1897 Human Embryology 23

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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 XXIII. The Urogenital System

The early history' of the urogenital system has already been given. Chapter XI., p. 2:J0. We have now to consider the differentiation of the male and female type from the indifferent condition. In order to render the complex changes clear, it ban seemed to me advisable to give first a general history of the metamorphoses, so as to bring out first the homologies in the two sexes, next to present the special histories of the single parts, and finally to append an account of the external genitalia.

I. General History

The Indifferent Stage. I

The early history of the urogenital system has been given in Chapter XI. ; nevertheless it will be convenient to present here a generalized diagram of the indifferent stage, for comparison with diagrams of the differentiated system male and female. The indifferent stage is characterized by all thd oi^ans being contained in two longitudinal urogenital ridgea Fig. il(i5, one on each side of the body and projecting from the dorsal wall into the peritoneal cavity. At the caudal end of the abdomen the two ridges draw closer together and finally come into contact with the anal region of the intestinal cunal. The ridge isconstituted chiefly by the WolflSan l)ody, ii'.b, and it therefore iimtains the Wolffian tubules and the Wolffian duct, IV. D, which is situated on the side of the ridge farthest from the mesentery, mex. Close alongside the Wolffian duct lies the Mullerian duct, M.D. Both ducts open into the cloaca, C7, or terminal division of the intestine.


Changes in Both Sexes

The essential or fundamental difference between the two sexes is the change of the genital ridge into an ovary or testis according to the sex. The secondary differences are chiefly in the modifi«rations of the ducts, and as regards these the most important changes are that in the male the Wolffian duct becomes the genital duct («hiet of the epididymis. vas deferens, and ductus ejaculatorius). while in the female the Miillerian duct l>€'ctiines the genital duct (Fallopian tutx? or oviduct, uterus, anil vagina). Before considering the changes more in detail it willlie oinveniont to divide them into two grouiw; 1, oomraon to both sexes; '.', charaoteristic of one sex.


1. Changes Common to Both Sexes

There are three important changes from the indifferent stage to be noted under this head : A, the union of the caudal ends of the urogenital ridges to form a single median genital cord; B, the anterior end of the. Wolffian body persists and undergoes modification in connection with the genital glands, by which two sei)arate organs are produced in each sex; C, in the course of development the genital organs become restricted to the lower (or caudal) end of the abdomen, and do not continue to stretch the whole length of the abdomen as at first. Another important series of changes is that by which the cephalic portion of tiie urogenital ridge acquires in the female a transverse position, in consequence of which the upper or cephalic end of the liillerian duct, or in other words the future Fallopian tube, runs transversely. This change occurs in the male also, but is less noticeable and is, to a certain extent, masked by the migration of the testis from the abdomen through the inguinal ring into the scrotum.


2. Changes Characteristic of One Sex Only

A. Male

The general plan of the urogenital ridge in the male is indicated in the diagram Fig. 26G. In the male, as stated above, the sexual gland becomes a testis by the development of seminiferous tubules, and the Wolffian duct becomes the genital duct. The connection between the Wolffian duct and the seminiferous tubules is established by means of the anterior tubules of the Wolffian body. There are special extensions of these tubules into the testis, which unite with the seminiferous tubules and form a series of anastomoses with one another within the testis (compare Fig. 2(30), constituting the rete testis, while the tubules proper of the anterior part of the Wolffian body remain to serve as the channels of connec^tion (vasa efferentia) between the rete testis and the Wolffian duct, which is thus enabled to serve as the spermiduct. A portion of the anterior Wolffian tubules persist as a separate group, which is known as the organ of Giraldes, or paradidymis of Waldeyer. The S|)ermiduct becomes differentiated into three principal divisions: 1, the coiled portion


Fig. 866. —Diagrami to Illustrate the Homolono are stthe testis COnstltutmg the gles of the Sexual Apparatus, i/i/d. Hydatid;

rhift of tho onididvTnis* "1 tboloilff ^- ^/' ^**^ efferentia: Ejxi, duct of epididy(lUCl OI Uictpiauniius, ^, iiit>ioug ^^^. jr. z). wolffian duct; 3f.A Mailer's duct;

