive tract between these two forms is in the time of union of the dorsal and ventral pancreatic anlagen which had taken
' The terms used by Scaininou ("lo) in (Icsciiiiing the ihicls of Ehismobraiichs have been used in tliis i)a|)er.
DEVELOPMENT OF LIVER AND PANCREAS
213
place in most of the 13 imn. Amblystoma embryos which I have observed, and is described in stage 42 (29 mm.) in the Normal Plate series of Necturus. Also the limbs, particularly the caudal ones, appear comparatively later in Amblystoma. Such a table, of course, can be only an approximate comparison.
TABLE 1 Correlation of Amblystoma embryos with the Normal-plate series of Necturus
FIGURES
EMBKTOS
NORMAL-PLATE SERIES
Figure
Length in mm.
Stage No.
Length in mm.
1
4.5
5
7
9
9
11 11.5 12.5 13 13
13.5 14
13.5 15 20
21
22-23
25
28
29
30
31
34
38 39 42 43 45 49
8
2
9
3
12
4
15
5
16
27-30
6
17
18
21
21
31, 45
18
25
19
26
8, 39, 40
7A, 9, 41
10, 33, 42, 46
11, 20, 43
29
30
32
39
II. THE DEVELOPMENT OF THE LIVER, HEPATIC DUCTS AND
GALL-BLADDER
1. Literature
The literature of the development of the great glands of the digestive tract of Amphibia can be conveniently divided into two parts covering two fairly distinct periods: first, the work of the early investigators who determined the position of these glands in the embryo and their relation to the lower germ layer; second, the series of contributions beginning with Goette's large monograph upon the development of Bombinator ('75) and dealing mainly with the detailed developmental anatomy of these organs.
THE .\MBU1CAN JOURNAL OF AXATOMY, VOL. 19, NO. 2
214 E. A. BAUMGARTNER
The following table gives a list of the authors, the dates of their publications and the material upon which their work on the development of the liver and pancreas was based.
Steinheim ('20) studied older embryos and observed the attachment to the gut. Rusconi ('26) investigated younger embryos and, as did Reichert ('40) and Vogt ('42), described the ventral growth of the intestine to form the liver. Remak ('55) and v. Bambecke ('68) differed from the above only in the number of lobes formed and noted the close relation of the gall-bladder to the right lobe.
According to Goette ('75), the liver in Bombinator originates as a ventral outpouching of the foregut posterior to the heart. This diverticulum becomes separated from the gut by a gradual cranio-caudal constriction, and the narrow connection which remains forms the common hepatic duct. The outpouching then grows by the production of folds or buds from its sides which form the primary hepatic columns. The lumina remain in these columns although they may be very small. Goette regarded the early anastomoses and formation of the net-like hepatic cylinders as aided by the ingrowth of a capillary network. The gall-bladder develops as an outpouching of the posterior part of the primitive hepatic duct caudal to which the ductus choledochus is formed.
Balfour ('81) made the statement that there is a single ventral diverticulum from the gut which later develops into two secondary branches and so forms the liver.
Shore ('91) in his study on the frog found that the liver takes origin as a ventral lengthening of the gut lumen into the mass of yolk-cells which lies posterior to the heart. The yolk-cells lining this lumen are transformed into hepatic cells and this mass becomes partially separated from the gut. This constriction is aided by the caudal growth of the sinus venosus. Later there is formed at the expense of the yolk-cells and by cell-division a large cell-mass into which the blood-vessels tunnel forming a tubular gland whose columns divide and anastomose producing a network interlacing with that of 'blood-lacunae.'
DEVELOPMENT OF LIVER AND PANCREAS
215
Marshall ('93) gave a brief account of the development of the liver in the frog in his vertebrate embryology. He described a caudo-ventral projection from the anterior part of the mesen
TABLE 2
Table of authors and the fo7-nis studied
Steinheim
Rusconi
Reichert
Rusconi
Vogt
Remak
Rathke
Bambecke
Goet te
Wiedersheim
Balfour
Shore
Goeppert
Marshall
Minot
Weysse
Stohr
Hertwig. :
Brachet
Hammar
Woit
Kolhnann
Gianelli
Choronshitzky
Reutcr
Gianelli
Piper
Weber
Braun
Eycleshymer and
Wilson
Baumgartner
MATERIAL
Rana
Rana
Rana temporaria
Rana esculenta
Salamandra
Alytes obstetricans
Rana temporaria Rana esculenta
"Vertebrates" Pelobates fuscus Bombinator igneus Salamandra perspicillata"Amphibia" Rana Salamandra maculata., etc.
Bufo vulgaris, etc. Rana
"Amphibia" Rana temporaria Rana esculenta Rana temporaria "Amphibia"
Review Rana
Rana temporaria Triton taeniatus, etc. "Amph bia" Triton cristatus Rana temporaria Salamandra maculosa, etc. Alytes obstetricans Triton RevieAv Review.
Alytes obstetricans -Necturus maculosus
Amblystoma punctatum
216 E. A. BAUMGARTNER
teron. The anterior wall of this depression is thrown into folds, blood-vessels penetrate between these structures and outgrowths from the hypoblast form the hepatic cylinders.
Weysse ('95) found in the frog that the liver-anlage is a dorsoventral cleft extending into the yolk-mass from the gut lumen. A caudal extension of this cleft forms the posterior hepatic duct, while the cranial hepatic duct is formed by a folding of the anterior wall of the hepatic anlage. The yolk-cells are transformed into the true hepatic cells and can be early recognized by the deposit of pigment within them.
Hertwig ('96) and Kollman ('98) gave only short descriptions, stating that in Amphibia the hepatic anlage is a single outpouching from the ventral wall of the duodenum.
Hammar ('97) who worked on the development of the frog's liver, has named the entodermal cell-mass posterior to the heart the 'Leberprominenz.' Into this extends an early lengthening cavity which is continuous with the lumen of the gut. This he termed the 'Leberbucht.' By a cranio-caudal constriction this hepatic anlage is separated from the gut. The cell-mass about the fundus of this anteriorly directed sac develops into trabeculae of the adult organ and the posterior part forms the ductus choledochus. The gall-bladder is developed very early as a diverticulum of the ventral wall of the common bile duct, and by further growth comes to be a pedunculated organ, consisting of a cystic duct and gall-bladder proper. He regarded the origin of the trabeculae as perhaps due partially to the developing capillary network tunnelling into the hepatic cell mass as suggested by Shore.
Choronshitzky ('00) showed the anlage of the liver in the salamander in a figure of a sagittal section of a 9 mm. embryo, in which there is a ventral fold in the wall of the foregut. This fold is lined with yolk-laden cylindrical cells which posteriorly pass gradually over into the polygonal yolk-cells which form a mass projecting into the lumen of the gut. In the anterior ventral wall of the gut is a second slight pouch which later forms the gall-bladder. The two omphalo-mesenteric veins crowd in on either side of the liver outpouching, thereby aid
DEVELOPMENT OF LIVER AND PANCREAS 217
ing the constriction of the lateral walls of the gut. These veins unite anteriorly and form the ductus venosus. The liver-anlage therefore first grows ventrally and then anteriorly below the horseshoe-shaped union of the omphalo-mesenteric veins and the ductus venosus. A similar sagittal section of a later stage shows the liver at the cranial end of a short ductus hepaticus which is continuous caudally with the ductus choledochus. From the ventral wall of the ductus choledochus there is now a very marked outpouching, the gall-bladder, which is united with the common duct by a short cystic duct. The primitive liver-anlage has thus grown cranialward and become separated from the gut. Choronskitkzy believes this process to be due to the growth and differentiation of the gut. The walls of the primitive liver-anlage have folded and these folds later develop into solid liver-columns. The liver grows around the developing ductus venosus even to its dorsal surface and in so doing produces many folds and columns which grow" through the ductus venosus and divide it into sinus-like branches.
Renter ('00) in his studies on the development of the intestine of the Alytes obstetricans made mention of the early origin of the liver. This develops from the 'Anfangsdarm' division of the midgut. In later embryos the liver develops very rapidly and is divided into three lobes.
Gianelli ('01 and '02) described the hepatic anlage in Triton as developing in two parts, the anterior giving rise to the hepatic tissue proper and the caudal forming the hepatic duct. The gall-bladder arises from a mass of cells belonging to the primitive hepatic outpouching. By the development of the intestinal folds the hepatic duct becomes attached to the dorsal side of the gut.
Weber ('03) stated that the observations made on the development of the liver in the frog and in Triton differ but little. In the latter the intimate relation of the anterior end of the hepatic outpouching and the blood-vessels account for the development of this part into the hepatic tissue proper.
Bates ('04) in a paper on the histology of the digestive tract of Amblystoma has described the hepatic and pancreatic ducts.
218 E. A. BAUMGARTNER
He has described a bile-duct which lies free in the body-cavity for a short distance and then enters the pancreas which lies between the liver and the intestine. Here it is joined by two hepatic ducts and just as this enters the intestine it is joined by two other hepatic ducts.
To summarize briefly, the early investigators described the liver and pancreas as developing at the same time from the ventral wall of the gut, and also considered that they were parts or lobes of the same organ. Remak ('55) first noted that the liver is separate and distinct from the pancreas. Goette first gave a detailed account of the development of the liver in amphibia. Most of the investigators from that time have agreed that the liver begins as a single ventral outpouching of the gutwall caudal to the heart. The question as to the origin of the gall-bladder, whether from the caudal end of the ductus choledochus or from the wall of the intestine in this region may be, as Piper ('02) stated, one of interpretation rather than one of observation. Whether the hepatic cylinders divide and the blood-capillaries then grow between them, or whether the capillaries grow into the solid hepatic anlage so forming hepatic cylinders seems not to have been definitely determined. Shore's ('91) observations support the latter theory. According to the observations of Weysse and others the yolk-cells are transformed directly into hepatic cells. Very little has been written about the development of the hepatic ducts. The common bile-duct is described as the constricted attachment of the hepatic anlage, or the posterior end of the hepatic outpouching.
2. Early development of the liver
The liver in Amblystoma first appears in embryos about 4.5 mm. in length, which corresponds roughly to no. 21 of Keibel's Normal-plate series. The digestive tract at this stage is quite simple. The pharyngeal cavity is large and extends anteriorly to the oral cavity. Caudally it opens widely into the mesenteron which is composed of a large mass of yolk-cells and extends backward to the proctodaeum. The yolk-mass extends dorsally to the notochord and bulges ventrally.
DEVELOPMENT OF LIVER AND PANCREAS 219
Posterior to the anlage of the heart a sagittal section shows a ventrally and somewhat caudally directed projection of the gut-lumen, (fig. 1) which extends backward near the dorsal side of the yolk-mass. The anterior wall of the ventrally directed extension of the gut-lumen is lined by yolk-laden columnar cells and its posterior wall is formed by the cells of the large yolk-mass. This cavity is quite wide transversely and is connected to the gut-lumen above by a wide cleft.
Fig. 1 Sagittal section of an Amblystoma embryo 4.5 mm. long taken at about the median plane. X 30. F.g., foregut; He, heart; Li, liver; Y, yolk mass.
Fig. 2 Sagittal section of an Amblystoma embryo 5 mm. long, taken to the right of the median line. X 30. F.g., foregut; G, caudal extension of gut; He, heart; Li, liver; Y, yolk mass.
