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Lockwood CB. Development and transition of the testis, normal and abnormal. (1887) J Anat. 21(4): 635-664. PMID 17231714
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The Development and Transition of the Testis, Normal and Abnormal
By C. B. Lockwoop, F.R.C.8., Hunterian Professor of Comparative Anatomy and Physiology, Royal College of Surgeons of England, &c. (PLATE XV.)
Method of Investigation
Since our ultimate aim is to elucidate human anatomy and pathology, it would seem an ideal plan to trace the development of the testicles and their appendages in the human embryo ; but unfortunately this is not entirely possible. A sufficient supply of such material is hard to obtain, and that which is procurable is seldom quite fresh and undamaged. The kindness of numerous friends has, however, enabled me to investigate many interesting points in human embryos, but not the very earliest, as these can only be traced in the lower animals. Before attempting this branch of the inquiry it may be bist to glance for a moment at the end which is achieved, and afterwards inquire how it has been accomplished. The fact that the body of the testis and the epididymis have different origins is too well known to call for comment. The epididymis arises from a part of the Wolffian body, the testis from a separate group of cells, The Wolffian body has a prior claim upon our attention, for it has attained a certain degree of perfection before the rudiments of the body of the testis have appeared.
Wolffian Body in Human Embryos
The smallest human embryo of which I have examined the Wolffian body was seven thirty-seconds of an inch long ?—about the size of a small house fly. It is hard to estimate the age of such an embryo as this, but it looks exactly like one which His
1 This article will consist of three lectures, delivered to the Royal College of Surgeons of England in February 1887.
2 I am indebted to Mr D’Arcy Power for this embryo.
considers to have been twenty-three days old! The most palpable of its external characteristics were a considerable flexure of the body and neck, an absence of limbs, and the presence of a very obvious tail. Longitudinal sections show that the body is divided into two compartments—a superior, containing the heart ; and an inferior, containing the liver, lungs, alimentary canal, and Wolffian bodies.
A glance at the accompanying figure (fig. 1) shows that, owing to the absence of the diaphragm, there is no separate pleural sac, but that the lungs lie in the upper part of a pleuroperitoneal cavity. The large size and the appearance of its glomeruli render the Wolffian body a conspicuous object. Occupying nearly all the back part of the pleuro-peritoneal cavity, it reaches nearly as far forwards as the heart and pericardium. Its front extremity is tapering and situated behind the lungs, whilst the posterior is larger and extends towards the hinder end of the pleuro-peritoneal cavity.
This brief description can hardly convey an adequate idea of the enormous proportionate size of this part of the embryonic excretory system. Perhaps it would not be incorrect to say that at this period (the 23rd day) the length of the Wolffian bodies is a third of that of the embryo, and that they constitute a sixth part of its bulk. Another most interesting feature is their extension towards the heart and pericardium, for it betrays their close resemblance to the permanent kidneys of the lower vertebrates, ¢.g., fishes.
Although this particular embryo serves admirably to display the size and importance of the Wolffian body, yet it affords but little information concerning its minute structure. In the section drawn in fig. 1, the organ consists of a groundwork of embryonic connective tissue, throughout which are numerous round and oval spaces. These spaces are lined with minute cubical cells, and a glomerulus protrudes from some part of the wall into the interior. This arrangement holds good throughout the whole length of the Wolffian body; but it may be noticed that anteriorly the glomeruli and the spaces which contain them are smaller than elsewhere. Besides these glomeruli various tubules lined with epithelium may be made out in the other sections, but, as these will soon be considered in detail, it is unnecessary to do anything more than merely mention them. Having now obtained a general idea of the Wolffian bodies of the human embryo, we may proceed to examine them in greater detail. For this purpose transverse as well as longitudinal sections are requisite, and a commencement may be made with specimens which have been made from a human embryo a little larger and more advanced than the last, and perhaps at about the 26th day of intrauterine life! The length of this specimen was nearly half an inch, and the fore and hind limbs had just begun to appear. The section which has been chosen (fig. 2) is through about the middle of the embryo, and the organs divided by it are the stomach, duodenum, liver, and Wolffian bodies. The last named occupy no inconsiderable portion of the back of _ the pleuro-peritoneal cavity, and their triangular shape is very unmistakable. The base of the triangle is towards the aorta and vertebral column,.and is continuous with the tissues which surround those parts; the sides are free, and look into the peritoneal cavity. The bulk of the Wolffian body consists of embryonic connective tissue, and its surface is covered with a layer of cells, which are thickest and most columnar round the most prominent part of the organ. Beneath this covering and near the apex of the triangle is a ring of concentrically arranged cells, surrounding a small lumen, and this represents the Wolffian duct. On either side, coiled tubules, the Wolffian tubules, are apparent; and on one side, the spectator’s night, the. tubule seems to open into the duct. Moreover, in all probability the tubule is connected with a large glomerulus, for on either side these structures are prominent objects. Since all the things which have been mentioned, duct, tubule, and glomerulus, can be described better hereafter, a detailed account of them will be deferred, and instead we will proceed to trace their mode of origin. Before attempting to do this the large . veins in the back part of the Wolffian body may be noted; they are the cardinal veins, and are the channels by which the blood from the primitive kidneys is returned to the heart.
1 His, Anatomie Menschlicher Embryonen, Leipzig, 1885, iii. ; Zur Geschichte der Organe, p. 238; Atlas, iii. Tafel x. fig. 7.
Fig, 1. — Hi., heart; P, lungs; L., liver; St., stomach; W.B., Wolffian body ; C, tail; 7, tongue; x 25.
Fig. 2. — St., stomach; Mg., mesogastrium; W.D., Wolffian duct; 7’, Wolffian tubule; Gi., glomerulus; C.V., cardinal vein; 4A0O., Aorta; S.M., spinal medulla; x 25.
1 This embryo was very like one depicted by His, Aédas, iii. Tafel x. fig. 9.
It is clear that it is impossible to pursue this branch of the inquiry upon human embryos, for it may have been noticed that the organs in question had already, in the youngest embryos at my disposal, attained a certain degree of perfection. The important réle which the Wolffian duct is destined to play is betokened by the early age at which it appears. This point, the origin of the Wolffian duct, is, as a preliminary, conveniently studied in chicks, and although knowledge gleaned from them is most valuable, yet perhaps it would be rash to assume its applicability to the human subject without supplementing it with information gathered from other sources.
A great many reasons might be adduced to show that in the rabbit the process of development approximates very closely that which has been observed in man. Moreover, that animal has many advantages which need not be mentioned, rendering it suitable for these investigations. Therefore I have availed myself of the embryos of this animal, and whatever they have taught will be mentioned in due course.
Wolffian Duct of Chick.
The phenomena which attend the development of the Wolffian duct in the chick are difficult to observe, and authorities are not entirely agreed upon their interpretation! In order to obtain satisfactory information an embryo at about the 48th hour of incubation may be chosen.