VaS aeiereilS rnnninK through the ^/.c, |?enital cord: rf.»«. uterus mascullnua; U»8tis: Rett', rete Halleri: Parad, paradi urogenital fold, to where the two dymis; /, fimbria; parr, parovarium or epo fnlil« mn'fii tn pnn^fif.ntpfhA craiiitjil ophoron: (.7, uterus: 6?. c, genital cortl: Vg^ rOKlS Uniie lO consiltute tne J^enuai vi^ina; or, ovary: puroophl i>aroOphoron.

cord; 3, the ductus ejaculatorius, developed below the point where the seminal vesicles are formed and within the genit^il cord. The Miillerian ducts remain rudimentary and their middle }X)rtions usually abort, leaving the upper fimbriate ends to develop into the so-called hydatids of Morgagni, and the lower or caudal ends to unite within the genital cord to form the so-called uterus masculinus (prostatic; vesicle), a rudimentary representative of the female uterus and vagina.


B. Female

The general plan of the urogenital ridge in the female is indicated by the diagram, Fig. 2iUt. In the female, as stated above, the sexual gland tecomes an ovar}% by the development of ovic follicles, and the Miillerian duct becomes the genital duct. The Wolffian duct remains rudimentary and in part disappears ; it persist^s in the genital cord as the duct of Gartner, but does not, so far as known, unite with its fellow; it persists also at its upper or cephalic end as the duct of the jmrovarium (epoophoron, organ of Rosenmiiller) , which comprises the group of Wolffian tubules in the female homologous with the vasa efferentia of the male. There also persists a group of Wolffian tubules, which has been named the paroophoron by Waldeyer and is homologous with the male organ of Giraldes. The Miillerian ductus unite within the genital cord to a single median duct, which enlarges greatly and is differentiated into the uterus and vagina; the upper or cephalic portions remain separate and form the Fallopian tubes or oviducts proper; the Miillerian funnel becomes tha fimbriate opening of the Fallopian tube.


Homologies between the Sexes. — These may be readily followed by means of the accompanying diagrams. Fig. 266, A and B, and the table given below. The diagrams call for no further explanation than is given on the figures.


Tabular View of the Homologies of the Human Urogenital Apparatus in the Two Sexes.

Imdifferent Stags. Genital rid^e. Wolffian tubules.

Wolffian (UirtH.

Mftllerian duct.

Urogenital sinus.

Genital (»mineuce. External labia.

Male.

Testis.

1. Epididymis.

2. Paradidymis.

3. Vas deferens. (Vesicula seminalis. )

4. Ductus ejaculatoriuK.

5. Hydatid of Morgagni.

6. (Usually alx)rt8. )

7. Uterus masculinus.

8. (Usually undeveloped.

9. Verum montauum.

10. Urethra.

11. Co\vi)er's glands.

12. Penis. 18. Scrotum.

Female. Ovary.

1. Epoophoron.

2. Parovarium.

3. Duct of Gartner.

4. (Usually aborts.)

5. Fimbriate opening.

6. Fallopian tuW.

7. Uterus. H. Vagina. J). Hymen.

10. Urethra and vestibule.

11. Bartholini's glands.

12. Clitoris and nynipha?.

13. Labia majora.

II. Special Histories of the Urogenital Organs

Sexual Glands

A. Male

The testis becomes recognizable bj' its histological character in the human embryo according to W. Nagel, 89.3, 300, at five weeks; according to Benda, 89. 1, at six weeks. It can be distinguished from the ovaries by its external form in the three months' embryo. By the al)ortion of the Wolffian body and by the growth of the testis the latter bec<mies the principal organ of the urogenital fold in the male. the Wolffian j)art of the fold remains to form the mesorchium, the lower or caudal portion of the fold remuininji as the j^iiberiiacnlum. By the fourth month the tt^Htis has ii.S8iiiue<l its permanent form, but its growth continues.

The role of tho Wolffian tubules in the genesis of the t^tis is ilescrilxHl below, p. 50<).

