1897 Human Embryology 11

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Minot CS. Human Embryology. (1897) London: The Macmillan Company.

Human Embryology: Introduction | The Uterus | General Outline of Human Development | The Genital Products | History of the Genoblasts and the Theory of Sex | The Germ-Layers | Segmentation | Primitive Streak | Mesoderm and the Coelom | Germ-Layers General Remarks | The Embryo | The Medullary Groove, Notochord and Neurenteric Canals | Coelom Divisions; Mesenchyma Origin | Blood, Blood-Vessels and Heart Origin | Urogenital System Origin | The Archenteron and the Gill Clefts | Germinal Area, the Embryo and its Appendages | The Foetal Appendages | Chorion | Amnion and Proamnion | The Yolk Sack, Allantois and Umbilical Cord | Placenta | The Foetus | Growth and External Development Embryo and Foetus | Mesenchymal Tissues | Skeleton and Limbs | Muscular System | Splanchnocoele and Diaphragm | Urogenital System | Transformations of the Heart and Blood-Vessels | The Epidermal System | Mouth Cavity and Face | The Nervous System | Sense Organs | Entodermal Canal | Figures | References | Embryology History

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Chapter XI. Origin of the Urogenital System

Tub outlines of vertebrate morphology were given, in the main, correctly by the older anatomists, except as regards the urogenital system. In 1875 Carl Semper announced the discovery that the excretory tubules of elasmobrancha have a funnel-shaped opening in the abdominal cavity — a fact discovered by Balfour, 78.3, at about the same time. Both authors recognized that this discovery was profoundly significant, but it is chiefly to Semper that we owe the reform of conceptions in this field. It is unnecessary to attempt a historical review; the reader will find in Max Furbringer's admirable monograph, 78. 1, a thorough, critical, and trustworthy revision of all that had been done up to uiat time. For notices of the subsequent literature see Riickert, 88.1, Van Wiihe, 89.1, and H. H. 'ield, 91.1. R. Semen's valuable memoir, 91.1, became accessible to me too late to enable me to remodel this chapter as his results render necessary.

Fundamental Parts of the Urogenital System

For a general explanatory devnption we may consider the fundamental parts to be four on each side of the vertebrate embryo, compare Fig. 131. The four parts are two longitudinal ducts: the pronephric or WolflBan duct, W. D, and the Mullerian duct or oviduct, M. D; and two ridges on the dorsal side of the body-cavity, Coe, into which they protrude; each ridge is covered by mesothelium resting on mesenchyma. The smaller Ei ridge. Gen, is called the genital, since it is transformed into the ,^ genital glands; it lies nearest the median line ; its cephalic end is probably identical with the so-called glomus of the pronephros. The larger ridge, Ex, is called the Wolffian or nephridial ridge; it contains the transverse excretory tubules (segmental tunai"ridit^i iitik, bules, nephridia) which are deim.Bomaiopieure: vejoped from the nephrotomes, jicn, arcnenteron. ^^^ expansion of which probably causes the bulging of the mesothelium, which results in the formation of the WolflBan ridge. The nephridia open into the pronephric duct. The cephalic end of the nephridial or Wolffian ridge give rise to the pronepliros, while the remainder of the ridge is for me chief part at least converted into the Wolffian body (primitive kidney, inesonephros, Uruiere).

Fig. 131.— Dlaerunuiatic CYom-.Stvi ion at a Vertebrate to show- the Fuodaraental Relations of the Urofceiiital SyKtem. Ud. H«lullary tube; <VrA. Dotocbord ; j4o, aorta; Cm. Keoltal rldae; W. D. Wolfllaii duct; M. P. Jfiller'duct ; Cr, excretory or WolfflaD tUik : l^illi

Head-kidney or Pronephros

The head-kidney beinj; the fint part of the urogenital system to be differentiated in the vertebrate embr^'O, must be regarded as the phylogenetically oldest part. It is found in the embryos of (probably) all vertebrates, but disappears before adult-life in selachians, some teleosta, and all amniota.

The head-kidney is always situated in the segments immediately behind the heart, ajid is a paired organ with a longitudinal duct, which finally opens into the cloaca or hind end of the alimentary tract; the duct has great morphological importance; its development is described in the next section. Tho head-kidney consists of from one to five or more transverse tubules which are differentiated from the nephrotomea and have on the one hand an opening into the ventral coelom or abdominal cavity, and on the other into the longitudinal duct. Each tubule consists of epithelium and when well developed takes a convulated course. The number of these transverse tubules is said to be greatest in Myxine; in Petromyzon there are four or five, in Torpedo sis, Priatiunis four, Ccecilia four, Anura three, Unxlela two; but in teleosts and cartilaginous ganoids one only. The head-kidney often protrudes somewhat into the body cavity, and the part of the body-cavity into which it protrudes may* liecome, as in teleosts and the lamprey, shut off from the remaining welom. There is also developed a so-called glomus, which is a fold uf the mesothelium arising near the base of the mesentery, and containing numerous blood-vessels. The structure of the organ is well illustrated by Fig. ??.

The development of the head-kidney varies considerably ia the ilifferent classes of verfobrates, so that we are still uncertain as to what art! the essential and typical features of its development. The confusion is probably due to the fact that it is only recently that we have gained the knowledge that between the myotome and the lateral plate comes, in every segment, the nephrotome, to which the origin of the transverse excretory tubule, both of the head-kidney and of the WolflSan body (mesonephros) has been traced in a number of cases. Since we have known that the essential question is, Do the pronephric tubules arise from the nephrotomes? sufficient investigations have not been undertaken. But it has been shown in several cases that the nephrotomes do produce the tubules. The typical mode of development both for the pronephros and mesonephros is, I thinks probably as follows: The nephrotomes typically contain a coelomatic cavity ; when they separate from the myotome the myotomic end of the nephrotomic cavity becomes closed, but the other end remains open and becomes the permanent nephrotome opening of the nephric tubules (Segmentalorgane of Semper) ; the nephrotome now lengthens out and unites secondarily with the pronephric or segmental duct. Until furi;her research of a far more thorough character than anything we yet have shall decide the question, this hypothetical account is the best that can be presented.

In the cylostomes, teleosts, and amphibia the pronephros is said to arise from the mesothelium of the ventral coelom ; but as this takes place so that the mesothelium is close to the myotome, it is more than possible that we have to do really with nephrotomic tissue. .' If Goette's account of the process in Petromyzon, 90.1, 54, 55, ho correct, then it may be that in the lamprey the nephrotomic anlago separates from the myotomes, and while still connected with the lateral plates undergoes segmental division. In the lamprej' (Goette, /. c), teleosts (Mcintosh and Prince, 90.1, 783-785), and amphibians (Fiirbringer, 78.1), the mesothelium, which produces the tubules, produces the longitudinal duct also, but in view of what is stated of other vertebrates this has been questioned. Our knowledge of the hejid-kiduey in am])hibia has been very much extended by the recent researches of Mollier, 90.1, Marshall and Bles, 90.1, R. Semon, 91.1, and H. H. Field, 91.1. My lack of personal familiarity with the amphibian pronephros makes me unwilling to attempt a critical summary of their researches.