Weysse ('95) has described this cavity in frog as a cleft in the ventral mass of yolk-cells, and Hammar ('97) has termed it the 'Leberbucht.' From the study of a slightly more advanced stage Weysse concluded that the caudal and ventral end of this cleft finally formed a caudal hepatic duct. He correlated this with the caudal hepatic duct described in the chick. That the caudal projection does not form a caudal hepatic duct in amphibia seems clear from a study of the later development. The reason for this error was probably, as Hammar has pointed out, that
220
E. A. BAUMGARTNER
Weysse did not follow the development beyond a very early stage.
In an embryo approximately 5 jnm. (fig. 2) long the anterior wall of this early ventro-caudal projecting cavity has become more prominent. The extension of the gut-lumen into this outpouching is a large cone-shaped cavity somewhat flattened in transection. The columnar epithelial cells lining it are nowfound farther caudal ward than in the preceding stage.
Fig. 3 Sagittal section of an embryo almost 7 mm. long. X 30. D.choL,
ductus choledochus; F.g., foregut; G. caudal extension of gut; G.B., gall-bladder;
He, heart; Li, liver; Y, yolk mass.
In a sagittal section of an embryo 7 mm. long there is shown a more advanced stage of the condition just described. From a comparison of this stage (fig. 3) with the previous one and the one following, it will be seen that the hepatic anlage has become more prominent by a cranio-caudal constriction from the gut. Folds have begun to form on the outer surface of the liver. The cavity of the hepatic diverticulum is widely connected with that of the gut. In the ventral, wall there is a slight median depression (GB) which is the earliest indication of the gall-bladder. This depression is at the caudal end of the liver-anlage in the region where the primitive ductus choledochus is forming.
DEVELOPMENT OF LIVER AND PANCREAS 221
The liver of another embryo 7 mm. long appears as an anterior and ventral outpouching of the gut. Figure 36 is of a plastic reconstruction of this region of the archenteron. That the constriction from the gut has proceeded caudally will be apparent by comparison with earlier and later stages. The cavity projecting into the liver-anlage from the lumen of the gut is now much longer, and there are indications of further projections from it on the right side as the lumina of ducts.
Choronshitzky noted this transverse extension of the lumen in the hepatic anlage of the salamander but did not follow its further history. At the posterior end in the median ventral wall is a marked outpouching which is the gall-bladder {GB, fig. 36). The opening of this outpouching; into the gut is still very wide laterally and shows no differentiation into cystic duct and gall-bladder. The evagination is wide transversely though not extending as far laterally as the liver. In ventral view the gall-bladder appears as a wide transverse outpouching. There is a slight furrow separating it anteriorly and laterally from the liver proper, and a more pronounced one separating it from the caudally placed yolk-mass.
In an embryo approximately 9 mm. in length (fig. 37) the liver is distinctly further advanced than in the preceding one. The caudal constriction from the gut has progressed rapidly (fig. 4). The original anterior convex surface of the liver has become markedly irregular showing numerous depressions or furrows between projecting masses of cells. Greil ('05) figures a model of the liver in a Bombinator embryo 7.5 mm. long with many secondary buds. A network of veins already occupies the spaces between the hepatic buds but Greil only states that it is present. The anteriorly directed cavity has become constricted dorso-ventrally and the division into ducts is more distinct. On the left side (fig. 37) there is a ventral {vl) and a dorsal {(I'm) projection of the lumen. On the right side the ventro-lateral extension is prominent. The median ventral evagination {GB) has become more pronounced. There is now the beginning of a lateral constriction of this evagination representing the formation of a cystic duct. The anterior lip of the
222
E. A. BAUMGARTNER
evagination has developed into quite a ridge separating the gallbladder from the developing hepatic ducts. On the ventral surface the anterior furrow separating liver and gall-bladder from yolk-mass is, as before, the more marked.
According to Shore ('91) in the frog the furrows found in the liver-mass are caused by the 'tunnelling in' of blood vessels. That it is not due only to this is apparent in Amblystoma where sections of this and other embryos show furrows in which there are no blood-vessels (fig. 4). It is important to note that Shore saw no vascular endothelium in these spaces which he regarded as blood-vessels.
Fig. 4 Sagittal section of an embryo almost 9 mm. long. X 30. Dhd.,
ductus choledochus; F.g., foregut; G.B., gall-bladder; He, heart; Li., liver ; Lu.,
lung; Y, yolk mass.
In another embryo of 9 mm. in length the liver in cross section (fig. 5) appears as a large oval mass with an irregular surface showing deep furrows separating the developing ducts. There is also a very marked dorso-ventral furrow separating the livermass into two unequal lateral portions of which the left is the smaller. The right portion is marked by two lesser furrows, one ventral, the other lateral.
In 10 mm. embryos a beginning of the network of anastomosing trabeculae can be seen. The development of the sinusoidal
DEVELOPMENT OF LIVER AND PANCREAS
223
capillary circulation in this network has progressed. In the 11 and 12 mm. embryos there is a confusing network of trabeculae and it is difficult to differentiate the main ducts from the hepatic columns. Shore believed that in the frog the tubules were first solid and that later a lumen developed. Goette expressed the opinion that a lumen was present from the earliest formation, though he admitted this was hard to demonstrate. The reason of the difficulty of proving this either way is apparent. However, from a study of sections of Amblystoma it would seem that a lumen is present from the earliest stages.
Fig. 5 Transverse section of embryo 9 mm. long. X 30. F.g., foregut; L, left portion liver; R, right portion liver.
Fig. 6 Transverse section of an embryo 11.5 mm. long. X 30. F.g., foregut; GB., gall-bladder; L., liver.
3. Position of the organ during development
At a stage represented by 11.5 mm. embryos there is a shifting to the right particularly of the caudal end of the liver (fig. 6). Such a shifting of the posterior part of the liver was noted at a later stage in Necturus by Eycleshymer and Wilson ('10) and others. The reason for this lateral ward shifting is probably the pressure of the rapidly growing stomach and duodenum which are beginning to take a ventral and sinistral position. It is possible also that the spleen which is now a prominent organ in the left dorsal region of the body cavity has some influence on this
7A
7B
7C
Fig. 7 A series of transverse sections in the region of the liver. A, embryo of 13.5 mm. X 20; B, embryo of 20 mm. X 15; C, embryo of 35 mm. X 10; G.b., gall-bladder; L, liver; P., pancreas; Sp., spleen; St., stomach; x, ostia of ductus choledochus into gut.
At level of anterior end of liver
About midway
between first
and 1 hird
drawing
Anterior end of
gall bladder
Level of
attachment of
cystic
duct to
Level of ostium of ductus
choledochus
A 13.5
Fig.
9
Fig.
8
Fig.
10 Fig.
11 Fig.
12
gall
bladder
B 20
^^
C 3")
1^
224
DEVELOPMENT OF LIVER AND PANCREAS 225
movement. Then, too, the ventral pancreas forms quite a mass in the median ventral region. Figures 7 A, B and C show the lateral and upward shifting of the posterior portion of the liver. The first drawing in each of the series shows a section taken near the anterior end of the liver which here is median and ventral in position and occupies somewhat more than onehalf of the area of a circle. The second drawings in figure 7 A and B show a beginning of a depression on the left side caused largely b}^ the change in shape and position of the stomach and duodenum as mentioned above. Figures 8 to 12 are cross sections of embryos 13.5 to 35 mm. in length showing the position of the liver at the level of the junction of gall-bladder and cystic ducts. Here the lateral and dorsal growth of the liver is marked. A somewhat further shifting is shown in the third drawing of figure 7 A and B and the second of 7 C. These sections were taken near the anterior extremity of the gall-bladder. In all of these the liver is crescentic in transsection and extends upward almost to the level of the dorsal wall of the stomach. The last drawing in figure 7 shows the relation of parts at the level of the opening of the ductus choledochus in the gut. In all cases a small portion of the liver is found dorsal to the duodenum in this region of the embryo. In an embryo 45 mm. long the anterior end of the liver is median and ventral as described above. There is a marked lateral and dorsal growth of the caudal end but in this embryo there is also quite a marked ventral growth which would indicate that from now on the shifting to the right will not be so noticeable, and that there is a growth to the left also.
4' Development of the biliary apparatus
a. Description of the hepatic ducts in the adult. A description of the fully formed biliary apparatus may be of interest before describing the development of the hepatic ducts.
The liver in the adult Amblystoma is a large organ extending fully one-half the length of the abdominal cavity (fig. 13). It has a ventral convex surface conforming to the wall of the abdomen
226
E. A. BAUMGARTNER
and is divided by an indefinite median line into a right and a left part of which the left is the longer and covers the left ventral surface and a part of the lateral wall of the stomach. The right portion or lobe though somewhat shorter, covers the ven
Fig. 8 Transverse sections of an Amblystoma embryo 14 mm. long, taken at level of attachment of cystic duct to the gall-bladder. X 35. D, duodenum ; G.B., gall-bladder; Li., liver; P., pancreas; Sp., spleen; St., stomach.
Fig. 9 Transverse section of an Amblystoma embryo 13.5 mm. long, taken at the same level as figure 8. X 35. For abbreviations, see figure 8.
Fig. 10 Transverse section of an embryo 15 mm. long, taken at the same level as figure 8. X 35. For abbreviations see figure 8.
DEVELOPMENT OF LIVER AND PANCREAS
227
tral surface of the stomach to the right of the midline and laterally extends well toward the dorsal wall of the stomach. There
Fig. 11 Transverse section of an embryo 20 mm. long, taken at the same
level as figure 8. X 30. For abbreviations see figure 8.
Fig. 12 Transverse section of an embryo 35 mm. long, taken as in figure 8. X 15. For abbreviations, see figure 8.
228
E. A. BAUMGARTNER
are usually one or two lesser indefinite furrows dividing the right lobe into two or three parts. The gall-bladder is embedded in the caudal end of the right lobe some distance from its ventral surface. Only a small part of its rounded fundus appears beyond the hepatic tissue. From the notch in the liver caused by the gall-bladder the one or two lesser furrows of the right lobe extend forward. The gall-bladder is a pear-shaped t sac with its larger end extending laterally and somewhat pos
Fig. 13 A dissection of an Amblystoma 12 cm. long. X 1. The ventral abdominal wall has been cut away and the gall bladder and main hepatic ducts dissected out. D, duodenum; D.choL, ductus choledochus; D.cy., cystic duct; D.h.d., right hepatic duct; D.h.s., left hepatic duct; L.L., left lobe liver; /B.L., right lobe liver; St., stomach.
teriorly. The smaller, medial and ventral end projects forward and connects with the short cystic duct. Only the large blind end of the gall-bladder receives a peritoneal covering, the remainder is embedded in hepatic tissue.
There are two main hepatic ducts. These unite to form a common bile-duct of variable length which may be joined by the pancreatic duct just before opening into the gut (fig. 14). Quite often, however, the pancreatic duct opened into the gut immediaately beside the ostium of the common bile-duct. The ductus
DEVELOPMENT OF LIVER AND PANCREAS
229
choledochus is embedded for some distance in the long narrow pancreas lying on the anterior surface of the duodenum and finally empties into the anterior side of the gut near the ventral surface.