1 Foster and Balfour (Elements of Embryology, 2nd 'ed. edited by Adam Sedgwick and Walter Heape, 1883, p. 94) say that the Wolffian duct may be seen
during the first half of the 2nd day. Mr Sedgwick (‘‘The Early Development of the Anterior Part of the Wolffian Duct and Body in the Chick, &c.,
In a transverse section through the middle of such an one it is easy to recognise the various rudiments. It is unnecessary to describe such well-known objects as the medullary canal and the notochord,. nor need particular stress be laid upon those important membranes, the epiblast and hypoblast, for the arrangement of the mesoblast, which lies between them, demands all our attention (Plate XV.fig. 1). Close to the medullary canal the mesoblast is thicker than elsewhere, and forms the mesoblastic somites or protovertebre; beyond these it divides into two layers, the somatopleure and splanchnopleure, separated by an intervening space, the body cavity. It is important to note that the somatopleure and splanchnopleure are united to the somites by a neck of uncleft mesoblast, which is called the intermediate cell mass, and which is said by many observers to give origin to the Wolffian ducts. So far, the presence of the latter in the section has not been mentioned, but nevertheless they are quite clear and distinct. Upon one side, the spectator’s right, the Wolffian duct consists of a ring of cells surrounding a small lumen. This ring is situated above the intermediate cell mass,” with which it has no connection, and lies in a bay made by the somite on the inner side, and externally by the somatopleure, the epiblast completing it above (Plate XV. fig. 1, W.D.). On the opposite side, although the position of the collection of cells, which afterwards forms the duct, is the same, it is questionable whether it has any lumen, and, moreover, it is, in the clearest way possible, united to the outer side of the adjacent somite. Perhaps we may pause for a moment to comment upon these facts. It is so clearly recognised that the Wolffian duct is at first a solid column of cells, which afterwards becomes hollow, that nothing more requires to be said upon that point, but its connection with the somite is another affair; because, according to some embryologists, the latter would be its normal origin, but others are of opinion that it arises from the intermediate cell mass. Under these circumstances it is interesting to note its connections in this particular embryo. Anteriorly the duct is united to the side of the somite (fig. 1), and, traced towards the head, gradually ends upon it, disappearing by degrees ; traced backwards, it first of all separates from the somite and becomes distinct, but further back it joins the intermediate cell mass, and terminates by mingling with it (Plate XV. figs. 2 and 3, W.D.). My own experience is insufficient to permit me to say whether there is a moment in the development of the Wolffian duct at which it always conforms to this description, but it does not seem unlikely, and the observation has the merit of reconciling conflicting views. However this may be, an important principle underlies these matters of detail, and it is that the Wolffian duct arises from mesoblast. As far as the chick is concerned, this principle seems to be supported by ample histological proofs. Nevertheless, we must not hastily assume that it. is true, because, after a little consideration, it will be acknowledged that there are-d@ priory reasons with which the observation does not readily conform. For instance, in the case of other glands it is possible to trace the origin of their epithelia either to the epiblast or hypoblast, so that in the case of the Wolffian duct of the chick there seems to be an exception to what ought to be the rule. The difficulty would disappear if credence could be given to the statements of Hensen and His, that in the chick the Wolffian duct commences as a longitudinal involution of epiblast.!. But the evidence to the contrary seems so weighty, and the mesoblastic origin so clear, that the very strongest confirmatory evidence must be forthcoming before the epiblastic parentage can be accepted. In truth, His, who at one time advocated it, has greatly modified his opinion.
Quarterly Journal of Micros. Sci., 1881, vol. xxi.) says that ‘‘the first trace of the Wolffian duct is visible in an embryo with eight protovertebre as a slight projection from the intermediate cell mass towards the epiblast in the region of the 7th and 8th protovertebre.”
1 1 am indebted to my friend Mr Ogle for the very beautiful specimens from which these conclusions were drawn.
2 Until the later stages of development are treated, all relations will be described as though the embryo was placed with its ventral aspect towards the ground.
1 Foster and Balfour, in their excellent manual, give a resumé of the views of embryologists on this point (Foster and Balfour, Elements of Embryology, 1st ed., p. 82, 1874). They say that Remak and Kolliker described the Wolffian duct as taking origin from the mesoblast of the somatopleure. Dursy regarded itas being derived from the protovertebre. Hensen, and for some time His, believed that the duct began as a longitudinal involution of the epiblast external to the protovertebre. Subsequently His took the view that it was a product of the protovertebre. Waldeyer says the Wolffian duct originates from the intermediate cell mass, But Waldeyer seems to think that the cells lining the duct are originally derived from the epiblast. Quite recently Romiti has described the Wolffian duct as being an involution of the epithelium of the pleuro-peritoneal cavity.
Epiblastic Origin of Wolffian Duct.
The origin of the Wolffian duct is a problem so interesting to the embryologist, and so important to the pathologist, that it has not been allowed to slumber. Quite recently observations have been published with the intention of proving its epiblastic origin in the guinea pig and in the rabbit. So faras the former is concerned, I am unable to adduce any evidence for or against, but as regards the latter, specimens are at hand suitable for investigating its truth.
A recent writer? has endeavoured to show that in the rabbit’s embryo certain thickenings of the epiblast separate and become the Wolffian duct. Embryos of the 9th day were used, and it may be stated that transverse sections through the region of the duct closely resemble the transverse sections of the chick (Plate XV. figs. 1 and 2). They are, however, much more advanced as regards the folding in of the splanchnopleure, and in the development of the aortz and body cavity. The epiblastic thickenings in question are situated just above the intermediate cell mass and external to the somite (Plate XV. fig. 5), and after examining many embryos, I find that it is impossible to doubt that they actually exist. Nothing could be clearer than the diagrams which Flemming gives to show how the cells which constitute these epiblastic enlargements become detached, and form the Wolffian duct; but, to my mind, the most important evidence, that afforded by actual drawings, is inconclusive. The matter is too important for summary dismissal, and perhaps the actual specimens are inadequately represented by the figures.
_ 1 Graf Ferdinand Spee, in his paper ‘‘Ueber directe Betheiligung des Ektoderms an der Bildung der Urnierenanlage des Meerschweinchens,” Arch. f. Anat. und Phys. 1884, p. 89, gives anumber of references to the literature of the subject. The epiblastic thickenings which he delineates are singularly like some which occur in both the rabbit and chick, and which will be referred to again presently. 8 <‘Die Ektoblastiche Anlage des urogenital Systems beim Kaninchen,” Von V. Flemming, Archiv. fiir Anatomie und Physiologie, 1886, p. 236, Tafel xi.