The pronephros of elasmobranchs tegins to develop in (Pristiunis) embr\'()s with twenty-seven segments ; the three foremost segments are 8ul)se(iuently included in the head, so the fourth is the first segment of the rump (Van Wijhe, 89.1, 473). In the first four (Pristurius) or six (Torpedo) of the rump segments the somatic mesothelium (wrongly termed somatopleuro by Van Wijhe) and nephrotome becomes thickened; these thickenings come into contact with one another, and, according to Riickert, 88.1, with the ectodenn; they subsequently acquire a lumen ; thus each nephrotome has an extension of its cavit>', and l)ecomes a canal with an oj)ening into the niain coelom, and extending in a curved line outward and backward toward the ectoderm. These four to six canals unite with the longitudinal duct which arises from the ectoderm. Our knowledge is based chiefly on Riickert's very detailed investigations, In amniota, and has been studied also by Renson, 83.1, Mihalkovics, 86. 1, Janosik, 86. 1, Wiedersheim, 90.3, and others. A few scattered observations are inserted by Hoffmann in his "Reptilien," p. 2047-2063 of Bronn's " Thierreich." Braun, 77.4, has shown that in reptiles the nephrotomes become much enlarged and appear as rounded vesicles of epithelium ; the anterior three or four of these vesicles retain the oi>en communication of their cavities with the ventral coelom, and these vesicles correspond to the pronephric anlage. Most of what little we know of their history is due to Mihalkovics, 86. 1, 41-47, 5o-CG. Each of the pronephric segmental vesicles acquires a communication by moans of a fine fissure with the longitudinal duct, which has meanwhile fonned between the nephrotomes and the ectoderm. The opening of the vesicle into the ventral coelom (pleuro-peritoneal cavity) is now closed, and the nephrotome is completeh' separated from the lateral ])lates or mesothelium of the body-cavity proper. The next change (Lacerta embryos of 3.5-4.0 mm.) is eflPected b}' the lateral wall of the vesicle sinking in, tlius converting the round vesicle into a tul>o bent into an S-shape and having its general course at right angles to the bodyaxis. It is uncert^iin whether Mihalkovics has described true pronephric tubules or merely anterior tubules of the Wolffian lx)dy. If the structures are j)ronephric there ought to be some trace of a ^leritoneal glomerulus near the opening of the tubule, which there is not. On the other hand, if they are Wolffian tubulins th(»re should be a glomerulus formed from the tubule itself, and this sei>ms to be the case, (see Mihalkovics, /. c, Taf. I., Fig. *J). R. Wiedersheim, 90.3, states that the head-kidney is very well develoj>ed in crocinlile embryos (10-12 mm.) and occupies the anterior end of the urogenital ridge; it merges into the Wolffian iKxly; the right pronephros has sixteen, the left thirteen, funnel-shajXH.! nejihrotomes; the glomus is the anterior continuation of the urogenital ridge, and is imperfectly segmentally subdivided ; the tailward end of the pronephros is shut off from the main coelom by a prolongation of the embryonic diaphragm (septum transversum) . In birds (Mihalkovics, 85.1, 58) the nephrotomes of the fourth to seventh s(»gments form the pronephric tubules, which o^xm into the !x)dy-cavity, and taking a transverse S-1 ike course empty into the lateral duct; on the mesenterial side of the j)eritoneal oi)ening of the tubule a glomerulus is formed; the relations can be seen in chicks of three to four days, or l)etter in ducks of the same incul)ation ; the pronephric tubules disappear the fourth day, but the glomeruli enlarge somewhat and do not disappear until the seventh day. Balfour and Sedgwick, 78.1, have advanced a different view; they state that the Miillerian duct, the development of which is described, p. 244, has three anterior openings at first, and these they homologize with the head-kidney ; but Sedgwick, 81.1, has withdrawn his opinion. Renson, 83.1, KoUmanu, 82.2, and Mihalkovics, 86.1, moreover, deny the existence of the three openings. In mam mala the head-kidney has been studied by Renson, 83. 1, 600, who states that in rabbits of ten days the S-shaped tubules with openings into the coelom are present and empty into the primitive longitudinal duct; they disappear very soon; nothing corresponding to the pronephric glomus has been recorded.

  • the head-kidney was first described by A. Seilgwick,
  • For a fuller review and discussion, see H. H. Field, 01.1, 272-281.

Our knowledge of the pronephros is unsatisfactory. The fullest review of the literature known to me is given by H. H. Field, 91.1.

The Pronephric Duct

The primitive longitudinal duct of the urogenital system is known by various names ; the most important are pronephric or segmental duct, (Vomierengang)^ and Wolffian duct, but it is doubtful how far the Wolffian duct of the amniota can be homologized with the pronephric duct of anamnia, compare below. Both ducts bear the same relation to the pronephros and Wolffian bodies, but diifer in their connection with the Miillerian duct, which in selachians seems to arise from the pronephric duct and in amniota arises independently of the Wolffian duct. For the purposes of this paragraph it is assumed that the pronephric and Wolffian ducts are identical, and the term pronephric is used for both.

The pronephric duct is the first longitudinal duct of the urogenital sj'stem to appear. AVlien first differentiated it always lies between the nephrotomes and the ectoilerm, Fig. 92, W, and always close against the mesodemiic tissue. It can be first seen (KoUiker, " Grundriss," 411) in chicks during the second half of the first day, in rabbits about the end of the eighth day in the region of the fourth and fifth segments ; it lengthens out very rapidly, so that in the chick the end of the second day it extends nearly or quite to the last segment formed. The intimate association of the duct with the mesoderm led to the general belief that it arose from cells of the intermediate mass (nephrotomes) or from the lateral plates (splanchnocoelic mesothelium) . This opinion was shared until recently by many good observers — see the citations of authorities by Fiirbringer, 78. 1, and Mihalkovics, 85. 1, 47-52. The first to call it in question seriously was Hensen (Virchow's Arch,^ XXXVII., 81, foot-note), who in 186C, definitely asserted the origin of the Wolffian duct, as W. His, 65.2, had previously suggested, from the ectoderm, which overlies the duct when it a])pears. The matter then was forgotten until attention was recalled to it by the very exact demonstration by Count Spee, 84. 1, that the duct is developed in the guinea-pig in connection with the ectoderm. Spec's discovery has since been confirmed by Flemming, 86.1, for the rabbit, by Bonnet, 87.1, for the sheep. But Fleischmann and Martin, 88.1, were unable to confirm it. No satisfactory evidence of the ectodermal origin in birds has come yet, although G. Brook, 87.1, has affirmed it; but for reptiles we have good evidence through Perenyi, 87. 1, Ostromoff , 88.1, and Mitsukuri, 88.1, while Strahl, 86.1, failed to find any. The best evidence of all is that furnished for elasmobranchs by J. Beard, 87.1, Van Wijhe, 86.1, 89.1, and J. Riickert, 88.1; the investigations of the last two authors appear quite conclusive. On the other hand, it must be mentioned that H. V. Wilson, 91.1, 247, expressly denies the accuracy of Brook's statements, and that H. H. Field, 91.1, reasserts that in amphibians the duct has no connection with the ectoderm.

The authors who defend the ectodermal origin of the duct essentially agree with one another; they find the tailward end of the duct has a solid cord of cells which ends by fusing with the ectinlerm, and where the fusion tiikes place the cells of the outer layer are often in karyokinesis, as if the cells were proliferating to be added to the duct. In the amniota the connection exists for a very short distance only, and may be easily overlooked; but the length of the fusion is variable, especially so in Torpedo, for Riickert states he found it there extending anywhere from eleven to twenty-five segments, and even differing in extent on the two sides of the same embryo. In a series of transverse sections — for a good figure see J. Riickert, 88. 1. Fig. 35 — we see running from back headward— ./fr^/, behind the end of the duct, the thin ectoderm consisting of a single laver of cells; necond^ the thickening of the ectodeim (see Flemming, His' ArchiVy 1886, Taf. XI., Fig. 7); thirds the inner layer of cells separated from the outer; this separation may take place in various ways, for the cells to form the duct may make a flat plate or a round cord, or there may even be a groove in the ectoderm, and when the groove closes it is separated as a canal; fourth^ a cord of cells lying within the ectoderm ; the cord is round in section, soon develops a central lumen, and its cells become distinctly epithelial.

In view of the remarakble unanimity of the descriptions by various ol)servers, I think it probable that it will be found ultimately that the pronephric duct is developed from the ectodenn in aU vertebrates.