The right hepatic duct is divided into lateral and medial rami. , The lateral ramus divides into medial and lateral branches. Generally the cystic duct opens into the latter (fig. 14 and 16). However, sometimes the cystic duct is one or
L.R.m.s Lafera/ brxrnch of /eff rnedia/ rornu3
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L./?./ 5 -lafera/ of /^/y /a/era/ ramus
-M.Rms Med/a/ branch of /eff ried'o/ ramus
M.RI d Media/ branch of r/ghf /afera/ ramus
/r'.rn.s -lef/ med/a/ ramus
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D h d -/?/ghf hepaf/c ducf
D.P. Pancreat/c duct.
\^ -V-D cho/- Ductus Cho/edochus
14
Fig. 14 Diagrammatic drawing of the gall-bladder and hepatic ducts of an Amblystoma.
THE AMERICAN JOURNAL OF ANATOMY, VOL. 10, NO. 2
230 E. A. BAUMGARTNER
even two divisions further removed from the common duct, as shown in figure 17 and 44. In a graphic reconstruction of the biliary apparatus of a 7 cm. embryo (fig. 15) the cystic duct joins the right lateral ramus as is shown also in figure 43. The hepatic radicle to which the cystic duct is attached shortly divides into trabeculae beyond this point. The right medial hepatic ramus divides and subdivides into branches as shown in figure 14. Its branches sometimes anastomose with the branches of the right lateral or left medial ramus (fig. 17).
The left hepatic duct is generally shorter and of slightly smaller diameter than the right one, as well as more ventral in position. It is divided as the latter into lateral and medial rami. The left medial ramus sometimes joins the right medial ramus as shown in figure 16, and this duct then subdivides as a single one. Frequently, however, the left medial ramus runs anteriorly subdividing into smaller branches of which some may anastomose with those of the right medial (fig. 17). The left lateral ramus is shortly divided into two of which the lateral either turns caudally (fig. 44) or sends out branches that go to the posterior portion of the longer left lobe.
b. Development of the ductus choledochus. The ductus choledochus in 9 mm. embryos is still very wide and short. The original caudalward projection from the gut cavity has disappeared and there is only the anteriorly directed common duct. In a model of an embryo 9 mm. long the ductus choledochus is wide transversely but constricted dorso-ventrally (fig. 37 and 38). It is attached at the anterior side of the now ventrally directed gut. At 11 mm. the duodenum has turned ventrally and folded to the right. A very much constricted and short common duct is attached to its superior anterior surface. In a 13 mm. embryo the conmion duct is attached to the anterior surface of the cranial fold of the duodenum. As before, the duct is small and short, soon dividing into right and left hepatic ducts. The epithelial lining of the duct still contains yolk-granules and except for a quite irregular but prominent lumen is very much like the hepatic ducts. Indeed the difference in the
DEVELOPMENT OF LIVER AND PANCREAS
231
lining cells of this duct and those of the hepatic trabeculae is not great.
Fig. 15 Graphic reconstruction (lateral view) of an Amblystoma 7 cm.
long. X 15. D.chol., ductus choledochus; D.cy., cystic duct; D.h.d., right
hepatic duct; D.h.s., left hepatic duct; D.P., pancreatic duct; G.B., gall-bladder;
L.Br., left branch of common ramus; L.R.l.d., lateral branch right lateral ramus;
L.R.I. s., lateral branch left lateral ramus; L.R.m.d., lateral branch right medial
ramus; L.R.m.s., lateral branch left medial ramus; M.R.l.d., medial branch
right lateral ramus; M.R.I. s.. medial branch left lateral ramus; M.R.m.d., medial
branch right medial ramus; M.R.m.s., medial branch left medial ramus; R.Br..
right branch of common ramus; R.l.d., right lateral ramus; R.l.s., left lateral
ramus; R.m.d., right medial ramus; R.m.s., left medial ramus.
Fig. 16 Graphic reconstruction (ventral view) of an Amblystoma 10 cm. long. X 15. For abbreviations, see figure 15.
Fig. 17 Graphic reconstruction (lateral view) of an Amblystoma 15 cm. long. X 15. For abbreviations, see figure 15.
232 E. A. BAUM GARTNER
In another embryo of approximately 13 mm. length, which is somewhat more advanced, the ductus choledochus is longer and of larger caliber (figs. 18 and 19). It is, however, still attached to the cranial surface of the anterior fold of the duodenum. The epithelium here is now definitely columnar in type, though yolk-granules are still present. In this case the pancreatic duct is attached near the gut to the common duct.^ In an embryo 13.5 mm. long the ductus choledochus (fig. 7-A) is attached in a fold to the left side of the gut. The duct here is large but shortly divides into the right and left hepatic ducts. The attachment of the duct to the left wall of the gut is to be seen in a less completely developed embryo 14 mm. long. From now on the common duct is attached to the left side of the gut which is faced somewhat cranialward, due to its growth anteriorly and to the right. The length of the common bile-duct before its division varies. In a 35 mm. embryo modelled the common duct is quite long and has a distinct turn shortly before it entered the gut. Here again the pancreatic duct opens into the common duct. There has been a continual change of position of the two ducts from the earliest stage to the fully developed one. In an embryo 13 mm. long a distinct pancreatic duct is seen ventral to the common duct. In the further development with the gradual rotation of the liver to the right there has been a change in position of the common duct until in the 35 mm. embryo it lies to the left of the pancreatic which is the condition found in the adult (fig. 44).
c. Development of the major hepatic ducts. The earliest indication of the hepatic ducts was pointed out in the description of the formation of the liver. In a model of an embryo approximately 5 mm. long, as previously stated, the cavity of the early hepatic anlage extends far laterally. On either side the cavity is constricted dorso-ventrally. From the drawings shown by
- In the further study of the pancreas it was found that this duct was attached by means of a small tubule to the left side of the ventral duct of the pancreas. The epithelial lining resembled that of the gall-biadder, for which this duct was mistaken at first. It might very well be a pancreatic bladder. The pancreatic duct in this embryo was to the right of the enlarged duct.
DEVELOPMENT OF LIVER AND PANCREAS
233
Choronshitzky it is probable his lateral cylindrical extensions are the early hepatic ducts. In Amblystoma these lateral extensions form only the lateral rami of the hepatic ducts. The medial rami are shown in the model of an embryo about 7 mm.
Fig. 18 Graphic reconstruction (lateral view) of the biliary apparatus of
an Amblystoma embryo 13 mm. long. X 100. D., duodenum; D choL, ductus
choledochus; D.h.d., right hepatic duct; D.cy., cystic duct; G.b., gall bladder;
R.l.d., right lateral ramus; R.l.s., left lateral ramus; R.m.d., right medial ramus;
R.m.s., left medial ramus; P.D., pancreatic duct.
Fig. 19 Graphic reconstruction (lateral view) of the biliary apparatus of an embryo approximately 13.5 mm. long. X 100. For abbreviations see figure 18.
long (fig. 36). On the right side in this model there is a lateral extension of the hepatic lumen. A longitudinal ridge in the floor of this side shows a beginning constriction into lateral and medial rami. The medial ramus is more dorsal in position and appears as a swelling on the outer surface. On the left side there
234 E. A. BAUMGARTNER
is a wide cavity. On the external surface there is a sUght dorsoventral furrow, an indication of the beginning division into lateral and medial rami.
In an embryo approximately 9 mm. long the right side shows a more marked lateral ramus. The medial still somewhat dorsal ramus is to be seen (fig. 37) . Here the left side shows a marked dorso-medial and a ventro-lateral prolongation. The outer surface of both sides of the organ shows many projections, the beginning of tubules from these main rami. The cystic duct though slightly to the right shows more of a constriction from that side. The anterior lip of the cystic evagination also is very prominent.
The rami are formed from the early hepatic ducts by a caudalward constriction and by elongation. Mitotic figures are to be seen at this stage but are more numerous in later ones. As is true of fishes (Scammon '13) there is a relative and actual reduction in the size of these ducts.
In another 9 mm. embryo the development of the ducts is seen to have progressed rapidly (fig. 38). Numerous mitotic figures are to be seen in different sections indicating a rapid growth of the ducts. There are distinct right and left hepatic ducts which show a marked growth. There is a medial longitudinal ridge in the ventral wall of the ductus choledochus indicating a caudalward progressing constriction and division (fig. 38). The cystic duct {D. cy.) is distinctly differentiated and attached to the right of the beginning constriction in the common duct. It extends ventrally and somewhat towards the right. The right hepatic duct as seen in figure 38, and in a figure of a model of the cavity of ducts (fig. 20) is divided into a lateral and a dorso-medial ramus. The lateral ramus is further divided into lateral dorsal and medial ventral branches. The left ramus also has medial and lateral divisions.
In embryos from 10 to 12 mm. in length, the trabeculae present a confusing network. The epithelium of both the hepatic ducts and trabeculae are heavily laden with yolk-granules, and that of the ducts is not yet differentiated into a distinct columnar type. However, the right and left hepatic ducts are clear. In
DEVELOPMENT OF LIVER AND PANCREAS 235
an 11 nim. embryo the right duct is distinctly divided into lateral and medial rami. A short cystic duct is attached to the caudal end of the lateral ramus and on its ventral side. In an embryo somewhat less than 13 mm. long the same arrangement of a short common duct and right and left hepatic ducts is present. The right duct is divided into the medial and lateral rami. The cystic duct here projects somewhat to the left and dorsal ward connecting as before with the right lateral ramus.
In a graphic reconstruction of a 13 mm. embryo (fig. 18) the right hepatic duct is divided into lateral and dorso-medial rami. The short cystic duct extends upward and opens into the
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.ms
Fig. 20 Anterior view of a reconstruction of the lumina of hepatic ducts and gall-bladder of a 9 mm. embryo. X 100. D.h.d., right hepatic duct; D.h.s., left hepatic duct; G.b., gall bladder; R.l.d., right lateral ramus; R.l.s., left lateral ramus; R.m.d., right medial ramus; R.m.s., left medial ramus.
right lateral ramus. A short lateral branch is the only other division of the right lateral ramus. The dorso-medial branch shortly breaks up into trabeculae. The left duct is also divided into rami. The differentiation of hepatic ducts from trabeculae is now clearer as the epithelium of the former is columnar in type.
In figure 19 from an embryo less than 1 mm. longer than the above, the formation of ducts is seen to have continued. The right hepatic duct is divided into lateral and medial rami, each of which is further divided into dorsal and ventral branches. The same holds true in a general way for the left hepatic duct and its divisions.