Hinfo</fiu'nin. — The Bubsecjuent account of the development of the testis follows Xagel, 89.3, closely, whose results I accept, both as roganls his observations and his criticism of previous investigations, iilthoiigh they rwj 11 ire modification owing to wliat luis become known <-onccriiing the gi'netic notation of tho mesenchyma to the mesotheliuiii of the genitiil lidgo, sec p. US. As describee! in Chapter XII. the genital mt'sothelium tlirows off ceils, which at first assume entirely the character of loose moseiichyma, and later remain crowded together with scarcely a tniccof division from tlio piireut epithelium; in this denser tissue apjwar large cells, tho so-called "Ureier." ( >ut of this aniage are develoixHl epithelioid oonls. the sexual cords, which include some of the ureier, and become more and more sei»iratwl fivim one another by the development of loose mesenchyma or embryonic connective tisane between them. Nagel finds that the male si'xual gland, Fig. 207, may be recoguized even in

an embryo <if i:[ mm. by the small number of the ureier aa compared with the ovary of cor i-espon ding age. In the testis at this stage (human embryo of i:! mm.), the se.vual conls are not yet very <listiiict and are ct»nnected with the superficial epithelium. In aa embryo of nine weeks, Fig. ^tlT, the sexual gland * is eoveretl by a regular cuboiilal eiiithelium, distinctly iNninded against the under •m iu-.-.Bnlniiiv Kltli K.illik.T-- rl..s<Ti|.ii.m iiiirl tljniiv rlnindrl**.- FIc- *«>. lliia vlaiid miulil hi" in iiviiry, hiil Vim A^'ki-ivii statcH tliui Kr.llltiT Ijhh limnjui JouUful in r«i^rd to hU

lying tissue, which is composed of mesenchyma with sexual cords, S.c, which are not connected with the mesothelium ; the submesothelial layer is the anlage of the tunica albiiginea; as no corresponding layer exists in the ovar}', its presence in the male gland at this stage establishes one of the most characteristic features of the testis; in the albuginea, connective- tissue fibrillaj are just beginning to appear. The central portion of the testis is occupied by sharply defined sexual cords, which frequently anastomose with one another and contain here and there an •* Urei,^' or sexual cell of Mihalkovics ; the sexual cells are clearer and larger than the other cells of the cords, measuring 11 /i with nuclei of 8/^ diameter. In an embryo of 35 mm. the general structure is much the same, but the albuginea is thicker and more fibrillar, and the cords are more regular in their arrangement ; the cords are about 32/x thick and their cells show a somewhat epithelioid arrangement; the few sexual cells thej^ contain now measure 14-16/*. In an embr}'o of ten centimetres a new feature is found in the presence of the interstitial cells. These are large cells which lie between the sexual cords, and are probably developed by the enlargement of the connective-tissue cells between the cords: they are spindle-shaped or polyhedral, with several processes each; their protoplasm offers a peculiar mat appearance; their nuclei are large, with one or two nucleoli and a distinct intranuclear network.

The cords are the solid anlages of the seminiferous tubules. The question has been debated at great length whether they are differentiated from the stroma or the epithelium of the genital ridge — compare the synopsis of opinions given by Nagel — but as the epithelium (mesothelium) produces the mesenchyma or stroma, the question appears to me insignificant. The further historj' of the sexual cords (future seminiferous tubules) has been most fully studied by C. Benda, 89.1, compare also Prenant, 89.1, 90.1. The cords remain solid throughout foetal life, the smaller cells having a radial position and epithelioid arr*mgement, but the nuclei are irregularly placed, so that it is difficult to decide whether the cells are in a single row or not around the centre of the conl. The large ureier are irregularly distributed — less irregularly in man than in other animals — but they are always completely imbedded in the other cells and show a tendency to lie near the periphery of the cord in man, rodents, dogs, and cats, near the centre in ruminants (ox). As to their numl)er, few ureier are found in the cords of man, while in rodents they are very numerous; dogs and cats occupying an intermediate position as to number. The condition described is attiiinenl in man about the sixth week, in the rabbit the seventeenth day, and persists with little change not only throughout the fcotal i)eriod, but until the time of puberty, when the cords changes to seminiferous tubules.

The conversion of the male sexual cords into the seminiferous tubules, being iH)st-f(etal, dot»s not fall within the scope of this work. The reader is referreil to the investigations of Prenant, 89. 1, Benda, 89.1, and F. Hermann, 89.2. According to Bonda the epithelioid cells give rise to the columns of S<M-toli (Benda's FnsszeUi'it) and the ureier to the spenuatocytes (Benda's Samenstamntzellen). This is in accordance with Benda's liypothesis that the spermatocytea have no genetic relation with Sertoli's column, an hypothesis which is not yet established firmly — compare Chapter III,

B. Female

The ovary becomes histolc^cally recognizable about the same time as the testis, i.e. six weeks; it can be readily distinguished from the testis in the three months' human embryo by ita external form. In consequence of the abortion of the Wolffian bodj- and of its own growth the ovary is alreadj- the principal organ of the urogenital fold at three months 4.s the greater part of the fold has thinned out to constitute the broad ligament the relations found in the adult are established at the age under consideration.