The backward growth of the duct is accomplisheil by the addition of cells from the ectoderm to its caudal end, and, when it reaches its hindmost extension (Pristiunis embryos of about eighty segments, with five open gill-clefts) it passes beyond the limit of the mesoderm; and hence, according to Van Wijhe's observation on elasmobranchs, 89. 1, 48G, comes into direct contiict with the entoderm or wall of the ckwica (hind end of the archenteron) ; it then fuses with the entoderm and separates from the ectoderm, after which it develops a lumen ; thus the duct comes to o]>en into the cloaca. The cloacal opening is invariably present in all vertebrates during a certain embryonic jK*riod at least; it is the ixjnnanent condition in anamnia.

The amniote Wolffian duct is round in cross-section when first formed, but soon becomes elliptical. By the changes in position, effected by the further development of the embryo, the duct is brought to run directly below the cardinal vein, and as it enlarges still farther its dorsal epithelium becomes flattened against the vein, and the cross-section of the duct becomes a triangle with the apex down; still later mesenchyma and Wolffian tubules grow Iwtween the duct and the vein, which finally becomes widely separated as in Fig. 137. The triangular section of the duct is retained for some time, but the elliptical section is gradually resumed.

The Wolffian Body

The Wolffian body (corps de Wolff, Urniere, mesonephros, primitive kidney) is the chief occupant of the embryonic Wolffian ridge; in anamnia it is the chief renal organ throughout life; in amniota it disappears during embryonic life, being rejJaced by the true kidney, except that a small part of the cephalic on<l is retained, as described in Chapter XXIII., and enters into sj^KK'ial relations with the sexual organs.

In its primitive fonn the Wolffian body appears to have consisted of a series of transverse tubules (Wolffian or segmental tubules) emptying into the Wolffian or pronephric duct. As was shown by Semjx?r's investigations on Plagiostomes, 76.2, there is primitively one tubule for each segment of the body, and each tubule begins with a funnel-shaped opening into the peritoneal cavity and takes a convoluted transverse course to the laterally situated Wolffian duct. Sedgwick observed that the tubules do not arise as evaginations of the mesothelium of the main ctBlom or splanchnoccele, which was the view held by Semper and Balfour, but from the intermediate cell mass, and that the cavity of the tubule ia the original coelomatic cavity of the nephrotome by which the ccmom of the myotome communicates with the splanchnoccele. Subsequently the nephrotome is severed from the myotome, and by elongation becomes a Wolffian tubule; the connection with the splanchnocoele is retained to form the funnel or nephrostome. The d is;"* position of the parts can be understood from the a caudal view of a Wolffian mm. The tubules begin at

accompanying figure. Fig. i:t3 tubule of an Acanthias embryo of If, immediately below the cardinal vein, VC, and runs obliquely

1, upward and outward to the glomerulus, 2, and then makes several convolutions, until it terminates in the longitudinal Wolffian duct, W. (/. At 2 the tubule is distended to make nx>m, for the glomerulus, which projects into it.

The development of the Wolffian body commences in Salamaiidra (Fiirbringer, 78. 1), with the formation of a series o£ solid cords derived from the nephrotomes ; in the cephalic end of the body, the cords remain connected with the splanchnoccelic mesothelium, but in the remaining segments the cords have no connection with the peritoneal epithelium. A. Sedgwick states that tlio cords are all without union with the peritoneum in the frog. The connection with the peritoneum, when present, is soon lost. The cords develop a cavity, which acquires a vesicular form ; the vesicle becomes flattened and S-shapetl ; the medial end develops into a Malpighian corpuscle as in the amniota (see below) , while the lateral end remains narrower, joins the Wolffian duct, and afterward lengthens out to form the coiled tubule ; at the junction of the tubule proper with the corpuscle a canal grows out in a ventral direction which meets the peritoneal epithelium and then develops a funnel-shaped opening (nephrostome) into the body-cavity; the epithelium of the funnel becomes ciliated. The anterior tubules are rudimentary-, the first fully developed Wolffian tubules being in the sixth segment b^ind the pronephros.

The tubules are more numerous than the segments — hence the nephrotomes must divide in some way, but just how is unknown. The tubules subseciuently become branched, tmd each branch develops a Malpighian corpuscle and a nephrostome; in the adult the Wolffian body or so-called kidney is studded over with numerous funnels as discovered by Spengel, 73.3. How the secondary branches develop is still to be ascertained. In amphibians, then, wo have two essentisd differences from the elasmobranchs — namely, the nephrostomes are not the retained oj)enings of the nephrotomes, but new formations, and the numl)er of tubules is greater than one for eturh segment ; this increase in numl)er implies a ver}' ])recocious sulxlivision or budding of the nephrotomes, and is a sec*ondary feature; for there is, of course, only one nephrotome on each side in every segment.

In all amuiota the nephrotomes all IxH'ome completely separated from l)oth the myotomes and jwritoneum througliout the region of the Wolffian Inxly, except that |H)ssibly in a few anterior segments the connection with the peritonemn is retained, as is suggested by Sedgwick's observations (Foster and Balfour's "'Embryology," 191) and Kolliker's (" Grundriss," p. 4i:]). Referring to the chick of the third day Balfour thus descrilK\s Sedgwick's results: "In front of alxmt the sixteenth segm(»nt s})ecial 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 j)art*j of the interme<liate (vU-mass attached to the peritoneal epitheliimi l)ec()mes converted into S-sha|)ed 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 Ixuly-cavity is for a short distance prolonged, so that this part constitatc»s a rudimentaiy f)oritoneal funnel leading from the body-cavity into the lumen of the Wolffian tubule.

  • (Foster and Balfour's " Elements," se(*ond edition, 101).

The following account of the development of the Wolffian Ixxly in amniota is based on ilihalkovics, 85.1. The tissue of the nephrotome is at first quite loose and not obviously epithelial; it becomes severed in each segment from l)oth myotome and peritoneum; the cells assume a radial arrangement and a cavity appears in the centre; the cavity enlnrges and forms a vesicle with epithelial walls; these* vc^sicles wt»ro called " Urnierenblaschen " by Remak, " Segmentalblaschen" by M. Braun, 77.4, 1.'33. In the lizards (Braun, /. c.) the number of vesicles corresi>onds with the number of segments, but in birds (Mihalkovics) the vesicles are more numerous than the segments ; this may lx> due to the nephrotomes separating from the myotomes and then expanding less than the muscular plates; Van Wijhe has shown, 89.1, that the numlx*r of Wolffian tubules is apparently increased in shark embryos by this process. The lateral wall of the vesicle very s<x)n comes into contact with the Wolffian duct, and the e))ithelia of the two structures fuse and shortly their cavities open through. Th(» dorsal wall of the vesicle now sinks in, and the connecting piece toward the duct lengthens out ; the tubule thus acquires an S-shapeil course. Fig. i:U; it runs inward from the duct, then do\\^lward and outward, and finally downwanl and inward again, ending in the ladl(»-shaped blind en<l, wliich is the anlage of the MalpiKhian corpuscle ; the dorsal epithelium of the anlage ia considerably thickened, or rather has retained its original thickness, while the ventral epithelial layer thins out rapidly. It is important to note that in ainniota the part of the tubule running from the glomerulus to the nephrostome ia never developed as in anamnia, all connection with the peritoneum being lost very early indeed. The further development proceeds by the differentiation of the Malpighian corpuscle and the lengthening and coiling of the tubule proper. The differentiation of the corpuscle takes place by the thinning out of the epithelium of the ladle-shaped blind end of the tubule; the two layers of epithelium. Fig. 135, lie close together, thus reducing the cavity of the corpuscle to a narrow fissure; both layers are convex toward the ventral side; the concavity of the upper layer is filled with mesenchj-mal tissue, the Fro i»-n ifflftu Tufuie f H ^ihi^p cells of which are at first looselv f?^1nV«™7n'^iis "("iJmlS'iubre together, but soon l>ecome densely ilft^o^MihSikovia' ■dSK™'"' crowded; into the dense mesenchyma \essels from the neighboring aorta penetrate and form a capillary plexus ; the rounded vascular mass covered by epithelium constitutes the glomerulus proper, while the lower layer of epithelium forms the capsule of Bowman, Fig. 138,

Fig. IM.— Section tbrough a WoUDanTi denn; r, TenacBTdliiKllB: ^o, urU: IV glomerulus: mtth. uiraothellum.