236 E. A. BAUMGARTNER
In the ventral view of the model of a 14 mm. embryo (fig. 39) the relation of pancreatic duct to the common duct is shown. The short thick common duct divides into right and left hepatic ducts (figs. 39 and 40). They lie in almost the same horizontal plane and are of about the same diameter, but the right is the shorter, dividing almost immediately into its lateral and medial rami. In a 13.5 mm. embryo (fig. 41) the right hepatic duct is of larger diameter than the left. In a 15 mm. embryo the common duct is very short (fig. 42). The right and left hepatic ducts here are very long as compared with those in other embryos. The left duct has come to lie in a more ventral plane due to the shifting of the whole posterior part of the liver and gall-bladder to the right. The same is true to a greater extent for the left ducts in the 20 and 35 mm. embryos (figs. 43 and 44). In a 20 mm. embryo the right hepatic duct is the shorter as it is in a 35 mm. embryo. In a 35 mm. stage the left hepatic duct is almost ventral to the right. The same holds true for a 45 mm. embryo. In the adult, however, the left duct is again more lateral to the right, but still somewhat more ventral.
d. Developmeyit of the minor hepatic ducts. Right lateral ramus. The right hepatic duct in a 14 mm. stage is divided into lateral and medial rami and the right lateral ramus is subdivided into lateral and medial branches (fig. 39). The short cystic duct is attached to the lateral branch. The medial branch (fig. 40) gives off several tubules in an oblique dorso-ventral plane. In a 13.5 mm. embryo the right lateral ramus is quite ventral to the medial one (fig. 41). As in the earlier stage, it is divided into lateral and medial branches. The cystic duct which is now directed almost horizontally, is attached to the right side of the lateral branch. The anterior portion of the lateral branch anastomoses with a duct from the right medial ramus. In a 15 mm. embryo (fig. 42) the right lateral ramus is shorter than in the preceding specimen. The right hepatic duct is, however, longer so that the cystic duct is attached to the lateral branch farther from the gut. The lateral branch here divides into dorsal and ventral branches. In a 20 mm. embryo the right lateral ramus is very short (fig. 43). In position it is now somewhat
DEVELOPMENT OF LIVER AND PANCREAS 237
dorsal to the right medial ramus. It soon breaks up into dorsolateral and ventro-medial branches. Both of these branches are very long. At the attachment of the cystic duct to the lateral branch there is a further division of the lateral again into medial and lateral radicles. The medial branch has anastomoses with the right medial hepatic ramus. Its further division is in a dorso-ventral plane. In a 35 mm. embryo the right lateral ramus divides into dorsal and ventral branches (fig. 44) . There is another division of the dorsal branch and the cystic duct is attached to the dorsal one of this last division. Frequent anastomoses are formed between the tubules of the dorsal and ventral branches, and between those of the dorsal branch and those from the right medial hepatic ramus, as also of the left medial ramus.
Right medial ramus. The right medial hepatic ramus of a 14 mm. embryo as shown by model is very simple (fig. 39). It joins with the left medial ramus, the further division of this common ramus is into right and left branches. The division of the medial ramus is very short and its lateral and medial branches long. Caudally directed tubules are given off from the lateral branch. The medial branch here is connected with the right lateral ramus. The medial branch divides dorso-ventrally into tubules. In a 15 mm. embryo (fig. 42) the medial hepatic ramus is again very simple. It is short and divides into lateral and medial branches of which the latter is given off almost at right angles and from its anterior surface are given off several tubules. The medial hepatic ramus in a 20 mm. embryo as in a 14 mm. one is joined with the left medial ramus (fig. 43). The resulting common ramus divides into a right dorsal {R. Br.) and a left ventral branch (L. Br.). From the right dorsal branch, dorso-lateral tubules are given off some of which are directed caudally. In a 35 mm. embryo (fig. 44) the right medial ramus is on the same horizontal plane as the right lateral. Its divisions are also into dorsal and ventral branches. Many anastomoses are found between the tubules of this ramus. Tubules from this ramus join those from the right lateral and from the left medial ramus.
238 E. A. BAUMGARTNER
Left medial ramus. The left medial ramus is joined to the right medial in a 14 mm. embryo (fig. 39). In a 13.5 mm. embryo the left medial is long and divides into dorsal and ventral branches (fig. 41). Also in a 15 mm. embryo is the left medial ramus quite long (fig. 42). It divides into medial and lateral branches both of which have dorsal and ventral tubules. The left medial ramus in a 20 nmi. embryo (fig. 43) is joined to the right. The left ventral branch of this combined duct divides shortly into dorsal and ventral radicles. In a 35 mm. embryo (fig. 44) the left hepatic ramus is quite long. Its anastomoses with the other rami have been noted. There are also several anastomoses with the left lateral ramus.
Left lateral ramus. In a 14 mm. embryo the left lateral ramus is very simple, dividing into medial and lateral branches (fig. 40). The left lateral ramus in the next stage shows further development and growth (fig. 41). In a 15 mm. embryo this ramus has lateral branches given off at quite an angle (fig. 42). It is shorter than the left medial ramus and divides into medial and lateral branches, the latter sending tubules far out to the side. The left lateral ramus in a 20 mm. stage is given off nearly at right angles to the left hepatic duct (fig. 43). It divides into dorso-medial and ventro-lateral branches. In this case the lateral branch is the longer. Several tubules go out laterally almost at right angles and from these tubules hepatic columns go posteriorly as well as anteriorly. In a 35 mm. embryo .(fig. 44) the left lateral ramus forms quite a network of ducts. The ventral branch makes an arch forward and is then divided into anterior and posterior branches. In an embryo 45 mm. long the main hepatic ducts are more nearly on the same horizontal plane. Of these ducts the left hepatic has extended farther to the left.
e. Development of the gall-bladder and cystic duct. The gallbladder appears somewhat later than the liver as noted by Hammar ('97). It arises as a median ventral outpouching caudal to or in the posterior end of the hepatic anlage. Choronshitzky has figured the anlage of the bladder in a median, sagittal section. The structure is shown as a slight depression developing from the gut, at the entrance of the common duct, and
DEVELOPMENT OF LIVER AND PANCREAS 239
a definite fold is shown between this and the ventrally extending lumen of the hepatic anlage. Greil ('05) showed the gall-bladder in a Bombinator embryo of 7 mm. length caudal to the hepatic tissue but more closely connected with the liver than with the yolk-mass behind it. In an embryo approximately- 7 mm. long, which is undoubtedly an earlier stage in Amblystoma (fig. 3) there is no distinct fold between the gall-bladder andliver-anlage. Only a slight median depression of the floor at the posterior end of the hepatic diverticulum is present. No difference is shown by ordinary stains in the epithelium lining this early cystic evagination and that of the liver. Not until later does the epithelium change into the low cuboidal type characteristic of the adult gall-bladder.
A little later the depression in the floor of the hepatic diverticulum is considerably increased (fig. 36). The position of the gall-bladder with reference to the opening of the hepatic anlage has not changed. In a model of liver and gall-bladder of a 9 mm. embryo (fig. 37) the evagination is quite deep. There is a distinct lateral constriction of the dorsal opening of the gallbladder and distinct anterior and posterior lips to the evagination, indicating the formation of a cystic duct (fig. 4) . There is also a deep furrow anterior to the evagination separating the gall-bladder from the hepatic anlage. The posterior furrow is even more marked. The gall-bladder is, however, still very wide laterally.
In another embryo approximately 9 mm. long the gall-bladder has a long cranio-caudal diameter. The furrow marking off the gall-bladder from the hepatic tissue laterally is distinct. The cystic duct is short and of large diameter and it, as well as the gall-bladder, lies to the right of the midline. The cystic duct projects upward and to the left (fig. 21).
A section of the gall-bladder of an embryo 11.5 nmi. long shows there has been a continual shifting to the right (fig. 6). The cystic duct has become longer but is still of wide diameter. It projects more to the left and upward. The gall-bladder, though embedded between hepatic tissue and caudal yolk-mass, is completely separated from both (fig. 22). In figure 23 is
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E. A. BAUMGARTNER
shown an increased cranio-caudal diameter, although the transverse is still the greater. The cystic duct here projects more to the left, still somewhat dorsally and slightly backward. The cranio-caudal diameter increases rapidly from now on, and the position of the cystic duct would indicate that there is a more rapid caudal growth. Figure 23 shows the model of a gall
Fig. 21 Transverse section of an Amblystoma embryo 9 mm. long, taken in the region of the gall bladder. X 30. D.chol., ductus choledochus; F.g., foregut; D.cy., cystic duct; G.b., gall-bladder.
Fig. 22 Sagittal section of an embryo 12.5 mm. long. X 30. F.g., foregut; G.b., gall-bladder; Li., liver.
Fig. 23 Drawing of a model of the gall-bladder of an Amblystoma 14 mm.
long. A, anterior view; B, left lateral view. X 40.
bladder of an embryo almost 14 mm. long. The cystic duct attached near the anterior end, projects to the left and dorsally. In two graphic reconstructions of embryos 13 and 13.5 mm. in length respectively (figs. 18 and 19), the gall-bladder is attached by a short and constricted cystic duct to a radicle of the right hepatic duct. In figure 18 the cystic duct leads from the anterior dorsal end of the gall-bladder to the left, caudally and
DEVELOPMENT OF LIVER AND PANCREAS 241
somewhat dorsally, the gall-bladder being distinctly to the right of the midline. In figure 19 the larger of these two embryos the cystic duct is not quite at the anterior end, but the craniocaudal length of the gall-bladder is distinctly greater. The general direction of the cystic duct is the same. The gallbladder is relatively as far caudally here as the one shown in figure 18. From the connection of the cystic duct to the gallbladder, it appears that there has been a marked growth cranialward.
In an embryo 14 mm. (fig. 39) long the gall-bladder has decidedly increased in its cranio-caudal diameter. In transverse section it is almost circular. The cystic duct is of very small diameter as compared with its earlier size. It projects now somewhat upward but almost directly to the left, due to the increased lateral shifting of the liver and the gall-bladder. In this embryo the cystic duct is attached to the extreme anterior dorsal end of the gall-bladder.
Figure 41 is of a model of a 13.5 mm. embryo. In this the general shape of the gall-bladder is the same as of the one just described, except that there is a slight increase in the vertical diameter (fig. 9). The cystic duct, however, is not attached at the extreme anterior end but to the left upper side. It extends towards the left as before but is now almost horizontal.
In a 15 mm. embryo the attachment of cystic duct to the gall-bladder is further caudalward than the previous one (fig. 42). This seems to mark the limit in its caudal attachment for all sizes examined. It would be difficult to say whether this shifting in attachment of the duct to the gall-bladder were due to a difference in the antero-posterior growth of the gall-bladder or to the rapidity of differentiation and growth of hepatic ducts. The cystic duct in this embryo extends toward the left, but now slightly ventrally, which can be taken as evidence of continued rotation to the right and dorsalward of the entire biliary apparatus (fig. 10).
Marshall ('93) has described the gall-bladder of amphibians developing as a lateral outgrowth from the bile ducts. From
242 E. A. BAUMGARTNER
its position at this stage one could easily be led to such a conclusion.
The gall-bladder of a 20 nun. embryo shows a very distinct dorso-ventral increase in diameter (fig. 11). With this there has been a marked cranio-caudal lengthening (fig. 43). The relative size of the gall-bladder is now greater. As before indicated, the cystic duct is here again nearer the anterior end, it extends towards the left and now distinctly ventralward (fig. 11). A right lateral and slightly ventral view of the gall-bladder is shown in figure 43.
In a 35 mm. embryo (fig. 44) the vertical diameter of the gallbladder has greatly increased. The cystic duct is now in the left anterior ventral end extending ventrally and to the left. In a 45 mm. embryo the gall-bladder has the same general shape as in the preceding, and the cystic duct has not changed in position (fig. 12).
In a graphic reconstruction of the biliary apparatus of a 10 cm. Amblystoma the cystic duct extends to the left, somewhat ventrally and anteriorly (fig. 16). The gall-bladder is pear shaped (fig. 13) with its large, blind end projecting slightly dorsally and to the right but mainly caudal ward.
/. Summary of the development of the biliary apparatus. In summarising the development of the hepatic ducts a table of the ducts as found in the various models will bring out more clearly their relations to the main duct. Table 3a to 3d shows the principal variations found in the hepatic and cystic ducts.
TABLE 3a Ductus choledochus
Left hepatic duct Right hepatic duct
Lt. lat. ramus, Lt. med. ramus Rt. med. ramus, Rt. lat. ramus
Branches — Lat., Med. Lat., Med. Med. Lat. Med. Lat.