Now, although in the rabbit the existence of these thickenings is indubitable, it cannot be without significance to observe that they are by no means confined to one position. The cells of the epiblast seem to accumulate wherever the underlying mesoblast presents an inequality of its surface, and in a specimen which had been taken from a nine days’ rabbit embryo (Plate XV. fig. 5) there were distinct thickenings at each side of the somites, and an additional one far away from the side of the Woffian duct in the bend which the somatopleure makes as it turns backwards, and in fig. 9 there is an epiblastic thickening exactly over the dorsum of the somite. It seems pertinent to ask whether we are to assume that all of these thickenings are destined to become Wolffian ducts. But however this may be, it seems quite clear that in the rabbit embryos which I have examined, . the Wolffian duct is in reality formed from a portion of the cells of the intermediate cell mass. Those cells are at first a solid column (Plate XV. fig. 10, W.D.), but this m a little while becomes hollowed out to form a real canal (Plate XV. figs. 5 and 9, W.D.). Of course this does not invalidate the idea that the epiblastic enlargement which overlies the intermediate cell mass may not participate in the production of the duct, but assuming that was so, it seems natural to infer that after producing the duct the swelling would alter. It is true that the figures and diagrams which have been mentioned show that this is supposed to be the course of events, but nevertheless the specimens which I have examined cast doubt upon the matter, because in them both the duct and epiblastic swelling coexist (Plate XV. fig. 5, T7.). Not only may this be seen in rabbits, but in sections through the hinder part of the forty-eight hours’ chick, which has been mentioned before (Plate XV. figs, 3 and 4), it is clear that the epiblast has thickenings analogous to those of the rabbit’s embryo, and, moreover, that the Wolffian duct arises independently of them from the somite and intermediate cell mass.
1 The specimens from which these observations were made are chromic acid preparations stained with picrocarmine, and obtained in the summer of 1886. Others have been prepared this summer (1887), which clearly bear out the views which have been stated in this lecture. The last embryos were hardened in corrosive sublimate and stained with borax carmine.
It must be acknowledged that it is not an easy task to observe the origin of the Wolffian duct in the rabbit’s embryo, because early embryos of that animal are exceedingly brittle and hard to prepare. But Kolliker? depicts the duct originating from the intermediate cell mass, and, as may be seen, my own observations are confirmatory of the correctness of that view. Fig. 6 shows the intermediate cell mass of an early rabbit’s embryo, and it may be seen how strikingly it resembles the analogous structure belonging to the chick; and I have little doubt but that it is the origin of the cell group from which the Wolffian duct, and, as will be seen presently, the Wolffian tubules originate.
Before proceeding further, a summary of the preceding considerations may conduce to clearness, and they may be tabulated as follows :—(a) In rabbit’s embryos the epiblast has thickenings, not only near the intermediate cell mass, but elsewhere; (6) none of these thickenings are altered by the advent of the Wolffian duct; (c) the chick has similar thickenings, although in it the Wolffian duct arises from the mesoblast.
Whatever may be the truth about the Wolffian duct, the preceding observations tend to invalidate the idea that its origin is from epiblast, but, without doubt, many will -be reluctant to admit their cogency. Such an admission is particularly hard for the scientific pathologist, who is tempted to assume that the epiblastic origin is true because it accords with @ priori conceptions. For instance, in the-case of the testicle the assumption that the Wolffian duct arises from epiblast has suggested a plausible explanation of the origin of testicular carcinomatas, but under the circumstances such an inference seems premature.? However clearly the epiblastic origin of the Wolffian duct is demonstrated, an obvious fallacy has to be guarded against. It is superfluous to point out that nothing but a very simple conclusion ought logically to be drawn from that demonstration— namely, that it may explain the origin of carcinomata from the parts developed from the Wolffian duct and tubules. Presently it will be seen what these are, but I may say at once that it is doubtful whether any part of the body of the testicle ought to be included.
1 Janosik, Sitzwngsberichte der K. Akad. der Wissenschaften, Band. xci. Heft. i. and ii., Wein, 1885, p. 113, repudiates Spee’s observations, and says he has not seen epiblastic thickenings which he could consider the origin of the Wolffian duct. Janosik’s monograph is clear, and gives a very good account of the bibliography.
2 Kolliker, Entwicklungsgeschichte des Menschen und der Hoheren Thiere, Leipzig, 1879, p. 277, figs. 197 and 198.
3 John Bland Sutton, An Introduction to General Pathology, 1886, p. 344. THE DEVELOPMENT AND TRANSITION OF THE TESTIS. 645
Prolongation of Body Cavity into Somite in Rabbit.
This seems a favourable place to mention an interesting peculiarity of one of the specimens. It is usual in rabbits’ embryos, at the end of the 6th or beginning of the 7th day, for the two divisions of the mesoblast to be continuous with one another at the intermediate cell mass, so that there the walls of the body cavity are quite unbroken. Moreover, the intermediate cell mass intervenes between the cells which bound the body cavity and those of the somite. The latter consists of a central area of round cells, surrounded by a wall of others of a columnar type (e.g., v. Plate XV. figs. 5 or 6). The meaning of this central area is not at a first glance particularly obvious, but the following observation throws considerable light upon its meaning. Ina transverse section through a rabbit’s embryo (fig. 7) the walls of more columnar cells surrounding the central part of the somite are particularly distinct and continuous with the somatopleure and splanchnopleure. In consequence the central area of the somite is a part of the general body cavity, to which it is joined by a narrow passage which lies in the centre of the isthmus, representing the intermediate cell mass.1
1 George Renson, Archiv fiir Mik. Anat., 1883, p. 600, ‘‘ Recherches sur le rein céphalique et le corps de Wolff chez les Oiseaux et les Mammiféres.” In this paper the author says he has seen this appearance in the chick, and calls the canal the ‘‘canal de communication.” He likewise quotes Kowalewsky (Die Bildung der Urogenitalanlage bei Hiihnembryonen, Warschaw., 1875, Jahresbericht, Hofmann und Schwalbe, 1875, p. 448), who first discovered this canal in the chick.
Doubtless an observation founded upon the appearances of a single embryo would not deserve to be recorded, if it was not in accordance with our previous knowledge. As far as I am aware, the body cavity is not known to be prolonged in mammalian embryos into the somites, but the appearances which have just been described are very significant, and when we read what Professor F. M. Balfour has written about elasmobranchs it must be acknowledged that a summary dismissal of the subject would be unmerited. This author points out that in some of those fishes the body cavity extends into the somite, and proceeds :— “The vertebral plate comes to consist of a series of rectangular bodies, the mesoblastic somites, each composed of two layers, a somatic and a splanchnic, between which is the cavity originally continuous with the body cavity. .... Between the vertebral and lateral plates there is left a connecting isthmus, with a narrow prolongation of the body cavity, which gives rise to the segmental tubes, and to other parts of the excretory system.”? This quotation shows that there is a strong resemblance between the development of the rabbit and of this particular fish, and it may help to explain the origin of the Wolffian duct. It may be remembered that the Wolffian duct, both in the rabbit and the chick, has been said to begin as a column of cells which afterwards became hollow. In seeking to explain the formation of this canal, it is suggestive to find that in some animals it is merely a shut-off portion of the body cavity, and further light is thrown upon the question by observing that, even in the rabbit, a part of the body cavity may once have occupied the place where the Wolffian duct and tubules afterwards appear. Supposing for a moment that we may assume that in rabbits, as in some elasmobranchs, the lumen of the Wolffian duct is a shutoff part of the body cavity another objection remains to be dealt with ; for if the assumption is admissible, why does the duct, in most specimens, look like a solid column of cells? We should expect it to retain the lumen when once it has been acquired. Although this is a weighty objection, it may lose some of its force if we consider a thing which is often overlooked. It is the custom to ascribe such preponderance to cells that the matrix in which they lie is forgotten. Properly prepared embryos may be seen to consist of cells embedded in a homogeneous matrix, and I am inclined to think that in it are foreshadowed many changes which occur in the arrangement of the cell elements. Therefore it is not impossible but that in the midst of cell groups apparently solid many unsuspected matrix changes are hidden by the cells themselves. However all this may be,1t seems possible that the force which hollows the Wolffian duct and tubules is actually the same as that which makes the body cavity, and that it is a part of the same process of cleavage.