The tubule lengthens rapidly and is characterized by a cylinder epithelium somewhat higher than that of the Wolffian duct, W. d; it retains for some time its simple S-shape, although the curves of the S become more and more exaggerated ; the Wolffian duct, W, d, at this time appears triangular in cross-section ; the base of the triangle is dorsal, being appressed against the overlying cardinal vein. The growth of the tubules and of the mesenchyma around them causes a rapid and increasing protrusion into the body-cavity, resulting in the formation of the Wolffian ridge, see Fig. 13G, and Fig. 137.

The transverse course of the tubules, their dilated medial ends and narrower lateral ends opening into the Wolffijm duct, can be readily seen in a fresh embryo dissected so as to expose the dorsal wall of the abdomen. The appearances thus obtained were familiar to the older embryologists and have been repeatedly figured.

Mihalkovics thus classifies the various views as to the origin of the Wolffian tubides: A, They are evagination of the Wolffian duck, Remak, 60.1, p. xxvii; Waldeyer, 70.1, ITJ; By They are evaginations either as canals or solid cords of the splanchnocoelic mesothelium (lateral plates or peritoneum) , Semper, 76.2; Spengel, 76.3; Braun, 77.4 ; A. KoUiker, " Entwicklungsg^. ;" Fiirbringer, 78.1, and others; (7, They arise ifrom the interme<iiate cell-mass or nephrotome ; this view was approached by many of the older writers, especially in Germany, who designated as Urnierenblastem the tissue, which we know now as the nephrotome. The correct view was first brought forward in 1880 by Adam Sedgwick, 80.1, 2, who clearlv recognized the significance of the intermediate cell-mass.

Multiplication of the Wolffian Tubules

After the first set of tubules is developed, secondary additional tubules appear. The origin of these is not certainlv known. Mihalkovics, 86. 1, 82, follows Bomhaupt and Balfour (Comp. Embryologj'") in tracing their formation to a now differentiation of the mesoderm of the Wolffian ridge; but the details of the process are not given by him, so that this view is nierely on opinion. Fiirbringer, 78.1, thinks the secundary tubules are developed, as he supposes the first to be, as evaginations of the peritoneum, but the evidence is drawn from the Amphibia, and seems to me questionable even for them. Braun, 77,4, 144, follows Spengel, 76.3, in assuming that the Malpighian corpuscles divide and that the division extends alone the tubule, thus accounting for the collecting tubules {Sunnnelrdfirchen) witli branches each ending in a Malpighian corpuscle. Still another metlicKl is suggested by Balfour's observation of buds growing out from the segmental vesicles or Malpighian corpuscles, and this explanation has l)een formally adopted, C. K. Hoffmann, for reptiles (Reptilien of Bronn's "Tliierreich," p. 2057) ; Hoffmann asserts that a Inid grows out from a primary corpuscle and forms a blind tube, which lengthens and twists until its blind end joins the Wi)lffian duct; Malpighian corpuscles with two ducts are, ho says, by no moans uncommon ; later the corpuscles divide and each tubulo then has its own corpuscle. I question the accuracy of this account.

However effected it seems certain that there is an increase in all amni'jta of the numlx?r of tubules oi>t»ning into the Wolffifin duct and also that some at least of the tubules become branching; it is probable that every tubule ends with a glomerulus. In the chick new corpuscles and tubules appear during the third day in the tenth to twentieth segments and usually on the dorsal side of the primary tubules (Mihalkovics, 85.1, 83) ; they have at first the form of segmental vesicles {Umierenbla.ichen), and become converted into Wolffian tubules by the same series of changes as the primary vesicles; the more advanced stages are always found headward, the differentiation progressing from in front tailward as with other organs. Tertiary vesicles (and tubules) arise either above, below, or between the primary and secondary tubules, and in sections (chicks five to eight days) one may see at once two or even three tubulea opening into the Wolffian duct. Still further tubules are formed in a similar manner; these do not open into the Wolffian duct, but into one of the three sets of tubulea already formed. The total number of tubules formed in each segment is at least five or six, probably more, in a chick of seven or eight days (Mihalkovics, 85.1, 88). In mammals it is very rare to see more than one tubule opening into the Wolffian duct in one section.

Structure of the Mature Wolffian Body

The Wolffian body readies its maximum development in t)ie chick of seven to eight days, in rabbits of IH-iO mm., in sheep and cow embryos of ib-'Mi mm., and in human embrj-os of the seventh week. The Wolffian bodies occupy nearly the entire length of the Wolffian ridges, but do not extend into the cephalic or caudal ends of the ridges; they appear, therefore, as two longitudinal protuberant masses on either side of the mesentery; they are suspended from the dorsal surface of the body-cavity and stretch from near the rudimentary dian™ i...^ -™ oifacofj pi« «. Pbrngm or septum transversum immemouth Per iwriorrtiuni nV.anteri- diately behin<i the ticart mto the pelvic SftubSdj '(M '*^^ika!*ooni'*jii r<^ion. The Wolffian Itotly tapers towanl ri^iiitb^ffcN oml'iuiiS I'nel^k ^'^'^^ ^'*'^' ^"^ **" '*^ convex lateral sur\<Aaa /« In itttmniiK Lu iun«: facc Can be distinguished the Wolffiim Ht iK.n 4fieri«fl^ ^^pj_ ^^^ j^j^j. j,j^ Mullerian duct also; compare Fig Hb The organ consists of a nuinl>er of relatively wide branching, and contorted epithelial tubules, the general course of which IS trans^ erse to the axis of the body. ( )ne ejid of the tubule opens into the Wolffian duct, the other terminates blindly in a Malpighian corpuscle, which lies toward the medial and ventral surface of the organ, Fig. 137. The tubules are not of uniform structure; the portion which Joins the Wolffian duct has a low clear-celled epithelium. Fig. 138, while the rest of the tubule is wider in diameter, Fig. 137, and has a higher cylinder epithelium with more granular ceSs; it is customary to distinguish the two parts as the collecting and excretory divisions, but we possess no certain knowledge as to the functions o£ the tubular epithelium.

The secretory portion opens widely into the lateral side of the Malpighian corpuscle; its epithelium changes quite abruptly into the thin epithelium lining the cavity of the corpuscle, and which is known as the capsule of Bowman, Fig, 138; the epithelium is of ct)urB6 reflected over the surface of the glomerulus, which it completely covers ; the epithelium of the glomerulus is thicker than that of the capsule, and is characterized by crowded spherical nuclei, very granular in appearance. The glomerulus itself. Fig. 138, is very insular in shape, b?ing imperfectly divided into lobes and lobules; its interior is occupied chiefly with the wide capillaries of the vascular network, bet\veen which is a small amount of embryonic connective tis.sue. In not quite mature Wolffian bodies the distinction betwe^i the dorso-lateral tubular and medio-ventral corpuscular zone is very evident, especially in transverse sections. Fig. 137.

The first curve of the S-shaped tubule, or that portion which empties into the Wolffian duct, is probably converted into the collecting tubule, the second curve of the S into the excretory tubule. In a Bheep embryo of 35 mm., according to MihalkovicB, 86.1, 88-89, the collecting tubule asceuds from the duct on the lateral side of the body, then bends toward the median line, descends through the midst of the organ, passing just laterally of the Malpighian corpuscles, and turning upward again passes into the much contorted convoluted tubule, which after many turnings opens into the Malpighian corpuscle. The course of the tubules may ^so be studied by isolating after maceration with hydrochloric acid, as first practised by Dursy.