Histogenesis of the Oiary — According to Nagel the ovary may be difitinguished from the testis in human embrjos of only 13-13 mm. by the greater abundance of the de^el pea and developing ureter. In an embryo of 12mm. Nagel,89.8, 305, describes the ovary as consisting of the proliferated germinal epithelium; the proliferated cells areof two kinds, the more , numerous are smaller, and have more darkly stained nuclei; the less numerous are the young ureier with lighter stained granular nuclei. In an embryo of 20 mm. the ovary projects a little from the surface of the urogenital ridge, and is fiUetl with the cells from the epithelium, the two kinds being present, and, f □ sen as before, with numerous in transitional stajjes between them; the ureier measure 10-lC/i with a nucleus of 8it diameter—the smailer cells S/i with a nucleus of 5;i ; in the centre, spindleshaped connective-tissue cells are appearing. In an embryo of 30 mm. the ovarj" projects still more from the Wolffian body ; tiie ureier are lai^r, Ki,'*, and the connective tissue or stroma is more devel(jpe<l and has capillaries. Nagel ha.s studied also embryos between :i and 7 cm. in length, hut we uiay pass at once to the latter. In embryos of 7 cm. the ovary is triangular in section, the apex of the triangle corresponding to the attachment to the Wolffian body or future broad ligiiment. The connective tissue now forms partitions, which divide the remaining cells into groups, Fig. 2(18, but the partitions fade out toward the surface, which is covered by a single layer of cells, which has begun to assume the character of an epithelium entirely distinct from the underlying cells. In an embryo of II cm. the covering epithelium of the ovarj" has become more sharply boimded and the development of the stroma has extended quite to the surface, dividing the proliferated cells into rounded groups of small colls and ureier, which are at this stage very numerous, and indeed outnumber the small cells in the balls. These balls are a highly characteristic feature of the yoimg mammalian ovary, but their arrangement and connections witli one another have been as yet only very imperfectly studied; nevertheless it seems safe to say that they are not separate masses, but, as seen under the microscope, sections of contorted fuid anastomosing cords. If this view is correct then there is an evident resemblance between the testis and ovary, there lx>ing in both cords derived from the germinal epithelium, containing ureier and separated from one another by vascular connective tissue. The ovary diffei's from the testis in having larger cords and a much larger absolute and proportionate number of nnni»r. That we have to do with sexual cords is evident in later stages, where the cords are very distinct and are found still connected with the covering mesothelium (WiilAcyev'H Keiniepithel) ; in their later stage, the ovarian sexual cords are known as Pfliiger's cords (Ptluger'schen Schlauche), bt»ing named after their discoverer, and they differ considerably from their earlier stage in that they include a large numter of small or follicular cells, which completely surround the ureier and se})arat<?> thc^m from one another, by constituting an epithelioid layer or follicle around each urei. The transition from the stiige of the balls, as we may call it, to the stage of Pliiiger's cords has not \kh}i\ clearly ascei-tained, because the origin of the small or follicular cells is still uncertain but I agreewith O. Hertwig ("Lehrbucii d. Entwickelungsgesch.," :5te Aufl., 3*^1) that they are cells of the original cords derived from the mesothelium of the ovary, although Rouget and so eminent an authority as Kolliker (**Grundriss," 423) have maintjiineil that the medullary cords grow around the ureier and pnxluce the follicles; Kolliker seems to me not to have offered sufficient evidence to render his view prolwible. Another view is that advocated by Foulis, 76.1, who iK^lieves that the ureier bi^comes entirely free and that the follicles are d(^veloj)ed trom the stroma cells — a conception which cannot be maintaineil. If we assume, as we apparently must, that the follicular cells arise from the sexual cords, tiie question would still remain, whether they are derived from some of the original small cells or from the ureier; that the latter derivation is the actual one is to my mind probable, because there appears to Ix* a stage in the developmc^nt of the sexual cords of th(» mammalian ovaiy in which all the cells are converted into ureier; but luitil further investigations shall have decided it, the (juestion of the origin of the follicular cells must ]>e considered an o|K»n one. Mihalkovics, 85.1, 441), discusses carefully the origin of these cells, but owing io the distinction he <lraws lx»tween the sexual (»ords and the proliferation of the germinal (Epithelium to form the ureier, it is im|H>ssible to follow his own nccount : Mihalkovics also gives an admirable review, pj). 4"^:i-4"3S, of the liti^ature upon the development of the ovary.