Cystic duct
DEVELOPMENT OF LIVER AND PANCREAS 243
Or, in case of anastomoses of the medial rami, as was found in two embryos of 14 and 20 mm. length and two older Amblystoma of 7 and 10 cm. length respectively, the following table is given:
TABLE 3b Ductus choledochus
Left hepatic duct Right hepatic duct
Lt. lateral ramus Common medial ramus Rt. lateral ramus
Branches, — Lat. Med. Left Right Med. Lat.
I Cystic duct
The cystic duct is attached as here shown;
TABLE 3c Right lateral ramus
Medial branch Lateral branch
I '
Cystic duct
TABLE 3d Right lateral ramus
Medial branch Lateral branch
I I
Medial radicle Lateral radicle
I Cystic duct
or as found in a 35 mm. embryo and one of the larger Amblystoma.
From these tables it will be seen that sometimes the right and left medial rami are joined. The division of the common medial ramus is into right and left branches. In their position
244 E. A. BAUMGARTNER
and final division these branches are the same as the right medial and left medial rami. As will be seen in figures of the different models, the smaller embryos did not have all of the divisions and subdivisions marked in the tables. In figure 39, for instance, the right branch of the common medial ramus shows no further division, the left branch only one. Further division of both is seen in the 20 mm. stage (fig. 43). The division here, however, is more into dorsal and ventral radicles, due to the more marked lateralward shifting of the liver and the ducts. The extreme of this lateral shifting is seen in figure 44, where the left hepatic duct is almost ventral to the right. The left lateral ramus in a 45 mm. embryo does not hold such a ventral position with reference to the left medial.
There seems to be no definite rule in regard to the anastomosing of ducts. In a 35 mm. embryo they are the most frequent and here apparently because the ducts were crowded so close together. That the right and left medial rami sometimes join and form one duct is seen in the models of a 14 and a 20 mm. embryo, also in the graphic reconstruction of a 7 cm. and 10 cm. Amblystoma. It would seem this fusion of the ducts is quite probably du£ to crowding.
The definite position of the hepatic ducts with reference to the portal vein is seen for all embryos (figs. 8 to 12). The same relation is also found in the adult. As a rule there is a branching of the hepatic ducts corresponding to the division of this vessel. In the developing embryo the ducts are found usually to the right of and ventral to the portal vein.
From the usual description of the biliary apparatus in the frog it would seem that there is a fairly close correlation in the main features between these two amphibians. The figures of Ecker, Wiedersheim and others show a gall-bladder connected to a right hepatic duct. There is also a left hepatic duct, the two uniting in the pancreas and forming a ductus choledochus which, as usually described, is joined by the pancreatic duct. In no case in Amblystoma were two cystic ducts found as is shown for the frog. The di\dsion into rami in the frog as far as the ducts have been figured, seems to be somewhat different from that
DEVELOPMENT OF LIVER AND PANCREAS 245
found in Ajnblystoma. The more marked divisions of the liver into several lobes may partially explain this. The duct-system as found in Necturus is quite different. Kingsbury here described three hepatic ducts opening into the gut. These anastomosed with each other and two were joined by the ventral pancreatic ducts. The third is a duct direct from the gall-bladder which, however, anastomoses with the other hepatic ducts. Gronberg ('94) described three hepatic ducts which unite with the cystic duct and form a ductus choledochus in Pipa americana.
Bates ('04) has described the hepatic ducts in Amblystoma. According to his description there are four heaptic ducts, two of which join the bile-duct in its course through the pancreas and the other two just as it opens into the intestine. It is possible that the two he found joining the bile-ducts are the right medial and lateral rami, and the other two, the left medial and lateral rami. In that case the ductus choledochus and the right and left hepatic ducts were very short as was found in some of the material used in this work. Or it may be that the two ducts which joined the bile-duct as it opened into the intestine are the two pancreatic ducts which have not fused until just at the ostium of the hepatic duct. The first two ducts then would be the right and the left hepatic ducts. I have never seen the cystic duct (bile-duct as Bates terms it) open directly into the common hepatic duct.
From the models and drawings it will be seen that the gallbladder at first has a wide dorsal communication just caudal to the hepatic lumen. As this communication constricts there is formed a short large cystic duct extending dorsally into the right hepatic duct. With further growth and division the cystic duct extends more and more to the left until at the 15 mm. stage it is almost horizontal and at the 20 mm. stage projecting ventrally and somewhat anteriorly. Its earliest attachment is to the ventral surface of the common bile-duct, but in the lateral ward shifting of the whole liver its attachment goes to the left side of a right hepatic radicle. The connection of the cystic duct to the gall-bladder in early stages is to its dorsal surface about midwa}^ between cranial and caudal pole. Somewhat
THE AMEKICAN JOURNAL OF ANATOMY, VOL. 19, NO. 2
246 E. A. BAUMGARTNER
later the connection is nearer the cranial end and usually reaches the extreme anterior end. The cranio-caudal growth of the gall-bladder has kept pace with the lengthening and differentiation of ducts in the 13 to 14 mm. stage. From the relations in a 15 mm. embryo it appears that the gall-bladder has shifted anteriorly. In this case the hepatic ducts have lengthened more than the gall-bladder. At 20 mm., however, there has been a marked increase in cranio-caudal growth of the gall-bladder so that it is ahnost as long as the ducts.
Beginning about at this stage the cystic duct is again attached nearer the anterior end of the gall-bladder. This may be taken as evidence that the cystic duct really shifts in its attachment to the gall-bladder. This seems to be borne out in some cases by the fact that its attachment to the hepatic ducts is to a division of the lateral branch of the right lateral ramus instead of to the lateral branch proper. In some cases where the lateral branch is quite long the attachment may have remained to it.
Whether the gall-bladder originates from the early hepatic anlage or from the gut has caused much discussion. As said before Piper ('02) thought this a matter of interpretation. The more marked furrow caudal to the gall-bladder might be taken as evidence of its belonging to the hepatic anlage, also the fact that the same type of yolk-laden cells form hepatic tissue and gall-bladder. That it, at least is directly caudal to the hepatic anlage is proven by the early connection of its duct to the common bile-duct.
The connection of the cystic duct probably depends to some extent on the extent of growth and division of the hepatic ducts. It will be remembered that in the earlier stages the cystic duct opens into the common duct, then into the early right hepatic. In the further growth and division of the right hepatic duct the cystic duct becomes attached to one of its radicles. As noted above, the cystic duct opens into the lateral branch of the right lateral ramus in all of the embryos studied except one, which was 35 mm. long.
That there is considerable variation in the relative dorsoventral position of these main hepatic ducts is to be expected.
DEVELOPMENT OF LIVER AND PANCREAS 247
However, in general, a study of the models shows a close shnilarity in their positions. Theue is a constant rotation of the liver towards the right and with this is a similar one of the hepatic ducts. In this rotation the right ducts come to be more dorsal in position, the left more ventral. The right lateral divisions would thus be dorsal to the right medial and the reverse should be true for the left. In general such an arrangement is found. A variation in the length of the different ducts is present. However, there is qute a definite relation in the total lengths of ducts in the different embryos. In a 15 mm. embryo the common duct is quite short but the greater length of the hepatic ducts compensates for this reduction. In a 35 mm. embryo the common duct is long, the hepatic ducts and their radicles divide shortly.
III. THE DEVELOPMENT OF THE PANCREAS AND PANCREATIC
DUCTS
1. Literature
The literature concerning the development of the amphibian pancreas like that regarding the liver is divisible into two periods, and Goette's work ('75) may again be said to mark the beginning of the newer one. The older observers mainly considered the pancreas as a part of the liver, or a modified lobe of that organ.
A list of the investigators describing the development of the pancreas will be found included in the tabular classification of the literature on the development of the liver (table 2).
Goette ('75) in his studies on the development of the Bombinator recognized three distinct pancreatic anlagen, two ventral and one dorsal. The dorsal one he described as placed just caudal to the gastroduodenal loop. The two symmetrical ventral anlagen develop from the primitive hepatic duct. Of these the right grows dorsalward to join the ventral growing dorsal anlage. The right duct changes in position until it opens into the left side of the hepatic duct. The united right and left duct then separates from the common bile-duct. Apparently
248 E. A. BAUMGARTNER
Goette considered the left outpouching as a rudimentary one. Later the dorsal duct disappears, thus leaving but one permanent pancreatic duct.
Balfour ('81) and Hertwig ('88) described a dorsal outpouching of the gut wall caudal to the level of the common bile-duct.
The development of the pancreas in both Urodela and Anura was described by Goeppert ('91). In both he found as Goette had described, one dorsal and two symmetrical ventral outpouchings. A constriction of the early dorsal outgrowth forms a duct, while folds and ridges developing on the blind end give rise to the glandular tissue. The right and left ventral ducts unite on the right side of the common bile-duct. However, he found two pancreatic ducts opening into the common bile-duct in an adult, also three pancreatic ducts that fused immediately before opening into the common bile-duct. The numerous dorsal ducts which he found in the adult urodeles he explained are of secondary origin. He, too, found only one pancreatic duct persisting in adult Anura.
Marshall ('93) stated that in the frog the pancreas developed as a pair of hollow outgrowths caudal to the liver. Later the ducts shift and open into the bile-duct instead of, as at first, into the intestine.
Minot ('93) in his text book of embryology, mentioned that in urodeles the dorsal duct persists, and in Anura only the ventral duct.
Weysse ('95) and Stohr ('95) both agreed with the description of Goette and Goeppert. Stohr was especially interested in the dorsal pancreatic anlage. He did not find a double dorsal pancreas as had v. Kupffer ('92) in one of the ganoids. He believed that the caudal dorsal pancreatic anlage described by V. Kupffer is part of the hindgut. Brachet ('96) reviewed the descriptions of earlier investigators.
Woit ('97), a student of v. Kupffer, and probably influenced by his views, in his work on the development of the spleen stated that the dorsal pancreas gives rise to the spleen as well as to a part of the adult pancreas in urodeles. He described two persisting ducts in urodeles.
DEVELOPMENT OF LIVER AND PANCREAS 249
Gianelli ('99) described an intrahepatic pancreas" in Triton, the tubules of which are in intimate relation with the livertubules, and it is stated by him are continuous with them.
Renter ('00) made mention also of the early appearance of the dorsal and ventral pancreas. Both arise from the anterior part of the midgut region (Anfangsdarm) from the yolk-cells. The pancreas, as well as the liver, is found in the gastro-duodenal loop as soon as this is formed, and both are at first to the right and dorsal to the intestinal spiral.
Choronshitzky ('00) described in Necturus and the frog two lateral outpouchings from the early hepatic duct. These two lateral outpouchings form the ventral ducts, later they unite posterior (ventral) to the common duct. He described two ducts in the adult urodeles.
Gianelli ('02) described three distinct pancreatic anlagen in Triton. The dorsal anlage develops first. The ventral anlagen develop from the posterior end of the hepatic outpouching as two masses of vitelline cells into which the lumen of the hepatic evagination later extends. The right and left outpouchings both fuse with the dorsal pancreas. The ventral pancreatic duct formed from both pancreatic anlagen opens into the hepatic duct. The left pancreas remains in intimate relation with the liver.