The accompanying figures 137 and 138, illustrate the structure and relations of the mesonephros in the rabbit of thirteen days, some what before the organ has reached its highest development. The transverse section. Fig. 137, shows the Wolffian body hanging obHc|Hely downward into the body-cavity, Coe; it is close to the median aorta, Ao; overlying it is the cardinal vein, V, and on its median side is the much smaller genital ridge. The Wolffian duct, W. d., lies about the middle of the lateral side, close to the surface, and causes a slight bulging of the mesothelium, ni.tih, at that point. The lateral zone occupied by tubules is very distinct from that occupied by the glomeruli, ntp.c. The collecting tubules are readily distinguished by their thinner epithelium from the excretory tubules. The longitudinal section, Fig. i3K, shows that the cardinal vein receives numerous branches from the organ, and illustrates more fully the structure of the Malpighian corpuscles; the glomeruli are usually attached to the dorsal side of the capsule, and the epithelium of the capsule is somewhat thicker on the ventral side. Particularly noteworthy is the small amount of connective tissue. The fully developed WolflSan body of amniota has the tubules more closely crowded together, and by its expansion obliterates the genital ridge, Fig. 137, as a distinct protuberance; the Miillerian duct also appears running parallel with the Wolffian duct; finally the shape of the body is changed because the expansion takes place chiefly in the region between the Wolffian duct, W. d, and the cardinal vein, F., thus causing the surface along which the duct runs to face ventrally.

Fig. lM.—LODicttudliuilypnlcal Section nf the Wolffliin Bodv of

Days. Ve. Cardinal vein, witli lu endolhe emluB; cap, oavUy of Bowmaa's capsule.

Historical Note. — The following data are taken from Mihalkovics, 85. 1, 03. The Wolffian bodies were discovered by Casper Fr. Wolff, " Theoria generat," in 1759. They received their present name from H. Rathke, 20. 1, in 1820, but Rathke termed the same organs in mammals Oken^sche Korper, In 1824 Jacobson (K. danske Videnskab. Selsk., Kjobenhavn) introduced the i^aine Primordialniere^ and discovered that in birds the bodies secreted uric acid. The bodies were recognized in man by J. Fr. Meckel (" Beitr. z. vergl. Anat." I., 71-72) and Johannes MuUer, 30.1. The older writers held them to be either beginnings of the kidneys, or spermiducts, or horns of the uterus, etc. Rathke, /.r., by discovering the origin of the true kidney, led the way to true conceptions. The glomeruli were discovered by Johannes MiiUer, 30.1. The next important advances were made by Bomhaupt, 67.1. Semper, 76.2, and Balfour, 78.3, founded our present morphological notions of the organs, and Sedgwick, 80. 1, 80.2, Van Wijhe, and others have elucidated the genetic relation of the tubules to the nephrotomes. Mihalkovics' fine monograph, 85. 1, is the most importimt recent publication.

Resorption of the Wolffian Bodies

The cephalic end of the Wolffian body is retained in the adult and enters into special relations with the sexual organs to be described later. The remainder of the organ is resorbed, leaving only a few insignificant remnants. The resorption begins immediately after the Ixxlies have attained their full development, in the chick by the ninth da}', in rabbits of 18-20 mm., in human embryos by the eighth week; in man the relations are nearly those of the adult by the sixteenth week. In lizards the bodies are functionally active, at least during the first year of life, so that as we ascend the vertebrate scale we find the functional period of the mesonephros gradually diminishing.

The resorption proceeds as follows : The formation of new tubules and corpuscles ceases, the wide blood-vessels become smaller and their space is taken by interstitial connective tissue. In the secretory tubules the cells lose their characteristics, becoming indifferent cylinder cells, their protoplasm staining deeper than before. The canals remain in this condition until the sixteenth or seventeenth day in the chick and in rabbit embryos until they are 3.5-4.0 cm. long, when the epithelium of the tubules begins to degenerate; the tubules shrink; fine fat granules appear in the cells; the cell boundaries hecome first indistinct and are then lost; the cells break down to a fatty detritus, which fills the tubiiles, which thus l)eoomo solid cords of more or less imperfect cells. Meanwhile the Malpighimi corpuscles are also degenerating; their vessels contract and the shrunken glomeruli no longer fill the Bowman's capsules; gradually the corpuscles shrivel up.

The diminution of the mesonephros is accompanied by an enlargement of the sexuid ridj^re, so that the proportion in size of the two structures is reversed, and instead of the sexual anlage forming a small strip on the medial side of the much larger Wolffian body, the Ixnly forms a diminishing protuberance along the base of the enlarge<l sexuid aidage (genital ridge, / . p. testis or ovary) . The base of the genital ridge, by which it is susjKjnded from the dorsal wall of the splanchnoca»le, by being narrowed, is converteil into the suspensor}' membrane (mesovarium or mesorchium) of the genital ridge; it is from the lateral side of this membrane (Gekrose) that the contracted Wolffian body projects, Fig. 130. As it is converted into connective tissue, and as the Wolffian, IT. c/., and illillerian ducts, M. r/., nmning in it lengthwise are retained, the mass of tin* Wolffian l)ody l)ecomes merely the wing connecting the two ducts with the l)aso of the genital ridge; this wing is known in the adult male as the li(ji(ineuiiDii epiilidywis^ in the female as the aid rcsixniilionis: the detaileil history of these changes is given in Chapter XXII.

Fig. 130. — S<.'Otionthrou>rh thi* Testis of a Human Kniliryo nf Sixi.v-thn'v to Sixtv-tM^ht DnvH (Mfnot Coll. i:^H). Or. Ti»8ti« with sexual cords; vinch^ mesbrchiuDi ; U'.b, Wolfllau Ixxly. 3M/i. iiiesotheliuni, roprt»euted by a dark line.

The Mullerian Duct

This duct, which is the primitive female duct, arises in all amniota from the mesothelium of the Wolffinn boily close to theAVolffian duct, see Fig. 141, M.D. It appeal's relativelvlate: in chicks alK)ut the fifth dav, in rabbits aln nit the sixteenth or somewhat l)ef ore the Wolffian Injdv is mature; it is developed throughout its entire length as soon as the Wolffian Inxly attains its full size. Mihalkovics, 85.1, '^sr), has (lescril)e(l its relations at this .stage for j)ig embryos, 5-r, em. long; on the lateral side of the Wolffian body there is a longitudinal whitish band, due to a thickening of the mesothelium ; this band is called the Tubenfalte by M. Braun, the Tubenleiste by Mihalkovics, because it is along this band that the Miillerian duct is differentiated and from a part of that duct the tuba Fallopii is developed. The epithelial band stretches on to the rudimentarj" diaphragm (septum transversum) ; this part of the band has been described by KoUiker (" Entwickelungsges. , " 050) aatho Z we rchfellhand der Urniere; the band also stretches caudally along the Wolffian ridge beyond the Wolffian body proptn*. The "Tubenleiste" quickly becomes more prominent, and its interior is filled with connective tissue (mesenchyma) in the midst of which runs the Miillerian duct close to the Wolffian duct. The Miillerian duct, Fig. 141, 3/.Z>., is merely a small tube of epithelium; if we follow it forward it is seen to bend down, join the mesothelium and open into the splanchnoccele close to the anterior end of the genital ridge and to the septum transversum ; this ostium ahdoniinale lx?conies the fimbriate opening (Morsits diaholi) , of the Fallopian tul)e; it is sometimes called the Miillerian funnel.