Gubernaculum, Processus Vaginalis, and Descent of the Testis. — The descent of the testis begins very early, the change in position l)eing evid(»nt by the tenth we(*k, but the j)assage into the scrotum does not lx»gin until the seventh month. Thi» testis makes three movements: 1, backward to near the inguinal ring; 2, forward a short distance, during the period of the formation of the muscular gubernaculum ; 3, downward into the processus vaginalis. The processus does not extend completely into the scrotum during foetal life, hence the foetal scrotum has no cavity and never contains the testis, but on the contrary is fiUed by a very vascular connective tissue like the labia of the female. At birth the processus lies partially in the scrotum.


The cause of the descent of the testis has been much discussed and many fanciful explanations have been propounded. There is no reason for supposing that these movements are in any wise diflFerent from the numerous other movements of organs and changes of form occurring during the course of development. These changes are all due to inequalities of growth in the tissues, but the causes of these inequalities are not yet ascertained. A long-prevalent tendency has tainted the study of the generative organs with mysticism, and it inust be attributed to this tendency that so many far-fetched explanations of the descensus testiculorum have been published. The changes in the gubernaculum are probably the immediate causes of a part of the changes in the position of the testis ; the growth of the gubernaculum accounts for the forward movement, and its atrophy for the passage along the wall of the processus vaginalis ; it must be added here that the testis also accompanies the downgrowth of the processus, and is not dragged down merely by the shortening of the gubernaculum. Some writers have supposed that the muscles of the gubernaculum effect the descent by their contraction, but this view lacks foundation.


The descensus has been carefully studied in the human embryo by Bramann, 84.1. The details of the process are as follows: The urogenital fold is a long structure reaching to the posterior or caudal end of the abdomen. The greater part forms the Wolffian body, and when this atrophies the fold is much reduced ; toward the head end it contains the testis and the remnant of the Wolffian body (epididymis) , the portion of the fold dorsal of these acting as a suspensory membrane to which the name of niesorchium has been given, and which is comparable to the mesentery ; it is quite thick, but finally disappears. The part of the urogenital fold tailward of the testis contains the Wolffian duct (vas deferens) and runs to the point of the abdomen, where the inguinal ring is developed. A portion of this region of the fold is converted into the gubernaculum Hunteri, by an ingrowth of muscular fibres from the obliquus intemus and obi. transversus. The mesorchium, together with the posterior part of the fold, including the gubernaculum, is the homologue in the male of the broad ligament of the female. To complete the statement of the homologies, it may be added that the gubernaculum becomes the cremaster, and is said to be the equivalent of the round ligament of the uterus in the other sex ; the latter identification needs confirmation.

Km, sm (Tn Illunrslfl t


a'lllumi. B, FcBliii lir the Hnit half of th n»l Is opened, re. Testis; i(>il, Pi'l'llrtytnls; i " Mm: r, 11. Iiladilw; r.*ii, vasa Hpvniiatlua. .


il.— A, Fo-tus f.( rourtwn to FlMmn Tenth moDth wllh lh« |irrH»TMii« vajci!iviis: ji.i\ pronTMUK TaglDulU: r. nt