Braun ('06) described the early development of the pancreas in Alytes obstetricans. He found a dorsal and two ventral pancreatic anlagen, which are first to be recognized as swellings of the yolk-gut wall and by the more numerous nuclei. The ventral anlagen are caudal to the anlage of the hepatic duct. The right ventral pancreas is somewhat more caudal than the left, joins the dorsal pancreatic outpouching and later joins with the left ventral pancreas. The dorsal outpouching looses its connection with the yolk-gut soon after coming in contact with the right ventral pancreas. The cells forming the pancreas are at this time still undifferentiated yolk-cells. Differentiation of the cells and glandular development take place at the same time. The early ventral outpouchings develop into the pancreatic ducts which unite just before opening into the gut to the right of
250 E. A. BAUMGARTNER
the hepatic duct. The pancreas in the adult Hes in the gastroduodenal loop.
Eycleshymer and Wilson ('10) described the two ventral anlages as dorso-lateral to the ductus choledochus. These appear some time after the single dorsal anlage, arid union with the dorsal pancreas does not take place until the embryo reaches a length of about 29 mm. They found that the dorsal duct opens into the duodenum just caudal to the stomach, and also mentioned two ventral ducts.
It is generally agreed by those who have described the duct system in the adult urodeles that at least two ducts persist. Hyrtl ('65) by means of injection in adult Cryptobranchus found two pancreatic ducts, one of which joined the hepatic duct. Oppel ('89) in Proteus has described an anterior and a posterior set of ducts, the latter emptying into the ductus choledochus. Kingsbury ('94) found one anterior duct opening just behind the pylorus, and two caudal ducts which open separately into the ductus choledochus. One pancreatic duct has been described as persisting in adult Anura. Bates ('04) stated that the pancreatic ducts join the hepatic ducts as they pass through the pancreas.
2. Early development of the pancreas and pancreatic ducts
• As stated in the description of the liver, the pancreas develops later than that organ. A well marked dorsal pancreas is to be found in embryos 8 mm. long. A mass of cells in the dorsal wall of the enteron is separated by a distinct transverse furrow from the anlage of the stomach in front and from the yolkmass behind. Mitotic figures are to be found in this mass of cells.
In embryos about 9 mm. long there are three pancreatic anlagen, two ventral and one dorsal, as has been described for other amphibia. The two ventral anlagen appear as evaginations posterior to the hepatic outpouching and caudal to the ventral-lying gall-bladder. The evaginations of the pancreas on the ventral wall of the gut extend in a longitudinal direction for
DEVELOPMENT OF LIVER AND PANCREAS
251
some distance. Anteriorly there is quite a distinct furrow between the Hver and the pancreas. A model of this stage (fig. 24) shows the outpouchings of the pancreas and of the gall-bladder and liver anteriorly. Posteriorly there is no sharp demarcation of the pancreas from the yolk-mass and gut. A model of the lumina of the pancreatic evaginations and of the gall-bladder and hepatic ducts makes the position of the different parts with reference to the antero-posterior plane more clear (fig. 25). At this stage the pancreatic anlagen are caudal to the gall-bladder which is directly anterior to the right pancreatic evagination.
Fig. 24 Lateral view of a reconstruction of the pancreatic anlagen of a 9
mm. embryo. X 40. D.pan., dorsal pancreas; Li., liver; St., stomach; V.pan.,
ventral pancreas; Y, yolk-gut.
Fig. 25 Lateral view of a reconstruction of the lumina of the gut and pancreatic anlagen. X 40. G.b., gall-bladder; other abbreviations, as in figure 24.
The pancreatic evaginations extend farther ventrally than do the hepatic ducts and the gall-bladder.
The right and left pancreatic anlagen are separated anteriorly' by a slight ventral furrow. Caudally the two evaginations are apparently fused as the area between them is bulged ventrally. The evidence of a division into two evaginations is much more clear in a view of the model of the lumina of the pancreatic anlagen (fig. 25). This is also brought out clearly by the figure of a section taken about 80/^ caudal to the gall-bladder (fig. 26). In this figure one sees the two very definite evaginations, and
252
E. A. BAUMGARTNER
that they are separated as far as the transverse width of the gut will permit. On the lateral side there is an indefinite furrow at about the level of the dorsal margin of the liver extending caudalward in the wall of the gut and yolk-mass. This marks the upper limit of the ventral pancreatic anlagen (fig. 24). In
Pig. 26 Drawing of a section through the ventral anlagen of an embryo 9
mm. long. X 40. F.g., foregut; V.Pan., right and left ventral pancreases.
Fig. 27 Drawing of a section through the ductus choledochus of an 11 mm. embryo. X 40. D.chol., ductus choledochus; <S/., stomach; V.Pan., ventral pancreas.
Fig. 28 Drawing of a section about 80^ anterior to the preceding. X 40. For abbreviations, see figure 27.
Fig. 29 Drawing of a section through the dorsal pancreas of an 11 mm. embryo. X 40. D., duodenum; D. pan., dorsal pancreas; Y, yolk gut.
DEVELOPMENT OF LIVER AND PANCREAS 253
figure 26 the lumina of the two veutro-lateral pancreatic diverticula open widely into a common lumen which connects dorsally with the gut-cavity. Anteriorly at about the caudal end of the gall-bladder this lower common lumen is separated from the lumen of the intestinal anlage as shown by the model of the lumina of the ducts (fig. 25) as well as by the figure of a model of the hepatic ducts (fig. 20). In the section figured (fig. 26) the lower part of the right evagination is separated from the main lumen by cells. The next section anteriorly shows the left lumen also cut off. The pancreatic lumina thus very early extend somewhat forward.
The dorsal pancreatic anlage, as shown by both models (figs. 24 and 25), is median and further caudalward than the ventral. As seen in figure 24 it seems to be an elevated portion of the wall of yolk gut. The anterior and posterior furrows separating the anlange from the stomach and gut are not so prominent in this specimen. A model of the lumen shows it to be directed forward (fig. 25). Stohr's statement that there are no evidences of double dorsal pancreatic anlagen in any stages as has been described in the ganoids by v. Kupffer is true also forAmblystoma. The dorsal anlage at this time is short in its craniocaudal diameter. It is, however, further developed than the ventral anlagen. Its ventral margin is limited by a slight groove at the anterior end. Caudally this groove is not present. Figure 24 as well as figure 25 shows that there has been quite an increase in the dorso-ventral diameter of the intestine. That the ventral part is becoming constricted from the dorsal is shown by both models and was pointed out in the description of the development of the liver. From the anterior end of the ventral part of the gut is the hepatic outpouching, caudal to this and on the right side the gall-bladder, and still farther posteriorly the ventral pancreatic anlagen. Caudal to these evaginations again the gut lumen takes a more dorsal position.
In 10 mm. embryos the dorsal pancreas is much more prominent. The furrow separating it from the stomach is quite deep. Also the caudal furrow is well marked. Mitotic figures are more numerous than before. The cells lining the evagination
254 E. A.'BAUMGARTNER
are columnar in type but still contain considerable yolk. The lumen extends a very short distance forward.
In an 11 mm. embryo there has been considerable- further development of the midgut region. The stomach has differentiated to some extent. It has flattened dorso-ventrally, and its posterior end is constricted and shifted to the left. The duodenum extends ventrally to the left and has an anteriorly directed portion which forms, with the stomach, the gastroduodenal loop. At its anterior end which Brachet ('95) has termed the 'seconde courbure' in Axolotl the duodenum turns to the right and is continuous with the caudal-extending yolkmass. Here at the cranial end is the ventral pancreas. The pancreatic area appears at this stage as a narrow zone of the gut marked off by furrows, anteriorly from the hepatic area and posteriorly from the duodenum and yolk (fig. 30) . The pancreatic ducts are short and extend ventro-laterally from either side of the common duct. These ducts are caudal to that part of the common bile-duct which gives off towards the right ventral side the cystic duct and anterior and somewhat dorsally two lateral hepatic ducts. The groove separating the anterior end of the pancreas from the hepatic tissue is well marked. The gall-bladder extends downward and to the right of the midline between pancreas and liver. A drawing of a section near the anterior end of the duodenal loop shows what appears to be a constricted forward projection of the gut (fig. 27). Somewhat anteriorly the cells lining this constricted gut are of a tall columnar type heavily laden with yolk as are the cells lining the duodenum (fig. 28). This is the caudal end of the common duct. Posterior to the section figured one can see the two lateral parts of the pancreas distinctly separated from the constricted anterior end of the gut where the common duct is attached (fig. 27). About fifteen sections of lOju anterior to this are the ventrolaterally projecting pancreatic ducts. The pancreas has grown both anterior and posterior to the ducts, but the greater growth has been forward (120/* caudalward and 140ju anterior). The left pancreas grows anteriorly sending a small projection to the left of the gall-bladder. A model of the pancreas of this stage
DEVELOPMENT OF LIVER AND PANCREAS
Joo
shows it as a cap placed over the anterior end of the gut, distinctly separated from it by a groove and closely united to the liver which lies in front of it. The right side shows only slight indication of its later dorsal and caudal growth (fig. 30) .
The dorsal pancreas forms an irregular elongated mass to the right of the gastro-duodenal loop, but extends somewhat caudal to it. To the right of the pancreas and ventrally lies the large yolk-mass (fig. 30). The dorsal pancreas is now relatively and actually nearer the ventral pancreas than in earlier stages. There is as yet little evidence of any ventral growth of the anterior
Fig. 30 Lateral view of a model of the pancreatic anlagen of an 11 mm. embryo. X 40. D.pan., dorsal pancreas; G.b., gall-bladder; Li., liver; St., stomach;
V.pan., ventral pancreas; Y, yolk gut.
Fig. 31 Lateral view of a model of the pancreas of a 13 mm. embryo. X 30. D.pan., dorsal pancreas; G.B., gall-bladder; St., stomach; V.pan., ventral pancreas; Y, yolk-gut.
end. The duct as in the younger stages lies mainly in the cranial part of the mass forming the dorsal pancreas. It extends to the right and dorsalward and is nothing more than a constricted part of the evagination. As seen in figure 29, it is attached to the archenteron near the large yolk-gut. The segment of the gut to which the dorsal pancreatic duct is attached is the caudal end of the duodenal loop in Brachet's ('95) 'ascending limb' which posteriorly is completely constricted from the ventral yolk-mass. Its attachment here, then as is shown by later stages, is to the dorsal wall of the duodenum.
256 E. A. BAUMGARTNER
The development of the duodenum and the gastro-duodenal loop has been described by Goette and others who described the changes which bring the opening of the dorsal pancreatic duct nearer to the pylorus than the ventral ducts. Goeppert ('91, p. 113) stated concerning this: — so erhiilt mann in den Schnitten die dorsale Anlage spater als die ventralen Anlagen. Wenn mann aber die schrag absteigende Richtung des vorderen Schenkels der Gastroduodenalschlinge beriicksichtigt, sieht mann leicht dass das dorsale Pankreas trotzdem einem noch etwas vor der Miindung des Leberstieles gelegenen Theil der Darmwand angehort . ' '
Brachet ('95) has described the position of the digestive tract in young axolotl. The duodenum continuing caudally from the stomach he has termed the first descending limb; the caudal turn, the 'premiere courbure;' then ascending limb, 'seconde courbure,' and second descending limb.
The dorsal and ventral pancreatic anlagen at this stage are composed of masses of cells still containing many yolk-granules. The ducts of both appear as constricted portions of the outpouching connecting them with the duodenum and the common bile-duct.