1. Francis M. Balfour, A Treatise on Comparative Embryology, 1885, vol. ii. p. 48 et seg., also fig. 21.
After this I need not continue to discuss the actual origin of the Wolffian duct, but will proceed with its subsequent history. First of all, it is necessary to describe the way in which it alters its position. It may be remembered that at the 9th day, in the rabbit’s embryo, the duct lay beneath the epiblast, just external to the somite and near the place where the somatopleure and splanchnopleure diverge to form the body cavity. However, owing to changes in contiguous structures, it speedily comes to lie beneath the cells which line the body cavity, and even from the first a slight elevation denotes its. situation (Plate XV. fig. 5). For reasons which will be clear after- . wards, this elevation may be called the urogenital ridge, and the important events which occur in it will, for a time, monopolise all our attention. Perhaps it may be said, in anticipation that the development of the Wolffian tubules and of the body of the testis are included in those events, but before beginning this part of the subject it may be as well to say how the Wolffian duct acquires its proximity to the body cavity. This is brought about very simply in rabbits, for it seems to be accounted for by an abrupt bending of the somatopleure backwards, and a downward shifting of the splanchnopleure (figs. 5 and 9). This is hardly the place to discuss the cause of these changes but it may be explained that when the embryo first appears it forms part of a blastodermic membrane, which is closely applied to the uterine wall, with which the dorsal surface of the embryo is in close contact. A little while after the advent of the Wolffian tube, the axial part of the embryo shifts away from the uterus, and consequently away from the plane of the peripheral part of the blastoderm. Now, both the body and gut walls of the embryo are continuous with the blastoderm, so that after the embryo has moved they are necessarily bent backwards towards the uterus. In the case of the body wall the bending is very abrupt, and when its relation to the duct is noticed it is easy to see how it makes the canal look more im relation with the lining of the body cavity than it did previously (fig. 5).
1 Kolliker, loc. cit., fig. 198, p. 279, portraysin the rabbit the Wolffian duct as a fissure, such as might be expected had it been originally part of the body cavity.
Whilst the urogenital ridge has been appearing, other events have happened. It may be remembered that originally the Wolffian duct was represented by a solid column of cells (fig. 10); but these after a time arrange themselves in a ring (Plate XV. figs. 9 and 12), and a lumen appears in their midst. When the canalisation is accomplished, the cells provide the duct with a lining of columnar epithelium, which ultimately becomes ciliated and surrounded by various coverings which will be mentioned hereafter. During the progress of these changes the duct has come to lie in the outer part of the urogenital ridge close to the posterior recess of the body cavity (fig. 12, W.D.), but another event of the greatest importance has occurred, for the Wolffian tubules have appeared towards its inner and ventral side. When this stage of development is reached, the situation of the urogenital ridge is at the back of the pleuroperitoneal cavity, and at either side of the newly-formed mesentery. Traced forwards, the ridge gradually dwindles, and terminates near the heart, and backwards it may be followed as far as the hinder part of the body cavity, but not quite to its end. It is therefore evident that the urogenital ridge of the rabbit has limits which correspond to those of the Wolffian body of the human embryo (Plate XV. fig. 1), and it clearly represents an early stage in the development of that structure.
The shifting of the splanchnopleure towards the ventral aspect of the embryo, which is seen in embryos of the 9th day, permits the urogenital ridge to protrude into a comparatively wide body cavity (fig. 12), and there is nothing to interfere with its growth, which goes on apace. By the 10th day it has grown considerably, and the embryo has begun to assume its more permanent characteristics. For instance, both the somatopleure and splanchnopleure have bent inwards towards the ventral aspect; the latter to form the gut and its mesentery, the former the body wall. At this stage (fig. 12) a large part of the urogenital ridge consists of mesoblast, in the midst of THE DEVELOPMENT AND TRANSITION OF THE TESTIS. 649
which the Wolffian duct and tubules are conspicuous. The former lies in the back part of the ridge almost in contact with the lining of the posterior recess of the body cavity, whilst the tubules lie towards its inner side. It is important to note that the epithelium which covers the ridge is slightly larger and a little more columnar than elsewhere. It is called the germinal epithelium. After these preliminaries, we have naturally arrived at a point where the question of the origin of the Wolffian tubules may be raised, and for a while that matter will occupy our attention.
Since it is unnecessary to comment upon the importance of this part of the subject, I will proceed at once to discuss the origin of the Wolffian tubules. Nearly all we know about this question has been learnt from the chick, so that it will be best to give a brief account of the phenomena seen in that animal, and afterwards describe those which I have observed in rabbits and in the human embryo.
What follows concerning the development of the Wolffian tubules of the chick is in accordance with the statements made in Foster and Balfour’s well-known work on “ Embryology.”! Since the early development of the Wolffian duct of the chick has already been briefly described, and, perhaps, it is hardly necessary to mention the changes by which it arrives beneath the lining of the pleuroperitoneal cavity. The process is very similar to that which has been seen in the rabbit, and the result is that, finally, the duct lies beneath the peritoneal epithelium. The tubules from their earliest appearance are very numerous, and, in a sense, developed independently of the duct. In its formation the latter does not monopolise all the intermediate cell mass, but merely a part, and the remainder becomes converted into the tubule. The process of conversion is not the same everywhere, and, without doubt, the following quotation will make the difference quite clear :—
In front of about the sixteenth segment special parts of the intermediate cell mass remain attached to the peritoneal epithelium on this layer becoming differentiated, there being several such parts to each segment. The parts of the intermediate cell mass attached to the peritoneal epithelium become converted into S-shaped cords, which soon unite with the Wolffian duct, and constitute the primitive Wolffian tubules. Into the commencement of each of these cords the lumen of the body cavity is for a short distance prolonged, so that this part constitutes a rudimentary peritoneal funnel, leading from the body cavity into the lumen of the Wolffian tubule.
1 Loc. cit., 2nd ed., edited by Adam Sedgwick and Walter Heape, p. 190; also Mr Sedgwick’s very able paper upon the subject has been consulted (Quart. Jour. of Micros. Sci., vol. xxi., 1881). 650 MR C. B. LOCKWOOD.