That the Miillerian duct arises from the mesothelium of the Wolffian ]x)dy was first maintained in 1805 by Dursy, 66.1, and confirmed by W. Waldeyer, 70.1, 1 '24-1 20, who conceived that in the chick a band of epithelium is invaginated ; Bornhaupt, 67. 1, 57, had, however, previously asserteil that it is only at the anterior end that the mesothelium is invaginated, and that the duct grows Imckward, an<l this view has been generallv adopted, see Gasser, 74.1, Si^moff, 74. 1, Fiirbringer, 78. 1, Braun, 77.4, Kolliker, ** Entwickehmgsges.," 07H, Mihalkovics, 85.1, etc. Sedgwick and Balfour (see Foster and Balfour, " Embryology," 215) have modified this view by asserting that there are three successive involutions of the Wolffian mesothelium ; l)ut it is probal)lo that these are accidental variations owing to the irregulnr tliickening of the mesothelium.

the development of the *' Tulx^nleiste" precedes the appearance of the Miillerian duct; it is produced by a gradual thickening of the mes()tlii»lium along a narrow band running lengthwise of the Wolffian IxkIv; in liziirds this band is ou the ventral side of the l)ody; in birds and mammals on the lateral side ; in the latter the '* Leiste" lxH\)m(>s more promim^nt by the increase of conncjctive tissue in it. After the duct is <leveloiKKl the epithelium of the *' Tubenleiste" again flattens out. Nothing is known as to the morphological significance of this jx^culiar mesothelial structure.

The actual development of the Mullerian duct in amniota may be descril>ed as follows : The mesothelium on the vcmtral (reptilia) or lat(»ral (birds and mammals) surface of the Wolffian body is thickened to form the THhenfalte. A triangular area at the cephalic end of the thickened longitudinal band is invaginated; when the invagination is complete we find an obli(iue funnel widely open to the splanchnocoele, and with its a])ex lying inside the mesothelium ; the connection of the Miillerian funnel with the mesothelium can be readily seen in sections. In birds the invagination is somewhat irregular, so that there may be more or less marked modifications of from one to three or even four invaginations ; no special significance attaches to this peculiarity, although Balfour has sought to homologize the anterior end of the Miillerian duct in amniota with the pronephros; but this homology is untenable in my judgment. The point of the Miillerian f imnel is closed and tapering ; it grows rapidly backward, elongating as a solid cord, which becomes a canal by the gradual backward extension of the lumen of the funnel into the cord ; in its groAvth the cord follows along underneath the thick mesothelium of the "Tubenleiste," and on the ventral side of the Wolffian duct. It continues (probably solely by the proliferation of its own cells) its growth backward beyond the Wolffian duct through the caudal extremity of the Wolffian ridge to the cloaca, with the entodermal lining of which it ultimately fuses, so that the completed duct opens into the cloaca. While it is developing, the duct continues to enlarge and is therefore for a time wider headward than tailward. In front it appears in cross-sections as an epithelial ring with a considerable lumen ; the further back we go t}ie smaller the cross-section becomes. The solid growing point is found in close contact with the epithelium of the Wolffian duct; this fact has led Balfour and Sedgwick, 79.1, to maintain that the Miillerian duct grows by cells added to its end from the Wolffian duct, but Mihalkovics, 85.1, 298-'299, has shoAvn that this assumption is erroneous. As to the time when the duct appears — the Miillerian funnel is developed in lizard embryos of 14-lC mm., or eighteen to nineteen days after the eggs are laid (M. Braun, 77.4, 182), in snake embryos of 15-18 mm. (Mihalkovics, 85. 1, 21K)), in ducks the fifth day, in chicks the end of the fourth day.

In elasmobranchs the Miillerian and Wolffian ducts are united in one, as first shown by Semper, 76.2, and consequently the former appears to be split off from the ventral side of the Wolffian or segmental duct. Semper 's observations have since been amply confirmed by several observers, Balfour, Van Wijhe, Riickert, and others. Spengel, 76.3, 13, has asserted that the duct arises in the same way in certain amphibians. Since this discovery there has been a strong tendency to accept the theory first advanced by Gegenbaur in his "Handbuch der vergleichenden Anatomic," that there was primitively a single urogenital duct, w^liich split into two. This theory is open to obvious objections; the facts upon which it rests are derived chiefly from the embryology of elasmobranchs, a ty|)e far removed from the direct lino of vertebrate evolution, and presenting many secondary modifications; the origin of the Miillerian duct in elasmobranchs has not been shown to agree with that in any other type, and is known to differ from it essentiallv in the onlv tvi>e in which the development of the Miillerian duct has been accurately worked out; and finally, even in elasmobranchs the Miillerian funnel arises from the splanebnocoelic mesothelium. Now as we see that in all vertebrates the Miillerian duct lies close to the Wolffian, and as the fonner is known to arise in part or wholly from the mesothelium, while the latter arises from the ectodenn, we must, I think, assume that the two ducts were primitively distinct and that their temporary union in elasmobranch embryos is a secondary modification, which recurs, perhaps, in no other vertebrate. The view here advocate<l has l)een suggested by Jimgersen, 89.1, ll>r», a j)upil of Sem|)er's, and is favored bv R. Wiedersheim, 90.3, 343.

The Genltal Fold

The genital fold is a small loi^tudinal rii^e wb ch appears on the dorsal wall of tl e splanchnoccele of the embrjo where t s s tuated between the Wolffian body and the base uf the mesentery }! g 4 (re In the elasmobranchs there is an early development of onnect ve t ssue n the r dge wh ch causes it to project cons de ablj n the pnm tive vertebrates th s a not the case so far aa we can ] dge from the development n Petromyzon and amiihibtans, but the ridge is produced chiedy by a thickening of the )K.'ritoiiual epithelium; this thickened band of mesothelium was first whoxtTi by Waldeyer, 70.1, to give rise to the egg-cells, and lijia since been shown to share in the development of the speriiiatoziia. hence it is called the {jermittal epithelium (Keitnepithel), and must hv regiirdetl as the primitive and essential part of the genitiil gliinds. In amniota the WolfiSan body enlarges so early and rapidly that it carrieB the g^ital fold along with itself, so that the latter hecomes merely a band on the medial side of the Wolffian body, Fig, 141. But in all tinmiota the genital fold is first clearly marked out by the differentiation of its mesotbelium from that of the peritoneum proper. In reptiles this differentiation takes place even before the Wolffian tubules have united with the Wolffian duct, but in mammals )t is deferred until the Wolffian body is quite large; hence in the mammals the genital ridge seems to be derived from the Wolffian body; but this must be regarded as a secoiidar} modification.

The genital ridgo extends nearly or quite the entire lengtii of t)ie abdominal cavity; ita ceplialic end is probably the anluge of the glomus of the pronephros; its caudal end remains inle urojteniui Fold of B. Chick different, fonuing the 80 umnryo oi tne J^ourtii Day, WD. Wolfflui duct- ., MuuerBcmct; ow, ™i<.m: IK, woiman tubSie- Called guberuacuhtm : the t of the ridge, that is, its middlo region, winch occupies the greater part of its extent, is the unlngo pn>i>er of the genital gland.

We can distinguish four stages in the development of the genital ridge: 1, the production of mesenchyma from the mesothelium ; 2, the ifevelopment of the medullary cords; 3, the appearance of tlm true primitive ovii; 4, differentiation of the sexual glands. 'La'I ua study these stages in order.