During the 12 and 13 mm. stages the dorsal pancreas comes in contact with the ventral one. The dorsal pancreas forms an irregular mass lying to the right of the duodenum and dorsal to the caudal yolk-mass. A small part extends anteriorly and somewhat ventrally and comes in contact with the right pancreas. In a 13 mm. embryo the two masses are fused (fig. 31). The acini ot the two actually fuse as is shown by figure 32. The ventral pancreas ig the smaller. A small part of the left ventral pancreas lies along the left side of the gallbladder and anterior to the duodenum (fig. 45). The hepatic ducts extend through the ventral pancreas anteriorly to the liver but have no connection with the former. The right ventral pancreas in figure 31 has not grown dorsalward to any extent to join the dorsal pancreas. The dorsal pancreas in this case has grown ventrally and to the right to join the ventral pancreas.
The dorsal duct is now well developed. It extends dorsally and slightly to the right from the right dorsal side of the duo
DEVELOPMENT OF LIVER AND PANCREAS
257
denum near its caudal turn. The duct di\'ides shorth^ sending out branches in all directions.
The ventral ducts in a model of a 13 nnii. embryo come off laterally from the hepatic ducts and immediately divide into smaller rami. As a rule the ventral ducts at this stage fuse into one tube ventral to the common duct. The ducts may join the ventral wall of the common duct or the anterior end of the gut, where the common bile-duct opens into the duodenum (fig. 45).
VPon
Fig. 32 Drawing of a section showing the united acini of the dorsal and right ventral pancreas. X 180. D.pan., from dorsal pancreas; V.pnn., from ventral pancreas; Bl., blood vessel.
Fig. 33 Lateral view of a model of the pancreas of a 15 mm. embryo. X 30. D.pan., dorsal pancreas; D., duodenum; Li., liver; .S^, stomach; V.pan., ventral pancreas.
In a 15 mm. embryo as well as in later stages the dorsal pancreas forms the larger part of the whole organ. It joins the right ventral pancreas by a neck of tissue, which is larger than in the preceding stages (fig. 33). The ventral mass is crescentic in transsection and lies just below the stomach, and anterior and somewhat dorsal to the anterior end of the duodenum (fig. 45). A part of the ventral pancreas extends anteriorly along the left side of the gall-bladder. The pancreatic duct opens into the gut ventrally and slightly to the left of the common bile-duct (figs.
258 E. A. BAUMGARTNER
42 and 46). The pancreatic duct directly divides into two branches, a right and left, which end shortly. The dorsal duct has the same position as in the preceding stage.
In the following stages there is an increase in the size of the whole pancreas. The dorsal portion comes to lie more and more dorsal to the duodenum and along the right wall of the stomach, while the ventral portion increases in size anteriorly, in front of the anterior duodenal loop.
A description of the parts in a 35 mm. embryo is given as typical of the further development of the pancreas. In this stage the anterior part of the pancreas lies ventral to the gallbladder and is embedded in the peripheral part of the liver (fig. 7, .B). This part later probably forms the intrahepatic portion of the pancreas described by Gianelli ('99). Somewhat caudally it is considerably larger in section and separates the liver into dorsal and ventral portions (figs. 11 and 12). In this region just caudal to the gall-bladder the hepatic and ventral pancreatic ducts lie embedded in the pancreas. The pancreas is rather prismatic in cross section, its medial side lying along the right side of the stomach, its lateral dorsal side bounded by the duodenum. Slightly anterior to the ostium of the common duct the pancreas is divided into two masses, one lying ventral and somewhat to the left of the duodenum, the other to the left of the duodenum between it and the stomach (fig. 7, C). This latter mass joins the anteriorly directed portion of the dorsal pancreas. The remainder of the ventral pancreas now lies to the lower right side of the stomach, with a part projecting caudalward along the ventral wall of the duodenum (fig. 34). The dorsal pancreas is caudal to the ventral and takes a more dorsal position, until it comes to lie above the duodenum which, has migrated downward. In cross section the dorsal pancreas extends from the lower right side of the stomach almost to its dorsal margin. About half a centimeter from its caudal end the pancreas forms a very small mass, triangular in section, dorsal to the duodenum.
The ducts of the ventral pancreas of a 35 man. embryo are shown in a graphic reconstruction in figure 34. The ventral
DEVELOPMENT OF LIVER AND PANCREAS
259
LtPon. d.
VPan.
34
35
' Fig. 34 Graphic reconstruction of the pancreas and pancreatic ducts of a 35 mm. embryo. X 20. Z)., duodenum; D.pan.d., dorsal pancreatic duct; D.pan., dorsal pancreas; Lt.jian.d., left ventral pancreatic duct; Rt.pan.d., right ventral pancreatic duct; V.pan., ventral pancreas.
Fig. 35 Graphic reconstruction of the pancreas and pancreatic ducts of a 20 cm. Amblystoma. X 5. For abbreviations, see figure 34.
260 E. A. BAUMGARTNER
duct arises with the common bile-duct from a fold in the lower left wall of the duodenum (fig. 7, C). It shortly separates from the common bile-duct and extends anteriorly along its right side. It is somewhat ventral to the hepatic duct and divides into dorsal and ventral ducts. The ventral duct shortly sends off branches caudalward to the left ventral portion and extends forward in the ventral anterior part (fig. 34). The dorsal of the two ducts divides several times into dorsal and ventral rami. Branches from the dorsal duct extend caudalward and to the right into the anteriorly directed portion of the dorsal pancreas. The dorsal pancreatic duct is given off from the upper left side of the duodenum near the caudal end of the pancreas and extends almost directly upward sending off several short branches anteriorly and posteriorly.
3. Description of the adult pancreas
In a 15 cm. Amblystoma the anterior end of the pancreas, which is somewhat triangular in section, is embedded in the liver to the right of the stomach. It lies along the upper concave border of the liver with the portal vein and has a small free surface lined with peritoneum. Caudal to this the pancreas lies to the left of the duodenum, between it and the stomach and enlarges in a dorso-ventral direction (fig. 35). Now it passes along the left wall of the duodenum but extends both dorsally and ventrally to it, the ventral portion being between the duodenum and stomach. About 5 mm. by sections from its anterior end, the pancreas divides into two masses, one along the left ventral surface of the duodenum and the other dorsal to it. The ventral mass remains in about the same position, and extends caudalward almost half the length of the body of the glajid. At the caudal end of the ventral portion of the pancreas the dorsal mass which has shifted somewhat to the right and lies above other loops of the intestine as well as the duodenum, is divided into two or three irregularly-shaped lobes, one of which is dorsal to the duodenum and directly to the right of the stomach. This part of the pancreas extends a considerable distance cau
DEVELOPMENT OF LIVER AND PANCREAS 261
dahvard. As the duodenuin shifts downward and to the left with relation to the stomach, this portion of the pancreas Hes more and more to the right of the chiodenum and for a considerable distance it lies ventral to the stomach. The pancreas extends caudalward almost to the gastro-duodenal loop.
A short distance from the caudal end of the pancreas the dorsal pancreatic duct is connected to the right side of the duodenum (fig. 35). This duct extends forward almost to the point where the dorsal pancreas divides into several parts and gives off small branches as it comes to lie more and more in the dorsal part of the gland. The duct of the ventral pancreas joins the common bile-duct (fig. 35) where it opens into the left side of the duodenum or beside it. The pancreatic duct is ventral to the common bile-duct and almost immediately divides into right and left ducts. The left duct is the more ventral and soon divides into branches which turn \'entral and caudally (fig. 35). The right duct extends anteriorly and upward and sends some branches into that part of the dorsal pancreas which joins the \'entral.
4. Discussion
As has been found in other amphibia, the pancreas in Amblystoma is developed from three anlagen, two ventral and one median dorsal. As in other forms the dorsal develops earlier and, as Stohr and others have stated, from only one evagination. This is found just caudal to the anlage of the stomach. In none of the embryos studied is there evidence of an outpouching toward the left as Goeppert ('91) described in another form. Greil ('05), however, in j^oung Bombinator embryos reconstructed right and left dorsal pancreatic outpouchings. Older embryos show that there has been considerable growth of the dorsal pancreas in a cranio-caudal direction with the lengthening of the duodenum, and a division of the anterior end into several processes, one of which joins the ventral pancreas.
In the very earliest stages the ventral anlagen are caudal to the gall-bladder. Goeppert ('91) and Ghronshitzky ('00) have described these anlagen as lateral to the cystic evagination
THE A^rEnICAN joukx.nl of axatomv, vol.. 19, xo. 2
262 E. A. BAUMGARTNER
in the forms studied by them. Greil ('05) reconstructed the pancreatic anlagen as well as the branchial pouches of 7 and 7.5 mm. Bombinator embryos and showed the ventral pancreatic evaginations as dorso-ventral to the attachment of the hepatic outpouching and described them as lateral to the hepatic anlage.
In Amblystoma the two ventral pancreases unite very early along their medial sides. Braun ('06) has described the union of the dorsal and the right ventral pancreas as occurring before the union of the right and the left ventral anlagen and Greil stated that the right ventral pancreas has united with the right dorsal in 7.5 mm. embryos. The appearance of considerable pancreatic tissue from the left side of the ventral anlage would indicate that there is growth from this evagination. Goette apparently thought this outpouching was rudimentaiy. The presence of a left duct in some 12-13 mm. embryos indicates that there is growth from the left side. Later stages show that there is more growth on the right side.
Although the lumen of the dorsal pancreatic anlage at first extends anteriorly, the duct later extends upward and in the adult again forward. Short lateral branches extend into the surrounding pancreas. The ventral ducts fuse to form a single one which divides into a right and left pancreatic duct. These again divide into smaller rami, of which some from the right side extend into the portion uniting with the dorsal pancreas. The dorsal and ventral ducts, however, always remain separate.
It is clear that in Amblystoma there is no complex pancreatic duct system as Oppel ('90) has observed in Proteus. In possessing a single dorsal duct Amblystoma resembles the Necturus as described by Kingsbury ('94). The posterior set of ducts emptying into the ductus choledochus as described by Oppel is quite different from the single ventral duct in Amblystoma. Nor does this conform to Kingsbury's description of two posterior pancreatic ducts each of which open into hepatic ducts. I cannot confirm Bates' ('04) observations concerning the several pancreatic ducts which he stated opened into the hepatic ducts within the pancreas. A table of the ducts as they have been described in \'arious urodeles may be of interest.
DEVELOPMENT OF LIVER AND PANCREAS
263
As is well known the duct of the dorsal pancreas does not ])ersist in Anui'a. However, (ioeppert ('91) and Voftt and "Win"; ('94) ha\'e described several pancreatic ducts in the ventral ])ancreas some of which joined the ductus choledochus or hepatic duct. It is seen from the table that there is considerable ^'ariation in the pancreatic duct-system of the urodeles. The com])lex system of some forms may be due, as Goeppert suggested, to a later union of the lesser pancreatic ducts with the duodenum or common duct.