In the foremost Wolffian tubules, which never reach a very complete development, the peritoneal funnels widen considerably. The section of the tube adjoining the wide peritoneal funnel becomes partially invaginated by the formation of a vascular ingrowth known as a glomerulus, and this glomerulus soon grows to such an extent as to project through the peritoneal funnel, the neck of which it completely fills, into the body cavity. There is thus formed a series of glomeruli belonging to the anterior Wolffian tabuli projecting freely into the body cavity. These glomeruli with their tubuli become, however, early aborted.” 4 |
Before going any further it may be wise to pause and deal with the questions which are raised by this quotation, and, following its order, the Wolffian tubules may be considered first.
Theoretically speaking the simplest way in which the Wolffian tubules could develop would be as outgrowths from the duct, but the statement in the first sentence, that they arise independently, disposes of this idea, and a little further on it is said that they proceed from the intermediate cell mass. In these particulars the rabbit’s embryo resembles the chick, for its Wolffian tubules are not protrusions from the duct, but are developed from a group of cells which is derived from the intermediate cell mass. The meaning of this will be clearer if we turn again to the rabbit’s embryo at the beginning of the 9th day. In such an one there is, in the position which the intermediate cell mass would naturally occupy, a collection of cells more or less surrounded by a delicate line of delimitation, and lying between the external surface of the somite and the body cavity (fig. 8). Before the end of the 9th day this group alters, and the cells of it, which are near the epiblast, become concentrically arranged to form the Wolffian duct, whilst the others remain as a mass which is ultimately converted into the tubules (fig. 9). At this stage the rudiments of the tubules lie towards the ventral side of the duct, and in close contact with the lining of the body cavity, but for the present the question of their continuity with the latter may be deferred.
1 Foster and Balfour, 2nd ed. p. 191.
Before describing how the Wolffian tubules grow and acquire a lumen, an interesting topic may be discussed. Apparently the group of cells from which the duct and tubules are developed extend without a breach from the posterior end of the body cavity to within a short distance of the head. Now it is not difficult to understand how a part of this column may become converted into a continuous thing like the Wolffian duct, but it is harder to appreciate the development of the tubules from the remainder. Possibly comparative embryology may throw a little light upon the point, and the researches of the late Professor F. M. Balfour show that in the elasmobranchs “ one segmental tube (or Wolffian tube) makes its appearance for each somite, commencing with that immediately behind the abdominal opening of the segmental duct, the last tube being situated a few segments behind the anus.” Although this segmental arrangement is not so obvious in the embryos of ‘other types,? yet it doubtless foreshadows the principle which determines the grouping of the cells which become the Wolffian tubules.
I regret to have been unable to obtain satisfactory evidence upon this point in rabbits’ embryos; but the appearances at the 11th day® were not uninteresting. At this time, owing to the spiral form of the embryo, it is hardly possible to procure good longitudinal sections, but occasionally a view of twelve
1 Comparative Embryology, vol. ii. p. 692.
2 Adam Sedgwick on ‘‘The Early Development of the Anterior Part of the Wolffian Duct and Body in the Chick, &c.,’’ Quart. Jour. of Mic. Sei., vol. xxi., 1881; also, Studies from Morphological Laboratory in the University of Cambridge, part 1i., 1882, p. 13 & seq.
3 Kolliker, Joc. cit., fig. 298, p. 476, depicts a ten days’ rabbit’s embryo, which corresponds to those which are here called eleven days’. The difference is accounted for by obvious considerations, which need not be mentioned. 652 MR C. B. LOCKWOOD.
somites and their adjacent tubules was obtained. In a section of this sort it was quite clear that, as a rule, two tubules belonged to each somite; but, here and there, it was hard to say whether this number might not be exceeded ; at all events, there were many more tubules than somites. Of course this merely indicates the arrangement at that particular moment, the 11th day. At earlier stages the conditions may resemble the more primitive type, but this is perhaps unlikely in an animal so high in the scale as the rabbit.
It has been said before that the Wolffian tubules are developed from a group of cells which lies in contact with the inner and ventral side of the Wolffian duct, and we may now endeavour to see how these acquire a lumen. In highly magnified longitudinal sections (fig. 11), the elements which form the tubules differ but little from those of the duct, and where both come together there is no line of demarcation. Afterwards, by a simple separation of the central cells, the lumen of the tubule appears; but at first the newly formed canal is without any opening into the duct. A junction is obtained by a continuation of the process which has just been described, for by a slight severance of the cells of the duct-wall the lumen of the Wolffian tubule becomes connected with the canal (Plate XV. fig. 12).
Before leaving this part of the subject, perhaps I may mention a circumstance which is shown in the drawing of the longitudinal section. It may be noticed that although the tubules are quite apart near the duct, nevertheless a little further off they are united by a string of cells. Assuming that this appearance is not due to any peculiarity of the sections, it seems reasonable to suppose that it is the last trace of a junction which, at one time, was much greater, perhaps complete. However, the appearance is but temporary, and at later stages there is no vestige of any union between the various tubules.
If the preceding description of the development of the rabbit’s Wolffian tubules be compared with the account which Foster and Balfour give of the same process in the chick it will be manifest that a very important question still remains to be discussed. It may be remembered that those authors write, that in the chick the anterior tubules are, from their earliest beginning, in continuity with the peritoneal epithelium ; as for the other tubules, they say “in the posterior part of the Wolffian body of the chick the intermediate cell mass becomes very early detached from the peritoneal epithelium, and at a considerably later period breaks into oval vesicles, which elongate into Wolffian tubules,
Evidence has already been adduced to show that in the rabbit the body cavity conforms with the conditions determined in elasmobranchs (fig. 7). Perhaps it is unnecessary to repeat how such an arrangement influences the relations of the Wolffian duct and tubules of that animal to the peritoneal epithelium and body cavity. Of course, if similar principles of development obtain in the higher vertebrates, as in the elasmobranchs, we should expect to find intimate relations between the developing Wolffian tubules and the body cavity.
So far as concerns the chick nothing more need be said upon this point, but as regards rabbits’ and human embryos, no apology is called for before the matter is fully discussed.
Clearly the question now is to tell whether the Wolffian tubules of the rabbit are, like the anterior tubules of the chick, continuous with the peritoneal epithelium. In order to ascertain this the nine days’ rabbits’ embryos, which have been mentioned before, are available.2 It may be remembered that one of these (fig. 8) showed a group of cells (the intermediate cell mass) lying between the somite and the body cavity, and from these the Wolffian duct and tubules originated. In a moment the basis for this assumption will be more clearly indicated, but our present object is to ascertain whether the cells are continuous with the peritoneal epithelium. It has been said already that they are clearly delimited everywhere else except near the peritoneal side, but, for the sake of simplicity, the point in question was not referred to. However, it may now be stated that, owing to the physical resemblances of the cells, it is impossible to say where the cells of the tubule end or where those of the peritoneum begin (figs. 8 and 9). In this particular embryo the intermediate cell mass reaches as a distinct group of cells only a little way along either side of the axis of the embryo; but, doubtless, this need not be taken to imply that the intermediate cell mass is of limited extent, but merely that the cells which compose it may not differ from their neighbours until about to undergo conversion into duct and tubules.