1. Production of the Genital Mesenchyma

The genital ridge is a product of that portion of the ^lanchnocoelic mesothelium which lies between the nephrotome or Wolffian tubule, iuid the Itase of the future embryo; in very young amniote embrj-os. Fig, 161, it is that part of the mesothelium neare^it the dorsal aorta. This germinal epithelium very early begins throwing off mesenchymal cells from its inner surface ; the process has yet to be studied carefully ; I find that it is going on both in the germinal epithelium and in the neighboring mesothelium in chicks at the time when the Wolffian tubules have just joined the Wolffian duct, as can be clearly seen in Pig. 134; the mesenchyma, mes, extends through the genital re^on and passes without demarcation into the mesothelium, msth. The continued production of mesenchyma has been specially emphasized for all classes of amniota by Mihalkovics, and that the germinal epithelium contributes to the ovarian stroma (in other words, forma

H.— Section of the U

ntesenchyma), was discovered as long ago as ISC/o by Borsenkow (Wurzburger Naturwiss. Zeitschr.^ IV.), and liiis been maintained by E^li, 76.1, Balfour, 78.2, K. Schiilin, 81.1, and others. The significance of the fact, "(vhich was obscure hitherto, is clear enough with our present knowledge of the genesis of the mesenchyma.

The mesenchymal proliferation of the genital mesothelium is ai^companied also by the appearance of enlarged clear cells with enlarged clear, distinctly nucleolated nuclei. These cells from their appearance have been termed by most writers primordial ova ( Ureter) from their assumed identity with the primordial ova of later stages. Mihalkovics, 85.1, has pointeil out that they entirely disappear before the development of the ova begins, and that they cannot l)e identified with a kind of cells which arise much later; ho pro|X)ses to call them sexftdl cells ((ieschlechtszellen), but the name is not felicitous; lx>f()ro giving thorn any 8i)ecial name it will bo well to learn more alnnit them. I think they may hiivo to do with the growth of the layer, for such enlargement <x»curs sometimes when a simple tissue is growing rapidly. The colls in question have l)een seen in nc^arlv all classes of vertebrates ; for sharks see C Rabl, 89.2, Taf. X., Figs. 7 and 8, Van Wijho, 89.1, and Riickert, 88.1; for Amphibia see (i<K»tt(% 75.1; for reptiles soe il. Braun, 77.4, 145; for birds and mammals see Mihalk«>vics, 86.1.

2. Development of the Medullary Cords

There appears very early in the mesenchyma of the genital fold ix>culiar cords of closely compacted cells which stretch up above the level of the fold alongside the cardinal vein. The sexual cor^^s are destined to generate the ova or spermatozoa according to the sex of the embryo. Our knowledge of the cords is still unsiitisfactory and confused. So far as I know the homologies of these cords have not been thoroughly studied in any of the anamnia, hence the following account is based on the study of amniotii. They were first distinctly ivcognizecl by Bornhaupt, 67.1; they were carefully studied by it. Braun, 77.4, in reptiles, who terms them HeifinentuLst nhnje the name medullary cords (MarLsfrntKje) was proposed by KoUikor ('* Entw.-Ges., 070) ; vSchmiogelow, 82.1, descril)es them in birds; in mammals they have l)een studioil by Balfour, 78.2, Ed. Van Beneden, 80.2, M. Nussbaum, 80. 1, and especially by Harz, 83. 1, whose paix?r is excellent. Mihalkovics, 85.1, gives a monograj)hic treatment of the cords, calling them Sex f i(t Is f range. \V. Xagol, 89.3, 3*23, has shown that the true sexual cords are concerned in the production of the ova and si)ermatozoa, and must l)e carefully distinguished from the remains of the Wolffian tubules, which can be observed, esi)ecially in later stages, in the genital ridge.

The cords appear in Lacerta embryos of 12-14 mm., in chicks of the sixth day, in shei^p of 1 0-1-2 mm., and have teen seen in human embryos of 15 mm., or about five weeks. In the lizard they are very large, Jind ap|K?ar one in esich segment as first shown by M. Braun, who accordingly has named them segmental cords; each cord n(^arly fills the interior of the genital fold, then stretches upward close to a Malpighian corpuscle of the Wolffian body and rises above to the level of the cardinal vein. Braun held that the cord arises from the epithelium of the Malpighian coq)Uscle, but Mihalkovics maintains that the two structures are always separatee!. In the chick the cords can be seen the sixth day, but they are smaller and more numerous than in the lizard; at first their borders a,re vague and they have no definite demarcation from the stroma of the ovary. For a description of these cords in the male chick of seven days see E. Schmiegelow, 82.1, IGl ; he terms them "Anlagen der Samencanalchen." In the sheep (10-11 mm.) the cords first appear without demarcation from the ovarian mesenchyma; in embryos of the sheep and rabbit of 12-14 mm. the cords are more distinct. Of the appearance of the fully formed mammalian medullary cords at the stage preceding the appearance of the primitive ova I know no exact description. They are three or four cells wide, with distinct outlines ; are twisted and branching ; stain somewhat deeper than the stroma proper of the genital ridge, of which they occupy a large part.

3. Appearance of the Primitive Ova

Since Semper's researches, 76.2, it has come to bo more and more generally admitted that the development of the genital glands leads in both sexes through an early stage characterized by the appearance of primitive ova (Ureiery Primordialeiev^ ovoblast). The primitive ova are merely enlarged cells of the germinal epithelium (or of the so-called medullary cords). They are readily recognized by being more transparent than the remaining cells and by having a larger and more transparent nucleus, which a little later has a well-marked nucleolus. These cells have long been known in the ovary, where they^ can be readily followed along in their development into egg-cells and mature ova, but in the male their history has still to be worked out satisfactorily. Most authors have assumed that in the higher amniota the primitive ova breiik up in the male and disappear very early, without wandering from their first lair, but Semper, 76.2, found that they are included in the embryonic seminiferous tubules of the sharks; in amniota no such process has been observed, but Mihalkovics has found in human embryos of 14-1 G cm., that the tubules contain cells resembling primitive ova, but in earlier stages there are no such cells. In fact wo must admit that the history of the male primitive ova is practically unknown, consequently I give now merely the description of the primitive ova as they appear in the germinal epithelium, deferring the further history of the genital glands to Chapter XXIII.

The primitive ova are modified cells of the genninal epitheliimi. In elasmobranchs the epithelium is at first a layer of cuboidal cells of imiform character, with small darkly stained nuclei ; as soon as the genital ridge begins to protrude (Acanthias of 19 mm.), the epithelimn becomes thicker and hero and there single ones of its cells become much larger, and their nuclei l>ecome much larger, sphericid, gi*anular, and stain more lightly; the thickening of the epithelium continues as the fold l)ecomes more prominent, but is confined to the medial side of the gland for a certain i>eriod; the thickening is accompanied by an increase in the number of clear cells, which tend to lie in groups of three or four, see Semper, 76.2, :5:}5-:J45, and Balfour, Comp. Embryol. I., 55-58. In reptiles, Braun, 77.4, 145, the history is essentially the same, except that the genital folil contains little mesenehyrna, and therefore the epithelium (Laeerta embryos 12-14 mm.) constitutes the greater part of the fold; the primitive ova resembles those of the elasmobranchs and are similarly imbedded in unmodified mesothelial cells. So far as known to me, there are no satisfactory descriptions or figures of the germinal epithelium with primitive ova at the indifferent stage of the genital fold of any bird or mammal whatsoever; it is to be hoped that this omission will soon be made good. There are brief references to the indifferent stage of the higher vertebrates in all the current text-books.

4. Differentiation of the Sexual Glands

The conversion of the fully developed genital fold into the sexual glands is described in Chapter XXIII

Evolution of the Urogenital System

The embryology of vertebrates renders it evident that we have, as stated in the introduction of this chapter, four parts as the primitive constituents of the urogenital system on each side: 1, the genital ridge; 2, the Wolffian ridge; 3, the Wolffian duct; 4, the Miillerian duct. A few words in regard to the probable evolution of each of these is in place.

1. The genital ridge is marked out essentially as a specialized area of the splanchnocoelic mesothelium, in which the primordial ova are developed. The protuberance of the ridge is increased by the stroma or mesenchyma developed below the mesothelium, but this is presumably a secondary modification. We may, therefore, regard tJie genital ridge as having been primarily a mesothelial band on the dorsal side of the body-cavity, bctwecm the r(X)t of the mesenter>- and the opening (nephrostome) of the nephrotome into the splanchnoccBle. This brings us not far from the condition in amphioxus and annelids, for in both tyj^es the genital products arise from the mesothelium.