TABLE 4 Table of the pancrentic (lucti< in the van'mis urodeles
Ventral pancreu.- 1, joining with ductus clioleiiochus
1, joininE the two hepatic ducts
9, 11 formin;; network with ductus choledochus
2, joining ductus choledochus '. 1, joining ductus choledochus
\ 3, joining ductus choledochus f 1, joining ductus choledochus
\ 1, opening near ductus choledochus
1, opening near ductus choledochus
2, joining separate hepatic ducts 1, joining hepaticocystic duct Several — joining various hepatic
ducts 1. may or may not join ductus choledochus
The glandular portions of the \'entral and dorsal pancreases fuse as is clearly shown in figure 33. This fusion of the glandular parts takes place inmiediately after the two parts come in contact. The union of the two parts is at first only a narrow neck of tissue, which remains small even in adults.
In a 35 mm. embryo and in smaller ones attention was called to a small part of the ventral i:)ancreas lying below the gallbladder and separating the li\'er into upper and lower parts.
The peripheral portion of the liver grows more rapidly and surrounds this part on the outer side. It then has a peritoneal surface only on the medial upper side. This corresponds to Gianelli's intrahepatic portion of the pancreas. However, as stated by Goeppert, the pancreatic and hepatic tissues are always clearly separate. Goeppert ('91) has given a description
l^'orm
Author
Dorsal
pancreas
Cryptobranchus japonicus
Hyrtl ('65)
1 (?)
Salamandra perspicillata
? Wiedersheim (75)
1 (?)
Geotriton fuscus
Proteus anguineus
Oppel ('89)
10, ;33
Menobranchus lateralis
Goeppert ('91)
Salamandra maculata
1 Goeppert ('OP
1
Salamandra altra
f Goeppert ('91)
1
Triton alpestris
1 Goeppert ('91)
9
Triton taeniatus
J Goeppert ("91)
Cryptobranchus japonicus
Goeppert ('91)
6
Xecturus maculatus
Kingsbury ('94)
1
Triton
Gianelli ('02)
1
Amblystoma
Bates ('04)
—
Amblystoma
Baumgartner CIS)
1
264
E. A. BAUM GARTNER
of the relations and lobes of the pancreas. The pancreas in Aniblystoma resembles that in those forms which he described in having a ventral part or lobe caudal to the liver and in intimate relation with it, and a dorsal or caudal part. Kingsbury described five more or less distinct parts. The following table shows a correlation of the lobes of the pancreas which various investigators have described with those of Amblystoma.
TABLE 5 Table of the -parts of the adult pancreas in Amphibia
Proteus
Oppel ('89)
Vordere
Theil
Mittlere
Theil
Hintere
Theil
Salamandra,
Rana, etc
Goeppert ('91)..
Dorsal o d e r
vordere Theil
Ventral o d e r
hintere Theil
Hinterste
Theil
Necturus
Kingsbury ('94)
Lobe along intestine
Central part
near gall
bladder
Lobe along
dorsal wall of
liver
Lobe along
splenic vein
Lobealong mesenteric vein
Triton
Gianelli ('02) ..
Corpo, estremitk posteriore
E s t r e m i t a
craniale
Pancreas intraepatico
Alytes obstetricanus
Kopf
Wurzel
Reuter ('06)
Amblystoma .
Dorsal portion
Ventral p o rtion
Portion along
dorsal concave border
of liver
Goeppert mentioned that there were usually several other prolongations of pancreatic tissue. This is also true for Atnblystoma, particularly from the anterior end of the dorsal portion were there several prolongations. It is to be remembered that Vordere' is used by Oppel and Goeppert to express proximity to the oral end of the intestinal loop and not in the ordinary topographical sense.
IV. GENERAL SUMMARY
1. The liver begins as a median ventral projection of the lumen of the gut, then as an anterior outpouching from this lumen.
DEVELOPMENT OF LI\ER AND PANCREAS 205
2. There is a later shifting of the postei'ior part of the liver to the right and dorsally, due to crowding of the stomach and development of the duodenum on the left.
3. A later growth on the left side results in an adult organ with right and left parts, the right side always remaining more dorsal on the lateral side of the stomach.
4. The ductus choledochus develops as the early anteriorly directed lumen from the gut.
5. The right and left hepatic ducts develop as divisions of the ductus choledochus and l:)y di\ision and growth form the hepatic rami and branches.
G. The gall-bladder begins as a median \'entral outpouching of the posterior part of the liver-anlage. It is first widest laterally, then becomes larger in its cranio-caudal diameter, then its dorso-\'entral and finally its longer axis is nearly transverse.
7. There is an early right lateral shifting of the gall-bladder as of the li\'er, due probably to the same causes. Along with this there is a constant shifting of direction of the cystic duct in keeping with the dorsalward migration of the gall-bladder.
8. The cystic duct is early closed off with the right hepatic and due to the caudalward growth and division of the hepatic duct is finally attached to the lateral branch of this duct.
9. The ventral pancreatic anlagen are \'entro-lateral evaginations of the gut caudal to the cystic anlage. The dorsal pancreas — a single median dorsal evagination — forms the larger portion of the early pancreas, later it is a narrow lobe dorsal to the duodenum.
10. The ventral pancreatic ducts are constrictions of the two ventral ]:»ancreatic anlagen. Later these unite and form a single ventral pancreatic duct which opens into the common bile-duct or into the intestine at the side of the common bile-duct. The dorsal duct remains a single stem with short lateral branches.
11. There are two main parts or lobes in the adult Amblystoma pancreas, a dorsal and a ventral, with one or more lesser projections from these. An anterior extension of the ventral lobe is constant.
266 E. A. BAUMGARTNER
V. BIBLIOGRAPHY
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DEVELOPMENT OP^ LIVER AND PANCREAS 267
Hammar, J. A. 1893 Eiiiige Plattenmodelle zur Beleuehtung der friiheren
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Jour. Anat., vol. 14. Shore, T. W. 1891 Notes on the origin of the liver. Jour, of Anat. and Phys.,
vol. 25. Steinheim, S. L. 1820 Entwickelung der Frosche. Hamburg. Stohr, Ph. 1895 Die Entwicklung der Hypochorda und des dorsalen Pankreas
bei Rana temporaria. Morph. Jahrb. 23. VoGT, C. 1842 Untersuchungen ueber die Entwickelungsgeschichte der Ge burtshclferkrotc (Alytes obstetricans) Solothurn. W'eber, J. A. 1903 L'origine des glandes annexes de I'intestin moyen chez
les vertebres. Paris. (Diss.) Weysse, a. W. 1895 Ueber die ersten Anlagen der Hauptanhangsorgane dea
Darmkanals beim Frosch. Arch. f. mikr. Anat., Bd. 46. Wiedersheim, R. 1886 Lehrbuch der vergleichenden Anatomic der Wirbelthiere. .Jena.
1875 Salamandrina perspicillata und Goetriton fuscus. Genua. WoiT, O. 1897 Zur Entwicklung der Milz. Anat. Hefte, Bd. 9.
PLATE 1
EXPLANATION OF FIGURES
36 Median view of right and left halves of a reconstruction of the liver of an Amblystoma emhryo 7 mm. long. X 70.
37 Median view of right and left parts of a reconstruction of the liver of an embryo 9 mm. long. X 70.
38 Posterior view of a reconstruction of the liver of an embryo 9 mm. long. X 100.
d., early anlage of duct G.B.. gall-bladder
D. chol., ductus choledochus Li., liver
D.cy., cystic duct Lt. Rt., left and right parts of hepatic
D.m., dorso-medial duct anlage
D.h.d., right hepatic duct c.l., ventro-medial duct
D.Ji.s., left hei)atic duct
268
DEVKLOI'MKXT OF I,l\i:i; AM) I'A XCKKAS
I'LATK 1
K. A. BAUMd ARTXHU
. .jMBiI III I *ii I III w a
afiwftiftaammifrmr/V!*'^ '
Lt GB
//
36 ^
Drn
4
^-3.
269
PLATE 2
EXPLAXATIOX OF FIGURES
39 Ventral view of a reconstruction of the hepatic ducts and gall-bhidder of an Amblystoma embryo 14 mm. long. X 100.
40 Dorsal view of the same reconstruction. X 100.
41 Ventral view of a reconstruction of the hepatic ducts and gall-bladder of an embryo 13.5 mm. long. X 100.
42 Dorsal view of a reconstruction of the hcijatic ducts and gall-bladder of an embryo 15 mm. long. X 100.
43 Right ventral view of a reconstruction of the liopatic ducts and gall-bladder of an embryo 20 mm. long. X 100.
D., duodenum M.R.l.d., medial brancli right lateral
D.chol., ductus choledochus ramus
D.cy., cystic duct .l/./?./..s\, medial branch left lateral
D.h.d., right hepatic duct ramus
D.h.s., left hepatic duct M.R.ni.d., medial branch right medial
D.P., pancreatic duct ramus
g.h., gall-bladder }f.Rjn.s., medial branch left medial
L.Br., left branch of common ramus ramus
L.R.I.d., lateral branch right lateral R.Br., right Ijranch of common ramus
ramvis /I'./.-s., left lateral ranuis
L.R.l.f^., lateral branch left lateral R.l.d., right lateral ramus
ramus R.tn.s., left medial ramus
L.R.m.d., lateral branch right medial R.in.d., right medial ramus
ramus Z.. extra duct in 13.5 mm. Am])lystonia
L.R.m.^., lateral branch left medial (■m])ryo
ramus
270
DEVELOPMENT OF LIVEK AXl) I'AXCUEAS
K. A. BALMGART.NEU
PLATE 2
Z/f/-?. ,^^'s
Rm.d.
39 C7.
.^Js.
LJ?/5
40
O.B.
43
M.t\md.
C7B.
D
41
PLATE 3
EXPLAXATIOX OF FIGURES
4:4 Right ventral view of a reconstruotion of tlic hepatic ducts and gallbhadder of an Ainblystoina cinljrvo '■]') mm. long. X 7(1.
D., duodenum L.Ii.in.^., hitcial Inanch left medial
D.chol., ductus choledofhus ramus
D.cy., cystic duct M.h'.l.d.. medial branch right lateral
D.h.d., right hepatic duct ramus
D.h.s., left hepatic duct .l/./^./..s'., medial branch left lateral
D.P., pancreatic duct ramus
(j.b., gall-bladder M.R.duI., medial l)ranch right medial
L.Br., left In-anch of common ramus ranuis
L.R.l.d., lateral branch right lateral M.R./n.s., medial l)ranch left medial
ramus ramus
L.R.I. s., lateral branch left lateral h'.Rr., right l)ranch of common ramus
ramus R.l.d., right lateral ranuis
L.R.m.d., lateral branch right medial R.l.s.. left lateral ramus
ramus R.iii.d.. right medial ramus
R.tii.s.. left medial ramus
DKVEI.OP.MKXT OF LIVER AXI) I'.WCRKAS
E. A. UAl'.M :AKT.VI;H
I' LATE 3
- 3B
PLATE 4
EXPLANATION OF FIGURE.'
45 Anterior view of the pancreas and ventral pancreatic ducts of a 13 mm.
embryo. X 60.
46 Anterior view of the pancreas and ducts of a 15 mm. embryo. X 60.
D., duodenum V.jxni., ventral pancreas
D.pan., dorsal pancreas Y, yolk-gut
G.B., gall-bladder D.cJr I., ductus clioledochus
Lt.pan.d., left ventral pancreatic duct V.j)nit.(i.. ventral pancreatic ducts.
/?/. pan. (/., right ventral pancreatic duct l'^)r (ither aljbreviations, see figure45.
St., stomach
274
DEVELOPMEMT OF LIVER AND PAXCHEAS
E. A. BAUMCARTXEn
PLATE 4
St
RPan.d.