1 Foster and Balfour, 2nd ed., p. 192.
2 These embryos were a little more advanced than those which Kolliker depicts, fig. 218, p. 298, and which are said to be of nine days and two hours.
It is a matter of some importance to follow this part of our subject step by step. As the peculiarities of the tubules are brought into view, fresh light will be thrown upon the question of the origin of the Wolffian duct; and this will further tend to invalidate the idea of its epiblastic origin. An embryo belonging to the same litter as the last is suitable for the purpose in view, and, curiously enough, this second embryo seems in every way more grown and developed than its fellows. In this case, instead of finding an uniform mass of cells (intermediate cell mass) between the somite and peritoneal epithelium, this situation is occupied by the Wolffian duct and rudimentary tubule (fig. 9). The duct is quite isolated from the surrounding structures, and no doubt as to its identity need be entertained. The rudimentary tubules lie towards the ventral side of the duct, and it is quite certain that where they are in contact with the peritoneal epithelium no line of separation can be discerned. Not only is this the case, but there is a distinct resemblance between the cells of the tubules and the epithelium which overlies them. In fact, observers are wont to say that the epithelium grows in to form the tubule; but, although such an expression would afford an accurate idea of the appearances in the rabbit, nevertheless it would convey a misleading impression of the process. It would be more correct to say, that as the cells of the intermediate cell mass differentiated into the Wolffian tubules they assumed a greater resemblance to the peritoneal epithelium. It must not be understood that the preceding statements apply to all the tubules seen in this nine days’ embryo, for some of those near the tail end are certainly not connected with the epithelium (fig. 10). Perhaps they are developed independently in the intermediate cell mass; and, if this be so, it would show that there is a striking similarity between the development of the rabbit and the chick, so far as their Wolffian tubules are concerned.
It is important to tell how many of the tubules are connected with the epithelium, but the endeavour has not yielded satisfactory information. Proceeding, therefore, with other questions, we notice that in embryos of the 11th day the junction between the front tubules and the peritoneal epithelium is very clearly marked, and on the 13th day not only is this the case, but fresh peculiarities have appeared. Before these are mentioned it may be as well to comment upon the significance of some of the preceding statements, and in doing so advance another step. First of all,a very simple inference may be drawn from the fact that in the second embryo the situation of the intermediate cell mass is occupied by the Wolffian duct and rudimentary tubules, viz., that the mass has actually separated into those two structures; or the matter might be put differently by saying that the cells of the intermediate cell mass nearest the epiblast simply became the duct, and that the others remained as the tubule. Since, therefore, the Wolffian tubules of the rabbit are continuous with the peritoneal epithelium, we may now proceed to ascertain whether their lumens communicate with the body cavity.
The comparative embryology of the vertebrate excretory system is so abstruse that it would be out of place, under such circumstances as these, to attempt its explanation. Those who have paid attention to the subject know that the Wolffian body, which so far is the only organ that has been mentioned, represents but a portion of the entire system. Sedgwick? in his able monograph, says, “The most peculiar feature of the excretory system of the vertebrate is the presence of three more or less distinct parts—the pronephros, the mesonephros, and the metanephros, or kidney proper.” The mesonephros is the organ which in these pages has been described under the name of the Wolffian body, but nothing whatever has been said concerning the pronephros. Obviously, before it is possible to say whether this organ is represented in the rabbit, it is necessary to ascertain by what characteristics it may be recognised, and for this purpose Sedgwick may be quoted. First of all, it is stated that “the pronephros when present always develops in continuity with the anterior end of the segmental duct.” The initial step in the inquiry is therefore exceedingly simple, for, since the Wolffian and segmental ducts may be considered the same, all that is requisite is to examine the front end of that canal. Next, it is said? that in the Amphibian the segmental duct arises as a groove of the parietal peritoneum, just ventral to the place where the body cavity is connected with the cavities of the muscle plates. This groove, which arises first of all anteriorly just behind the branchial region, is continued for a certain distance backward. It soon, however, becomes constricted into a canal lying between the ectoderm (epiblast) and parietal peritoneum. This constriction has been described as taking place in the following manner :—It first appears in the middle region of the groove, giving rise to a canal opening into the body cavity in front and behind. It then is continued backwards until the groove is completely converted into a canal behind, which soon acquires an opening into the cloaca. Anteriorly the wide opening meanwhile is divided up into two (urodela), three (anura), or four openings (cecilia), according to the species.
t On the Early Development of the Anterior Part of the Wolffian Duct and Body of the Chick,” &c., Quart. Jour. Micros. Sci., vol. xxi., 1881; also Gegenbaur, Elements of Comparative Anatomy, translated, &c., by Bell and Lankester, London, 1878, p. 603.
The canal immediately behind the last of these openings becomes coiled, and placed on the same level, but ventral to the openings. The part of the body cavity into which the openings of the segmental duct pass widens out, a vascular projection—the glomerulus—from the dorsal inner wall is formed, extending uninterruptedly from opposite the anterior opening of the segmental duct to as far back as the posterior. The dilated section of the body cavity in which the glomerulus lies, and into which the openings of the segmental duct opens, is partially separated from the rest of the body cavity. The whole structure, including openings of duct, ventral coiled part of duct, glomerulus, and dilated part of body cavity, is known as the pronephros.” ?
1 Gegenbaur (Elements of Comparative Anatomy, translated by J. Bell, 1878, pp. 601, &c.) calls the duct which belongs to the pronephros and mesonephros the ‘‘arichinephric duct,” and says that it originates from the mesoderm.
2 Upon this subject Balfour’s monograph ‘‘On the Nature of the Organ in Adult Teleosteans and Ganoids, which is usually regarded as the Head Kidney or Pronephros,” Studies from Morphological Laboratory in the University of Cambridge, 1882, p. 69, may be consulted ; also Messrs Balfour and Sedgwick’s joint monograph ‘‘On the Existence of a Head Kidney in the Embryo Chick,” &c., Quart. Jour. Micros, Sci., vol. xix., 1878.
Although the question only refers to Amphibians, the matter has been carried a step further by the discovery of similar organs in the chick and other birds.1 Not only are the above-mentioned openings represented in that animal, but glomerular structures protrude from them into the body cavity. However, in rabbits and human embryos similar protrusions cannot be found,? although passages leading from the body cavity into the Wolffian duct or tubules undoubtedly exist, and attention may now be given to them.
It may be remembered that the development of the Wolffian bodies has been traced step by step until the organs were found to consist of a duct, with rudimentary tubules towards their inner sides. These structures, together with their surrounding mesoblastic tissue, lay in a fairly prominent ridge (the uro-. genital ridge), situated at either side of the mesentery. Moreover, it was said that the urogenital ridge extended towards the front of the body cavity, dwindling gradually, and ended behind the lungs, and close to the sinus venosus. The great bloodvessel of the Wolffian body (the cardinal vein) courses along immediately behind the duct, for which it is a convenient landmark.