2. The Wolffian ridge is the protuberance prot'iuced by the increase in size and number of the Wolffian tubules, and its inclusion of the genital ridge is a secondary result of its enlargement in the higher vertebrata. Moreover, it does not at first protrude at all, so that we have to consider not the existence of an actual ridge bul the presence of Wolffian tubules as the essential feature. The tubules, as we have seen, are developed from the nephrotomes, and when the nephrotomes have a distinct cavity that cavity is preserved to make the himen of the tubule. In all true vertebrates, how^ever, the tubule or nephrotome has lost its connection with the myotome, but retains (in the anamnia) the opening (nephrostome) into the splanchnocoele. The conection with the Wolffian duct is secondar}% and the manner in which it was accjuired cannot be satisfactorily accounted for at present.

Out of the Wolffian ridge there arise three primary organs, the prone])hros (head kidney), the Wolffian body (anamniote or primitive kidney), and the true kidney of the amniota (metanephros). Of these the last is not a primitive vertebrate organ, since it is found only in the higher forms; its development as an outgrowth of the Wolffian duct is descrilxHl in Chapter XXIII. ; nothing is known as to its prob'ible evolution. As regards the ])ronephr<>s and Wolffian body or mesonepbroH the first question is whether they are parts of the same sseriet? of primitive organs or distinct organs. That they are fM^jjarate organs has Vjeen maintained by several rvcent writers, notably by Ruekert, 88.1, and Van Wijhe, 89.1. The former lays stress u[xin the lateral outgrowths of the nephrotomes of the pronephrr^s, but this is of little importance, especially as the nephrotomes of the mes^jnephrrjs expand (as in amniota) and their side walls fuse with the pn^nephric duct ; hence I cannot admit that there is any essential difference. Van AVijhe, /. c, 50.>-5(Ms states: 1, that the prr^uephrrjs develojis earlier than the mesonephros, but in all organs the cefilialic jiortions are more advanced in the embryo than the caudal, and we cannot on this ground any moi*e divide the series of nephrotomes than we can the series of nervc*s or myotonies : '2, the pronephric and mesonephric tubules differ slightly in origin, but he overUxjks that they are lx>th derived from the nephrotomes; :?, the pronephrif duct unites with the pronephric tubules as soon as it appears, but the union with the Wolffian tubules is retarded ; this {xjiiit is insignificant, for variations in the time of development <»f organs are siM.fially chanicteristic of einbr^'os, and, mor.^ver, MihalkovicH lias made* it prol>abl<» that there is a gnidual ti-ansition : 4, the presfmce of the glomus is characteristic of the pi-onephros, but since the gl«;mns is api>arently only the anterior end of the genital ridge and as the genital ridge stands in the simie relation as the glomus to the nephrostomes this difference can be assigned a secondary value only. It appc.»ars then that none of the argmnents in favor <jf an essential distinction have force. On the other hand, the facts — that all the tubules are develoi>ed fn^m the nephrotomes, that the nejihrotonies in all cases unite by their lateral walls with the pronephrirr duct, that pronephric and Wolffian tubules never both occur in the sanif? segin<*nt, that tli(Te is in some and pro])ably in all cases a gradual transition from the pronephric to the Wolffian Inxly — Sfjcm to nu; ample to establish the conclusion that the two organs are parts of a single series. The pronephros is distinguisheil princijmlly by the glomus or vascularization of the genital fold in its neighl>orluxxl, and it l)ecomes separated from the Wolffian IkxIv profKT by th(* alxn'tion of several tubules between the pronephros and m(*sonephros. Why this sei)aration occurred we do not know.

We have, if the view just defended l>e correct, to consider that the (jxcretory organs w(»re primitively a uniform series of nephrotomic tubulins (S<»in|)er's segmentiil organs), each beginning with an opc^ning (nephrostome) int4) the splanchnoccele without connection with the myotomes, but o]x»ning laterally into the Wolffian duct. SemjKjr has sought to homologize these tubules with the segmental organs of anm^lids; the latter are excretory tubules ojK^ning into the cfelom, one on ouch side for each segment, and emptying externally through i\u} ectcxlenn. Now if we assume that the line of these ext<^nial oi>enings In^came a groove and then a canal, we may pass at once from the annelidan to the vertebrate ty|)e, as suggested by Haduon, 86. 1, and " Practical Embryol.," '250, and at the same time ju'count for the c*cto<lermal origin of the Wolffian duct as a survival of a stfige intermediate betwe<ni th(» annelidan and vertebrate types. The hJemi)er-Had<lon hyiK)thesis has nnicli in its favor, and I should be strongly inclined to adopt it, were it not for the recent investigations of Boveri, 90.1, which suggest that quite a different line of homologies, to be traced through Amphioxus, may be the true one.

3. The Wolffian duct., as we have seen, is probably of ectodermal origin. As indicated at the close of the preceding paragraph, there are two hypotheses to account for this assumed origin, namely, Haddon's, oB.l, and Boveri 's, 90.1. Haddon, accepting Semper's hypothesis that the tubules are homologous with annelidan segmental organs, assumes that they opened exteriorly and that the line along which they opened became a groove and then a canal. Th. Boveri asumes that the genital chamber of Amphioxus corresponds to part of the nephrotome, and that ducts, which lead in the branchial region of Amphioxus according to Boveri *s discovery from the body cavity into the peribranchial chamber, represent the excretory portion proper of the nephrotome ; thus he is led to homologize the peribranchial chamber with the Wolffian duct in that both are of ectodermal origin and both receive the nephrotomic tubules ; he, however, regards the duct as representing merely a specialized part of the amphioxan l)eribranchial chamber. Boveri 's brilliant comparisons are certainly very seductive, but his observations on Amphioxus do not suffice as yet to carrj' his conclusion above the h}T)othetical stage. There is one difficulty which he has overlooked, namely, that by his hypothesis the sexual gland is further from the splanchnocoele and nearer the myotome than is the excretory tubule, whereas in true vertebrates the relations are the reverse of this, so that the sexual glands in the two types would not be homologous, if Boveri 's hypothesis were correct.

4. The Mullerian duct, as stated in the section on its development, is in my opinion probably not derived from the Wolffian duct as maintained b}" Gegenbaur, Semper, and their followers, but is a new organ developed from the mesothelium within the vertebrate type. Its evolution, however, is, properly speaking, merely a matter of speculation at present.

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Human Embryology: Introduction | The Uterus | General Outline of Human Development | The Genital Products | History of the Genoblasts and the Theory of Sex | The Germ-Layers | Segmentation | Primitive Streak | Mesoderm and the Coelom | Germ-Layers General Remarks | The Embryo | The Medullary Groove, Notochord and Neurenteric Canals | Coelom Divisions; Mesenchyma Origin | Blood, Blood-Vessels and Heart Origin | Urogenital System Origin | The Archenteron and the Gill Clefts | Germinal Area, the Embryo and its Appendages | The Foetal Appendages | Chorion | Amnion and Proamnion | The Yolk Sack, Allantois and Umbilical Cord | Placenta | The Foetus | Growth and External Development Embryo and Foetus | Mesenchymal Tissues | Skeleton and Limbs | Muscular System | Splanchnocoele and Diaphragm | Urogenital System | Transformations of the Heart and Blood-Vessels | The Epidermal System | Mouth Cavity and Face | The Nervous System | Sense Organs | Entodermal Canal | Figures | References | Embryology History

Cite this page: Hill, M.A. (2018, August 19) Embryology 1897 Human Embryology 11. Retrieved from https://embryology.med.unsw.edu.au/embryology/index.php/1897_Human_Embryology_11

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