After this recapitulation the peculiarities of the front part of the Wolffian body and duct may be followed with greater ease, and first of all its termination may be discussed. It has been found that embyro rabbits at the beginning of the 18th day afford the most satisfactory information, To give an idea of the development of such specimens they may be compared to chicks at the end of the 3rd day of incubation, and the comparison is borne out by the fact that the limbs of the rabbit have budded, and its crystalline lens has appeared. However, it is necessary to say that a mere statement of the number of days which may have elapsed since impregnation seldom conveys a correct notion of the degree of development any particular embryo
‘may have attained ; so that 1t may be asserted that is not possible to foretell what embryos will be like any particular time
1 Janosik says he has seen the fore kidney in the chick, quail, pigeon, and duck, ibid., p. 125.
2 Janodik is of the same opinion as regards rabbits, loc. cit., P. 128.
8 Foster and Balfour, 2nd ed., chap. vi. fig. 65, p. 188, give a diagram of a
8rd day’s chick, which is comparable to the rabbit’s embryo at the beginning © of the 13th day.
after impregnation, nor can anyone be sure that the individual members of a litter will be alike ; indeed, in the earlier stages they vary exceedingly. If now we return to the rabbit’s embryo at the beginning of the 18th day, and investigate the most anterior portion of its Wolffian duct, it is found that a short length of it is detached and separate from the main canal.} This isolated front part consists of a solid column of cells (fig. 13) wedged in between the cardinal vein and the peritoneal epithelium, but before it ends posteriorly it has a lumen which is probably connected by two apertures—nephrostomata—with the body cavity (fig. 14). The histology of these communications is quite simple, for, where they are found, the outer side of the duct seems simply blended with the neighbouring epithelium, and through the midst of this connection there is apparently a narrow channel.? A very little way further ‘back than this curious organ, the Wolffian duct recommences, and, almost directly after its new beginning, it is united to the body cavity by a process of cells almost the same as those belonging to the detached portion of the duct (fig. 15). But, in addition, the urogenital ridge in this region has a peculiarity, for its most convex part is slightly grooved (fig. 16). Some thickened epithelium which lines the groove reaches towards the Wolffian duct. The succeeding sections show that this epithelial thickening is continuous with a somewhat tortuous Wolffian tubule, which opens in the usual way into the duct. From this point the Wolffian duct and tubules, traced backwards, conform to the description which has been given in previous pages, _ Assuming the correctness of the foregoing account, it is clear that at the beginning of the 13th day the front part of the rabbit’s Wolffian body resembles in some respects the organ which, in the chick, has been called the pronephros. It is true that there is a difference, for the presence of glomeruli has not been determined. An observer! has stated that in twelve days’ rabbits he has seen “from place to place the pleuroperitoneal’* epithelium adjacent to this canal (Wolffian) present traces of a cellular proliferation, in which can be seen the last vestiges of external glomeruli.” Now, in the chick, the external glomeruli are very distinct structures, and not only have other observers 2 failed to discover their analogues in rabbits, but, judging from the specimens I have examined, the vestiges mentioned in the quotation are exceedingly dubious. Nevertheless, the absence of an external glomerulus is not an insuperable objection to the hypothesis that rudiments of a head kidney or pronephros exist in the rabbit, because, as Sedgwick seems to point out, they are not an essential feature of the organ. Before taking leave of this interesting question, the nephrostomata demand further notice. It is possible that those apertures represent Wolffian tubules in a less complete state of development than elsewhere, but if this is the case they must have become considerably modified For instance, they are joined to the outer side of the duct, and not to its inner and ventral side like the true tubules; secondly, a section sometimes shows a nephrostoma opening by itself into the duct, whilst there may be, in the same section, a tubule to the inner side (fig. 15); finally, the Wolffian tubules blend with the epithelium of the body cavity at the most convex part of the urogenital ridge, and not at its outer side, like the nephrostomata. Concerning the groove that has been mentioned (fig. 16), it would be improper to say more than that it has a certain resemblance to the grooves seen at the fore end of the chick’s excretory apparatus,and which are considered to be an important part of its pronephros.
1 Sedgwick (On the Early Development of the Anterior Part of the Wolfian Duct and Body in the Chick, p. 16) mentions similar appearances in chicks.
2 Upon this point JanoSik and Rensen may be consulted. They describe very similar appearances. JanoSik says that he has seen four nephrostomata, The specimen I have described had but three. |
3 T have never seen the Wolffian tubules of the rabbit open into the body cavity with the exception of the tubules which have been described, and which seem to possess nephrostomata.
The foregoing observations cannot be considered complete until it has been ascertained whether they are applicable to the humanembryo. It may have been inferred, from what has been said, and from the figures which have been given, that the human embryo has a Wolffian body which closely resembles that
1 Rensen, loc. cit., p. 607.
? Janodik, loc, cit.
3 Sedgwick, Karly Development of the Anterior Part of the Wolffian Duct and Body in the Chick, &c., p. 25 et seg.
‘If we may judge from Mr Sedgwick’s (loc. cit., figs. 13 and 14) figures the nephrostomata of the chick and rabbit are not dissimilar.
of the rabbit. A glance shows that in either case its configuration is the same, and no difference can be detected in its contents. Human embryos which, according to His’s data,! are somewhere about the 30th day of intrauterine life, betray the likeness in a striking degree. Specimens at this age are nearly comparable to thirteen days’ rabbits, or to chicks at the end of the 3rd day. Their newly budded limbs have none of the bends they ultimately attain, nor can digits be discerned; moreover, the crystalline lens is incompletely formed and still united to its parent epiblast. A transverse section through such an embryo as this has already been figured and described (p. 638, fig. 2), so nothing further need be said before dealing with the details of the various structures. The way in which the Wolffian duct ends anteriorly, and the relation of the tubules to the peritoneum may be ascertained, but the earlier stages of their development can only be inferred. For the sake of clearness, the description of the posterior termination of the Wolffian duct may be deferred, and for. the present only its course and anterior termination considered. After all that has been said, it suffices. to say that it runs forward in the urogenital ridge lying beneath its epithelium. Of course, in these respects it resembles the Wolffian duct of the rabbit and chick, and it in no way differs from them, unless, perhaps, in being situated a little nearer the front of the ridge (Plate XV. fig. 2).
So far, little has been said about the germinal epithelium which covers the urogenital ridge, because rabbits’ embryos have chiefly been described, and in that animal its characters are not very distinctive. However, in the chick the germinal epithelium is represented by a semiliine of columnar and large ovoid cells, which surround the most convex part of the ridge, and is continuous by its tapering ends with the rest of the lining of the body cavity. The human embryo has germinal epithelium arranged like that of the chick, and its appearance is shown in the accompanying figure, which has been drawn from the front part of the Wolffian body of an embryo at about the 30th day of intrauterine life (fig. 17).?
1 Loc, cit., part iii. p. 238 ; Atlas, part iii. plate x. fig. 9 to 12. 2 The germinal epithelium of the human embryo and of the rabbit will be described in Lecture IT. THE DEVELOPMENT AND TRANSITION OF THE TESTIS. 661
In human embryos the front ending of the Wolffian duct is the same as in the rabbit or chick, and is found in the same position, behind the lungs, and close to the venous end of the heart. In this region the urogenital ridge and germinal epithelium have almost dwindled away, but the cardinal vein shows where the duct may be found, or rather where the column of cells which represent it may be found, because it is very questionable whether the terminal part has any lumen. However, the ending is independent of the peritoneal epithelium, and lies between the latter and the cardinal vein (fig. 18). The next question is, whether the epithelium of the human Wolffian tubules is connected with the peritoneal epithelium and a section from the same embryo, but taken a little further back allows an affirmative answer (fig. 19), At the level of the section the urogenital ridge and germinal epithelium are both very distinct, and the Wolffian duct may be seen lying beneath the latter. But the most striking and interesting feature is a Wolffian tubule which is united by one end to the germinal epithelium, whilst the other dilates and encloses a large glomerulus. Between these points the tubule is narrow, but has a distinct lumen, which, in this specimen, can hardly be said to open into the body cavity. Nevertheless, the histological arrangement suggests very strongly that there may have been an opening, and likewise betokens a distinct resemblance to the anterior Wolffian tubules of the chick and rabbit.
This part of our subject has enough importance to warrant the production of more evidence concerning it. With this object the fore part of the Wolffian body of another human embryo has been delineated (fig. 20). The specimen from which this was taken was very like the last, which had, it may be remembered, budding limbs and a commencing crystalline lens, and was compared to a thirty days’ embryo figured by Hist In this case there is a very distinct inturning of the peritoneal epithelium, and the nephrostoma which it forms has the appearance of opening into the Wolffian duct.2 A third human embryo, a little more advanced than the last two, afforded valuable confirmatory evidence of the junction of the peritoneal epithelium with that of the tubules, and perhaps with the duct of the Wolffian body. So far, nothing but the knowledge gained from transverse sections has been brought forward, but the longitudinal sections of another human embryo are so interesting, that a brief allusion to them may not seem out of place. After so much repetition, it is needless to describe in detail the excretory organs of this embryo. Its head had been broken off before it came into my possession, but as its limbs had just budded I judged it had attained about the 30th day of intrauterine life, and of course the histological features assisted me in arriving at this conclusion. The interest of the specimen centred in the forepart of the Wolffian body, where three vascular bulbous projections jutted into the body cavity. The free end of each of these (fig. 21) is covered with a layer of cylindrical cells, longer than those which cover the adjacent surfaces. The characters of these projections are very definite, but their size varies. - The hindermost is the largest and most vascular, and then comes its next door neighbour; but the foremost prominence, that nearest the head, is quite short, and by no means so obvious as the others. Between the largest of these protrusions, or, in other words, between the middle and posterior, there is a deep recess which, in some sections, seems to open into a neighbouring Wolffian tubule (fig. 22). Unfortunately it is impossible to make an unqualified statement upon this point, but at all events the epithelial lining of the recess is continuous with that of the tubule. Human embryos seldom afford very perfect material, so that, as in’ this instance, although the outstanding processes are quite clear and distinct, yet the communication between the tubule and body cavity is not so certain. However, the appearances seen in these longitudinal sections acquire fresh significance when they are considered by aid of the information obtained from those which are transverse. Without doubt, after a while the human Wolffian tubules will be demonstrated as clearly in continuity with the peritoneal epithelium in longitudinal as they have been in transverse sections.
1 His, loc. cit., p. 239; Atlas, pt. 3, pl. x. fig. 9.
2 Since the genital eminence of this human embryo has probably appeared, this epithelial ingrowth might possibly be the opening for the oviduct (Muller’s duct), but this supposition will prove unlikely when the development of that canal is discussed in Lecture II.
The actual nature of these villous-looking protrusions is hard to determine. [It is impossible to ignore their likeness to the external glomeruli of the pronephros of the chick, and it is by no means impossible that a similar organ is in question. On a& priori grounds such an assumption is tenable, and I am inclined to think that the human embryo may have a rudimentary pronephros. Speaking relatively, the human embryo, that in which the protrusions into the body cavity were seen, was not more developed than a chick of the third day, and, as far as could be ascertained from longitudinal sections, it did not possess a genital eminence. In the next lecture reasons will be-adduced to show that it is unlikely that the projections have anything to do with the oviduct.
Explanation of Plate XV
Ao. Aorta. 8. Somite. Co. Cardinal vein. I.c.m. Intermediate cell mass. So. Somatopleure. W.d. Wolffian duct. Sp. Splanchnopleure. W.t. Wolffian tubule. Ep. Epiblast. N.s. Nephrostomata. Hy. Hypoblast. Gl, Glomerulus. B.w. Body wall. Ne. Notochord.
M.s. §Medulla spinalis. L. Liver.
M. Mesentery. | P. Projection.
Im, _ Intermediate cell mass.| J. Recess.
f. Epiblastic thickenings. | Sp.c. Spinal canal.
Figs. 1 to 4. Embryo chick, 48 hours. — Fig. 1. Connection of Wolffian duct with somite. Fig. 2. Connection of Wolffian duct with intermediate cell mass. Figs. 3 and 4. Epiblastic thickenings. x about 120.
Fig. 5. 9 days’ rabbit’s embryo. Epiblastic thickenings and Wolffian duct and tubules coexist. x 80. |
Fig 6. Rabbit’s embryo, 8th day. Intermediate cell mass. x about 50.
Fig. 7. Rabbit’s embryo, 8th day. Communication between body cavity and interior of somite. —
Figs. 8, 9, and 10. Rabbit’s embryo, 9th day. x 180. — Fig. 8. Intermediate cell mass and its connection with lining of the body cavity. Fig. 9. Origin of Wolffian duct and tubule. Connection of tubule with the peritoneum. LEpiblastic thickening over somite. Fig. 10. Hinder tubule ununited to peritoneum.
Figs. 11 and 12. Urogenital ridge of a rabbit’s embryo of 10th day. x 250. — Fig. 11. Longitudinal section of Wolffian duct and tubules. Fig. 12. Transverse sections. To show the formation of lumen in tubules.
Figs. 13, 14, 15, and 16. x 120. Urogenital ridge of rabbit’s embryo at commencement of 13th day. Termination of Wolffian duct and structures supposed to represent a pronephros.
Figs. 17, 18, and 19. Urogenital ridge of human embryos. x about 100. — Fig. 17. Germinal epithelium. Fig. 18. End of Wolffian duct—Solid column of cells, Fig. 19. Wolffian tubule united to peritoneal epithelium.
Fig. 20. Urogenital ridge of human embryo about 30th day. Connection of epithelium of the body cavity and that of the Wolffian duct.
Figs. 21 and 22. Human embryo of about 30 days. Fore-part of Wolffian body. Projections into body cavity, and joining of peritoneal epithelium with those of the Wolffian tubules. Vol.XXT) py, vy
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