The first change which occurs is the nearly complete disappearance of the long piece of the urogenital fold, which lies tailward of the testis. Accordingly we find the male gland at the end of the second month has moved into the immediate neighborhood of the inguinal ring, with which it ia connected by the short remnant of the fold, Fig. 269, A. The vaa deferens has a nearly horizontal transverse course. The second change is the conversion of this hind remnant of the urogenital fold into the gubemaculum, a process which begins with the fourth and enda with the sixtli month, it requiring about two months for the gubemaculum to attain its maximum size. To form the structure in question, the fold behind the testis enlarges both longitudinally and transversely until it measures 8-iJ mm. by ;i-I ram. ; the testis moves forward meanwhile a corresponding distance. At first the gubemaculum consists only of the peritoneum and the enclosod connective tissue, but soon muscular fibres appear in its caudal portion ; thcso fibres can be traced to a connertion with the obli<mu8 intemus and obliciuus transversus; they form a sheath or mantle underneath the peritoneum and around a central core of eonneutive tissue; at first they do not reiich to the testis, hut stop at that point where the gubemaculum is crossed by the vaa deferens. They appear to extend farther forward later. The fihrcs are not iiamllel, but quite irregidar in their courses. At the lower end of the muscle a buudlo of connective fibres extends beyond the gulwrnaculum into the side of the processus (see below). The gnl)ernaculum is now completely differentiated. This stage of the organ is iiermanent in some rodents and othi.'r mammaLs, low in the si'ries, and must bo ('onsidered of great phjdogenetic significance. Attention is directed to the fact tlrnt its muscular fibres are striped, and to its shajw shown in the figiin?, I)ecau«e Ixith points accentuate the n-wmblancc tn the rodent cremastor. While the gubernaeulum is being formed there appears at its caudal end a little ]X)uch made by an evagination of the iX'ritoiuHtm at the inguinal ring. This is the anlage of the processus rat/initHs; it lit^* latoridly and ventrally of the end of the guhtrmaculum ; it enlarges veri' slowh' up to the end of the sixth month, but after that more rapidly. The third change is the true descent of the testis ; the evagination of the processus inclutlea not only a considerable |>crit»ncal surface, but also the gubemaculum, and later the testis ; in other wtinls the urogenital fold extends dt>wn the processus and forms, indetnl, the dorsomedial wall of the sac; as the sac grt)Ws do\vn, the fold (gubemaculum and testis) grows with it, Fig. 360, B. In a tranaverse section the lumen of the processus vaginalis appears somewhat crescent-like, the concave wall corresponding to the protuberance of the urogenital foki, the convex wall to the peritoneal covering. The mesorchium disappears during the descent into the processus. Early in the seventh month, the testis is drawn into the mouth of the sac, Fig. 269, B, and shortly after lies wholly in the interior ttiereof. But the testis descends t« the bottom of the processus; this translation is accomplished during the seventh and eighth months, first by a shortening of the g^bemaculum, second by a slipping down of the testis over the muscles; the portion of the gubemaculum between the testis and the base o£ the processuB is reduced to an inconspicuous band of connective tissue. The muscle now lies between the testis iind the base of the penis and is developed in that position into the creinaster. At last the processus enters the scrotum, and an entirely new relation of parts is established; the transportation of the testis into the scrotum represents a very advanced stage, since it takes place only in the higher mammals, and accordingly we find it to occur very late in the development of man.


The Broad Ligament. — The broad ligament of human anatomy is the persistent urogenital fold, reduced to a relatively thin suspensory membrane for the ovary and uterus by the abortion of the Wolffian tubules. The fundamental relations here involved may be readily understood from Fig. 27(J, which represents a section through the urogenital fold of a human embryo of the i third month; the fold is suspended from the dorsal wall of the abdomen the Wolffian body is considerably alMirted and divided into two parts, -cru»a section or the o»ary one of which, near the base of the fold, ami WolrnsnBody of a Human Embryo is the nnlago of the parovarium. Par, p,,,'!'t,Bul*oron"* the other, near Muller's duct, J/t/, is the aulage of the ejKxiphoron, Epo.

The ovary, Ov, projects from what was originally the medial side of the Wolffian bo<lj', with which it is connected by a thin mesovarium. As in the adult the broad ligament contains the parovarium and e|)iM>phoron, it is evident that it is really the Wolffian body, converteil into a suspensory membrane, most of the Wolffian tubules Ix-ingalmrtcd.


The development of the broad ligament is accompanied by a change of jKisitiun, first of the whole genital fold, second of that part of the fold which forms the ligament. It will be remembered that the two folds unite in part to form the genit;il cord, out of which the uteni.s and vagina are developed; the remainder of each urogenital ridge is transformed in the female into the broad ligament and ovary. As the ftetiis grows, the urogenital ridge fails to grow proportionately, and after the second month becomes m<tre and more restricted ti) the caudal or pelvic end of the abdomen. Its relative position is no rapidly shifted that by the end of the third month it already occupies its permanent situation. While this modification is beinp^ established the WolflSan body in large part aborts, and the portion of the fold in front of the genital cords comes to occupy an oblique and finally a nearly transverse position, which is permanently retained, so that the broad ligament is always obliquely transverse. At three months the ovary is as long (3 mm.) as the Fallopian tube, and stretches in an obliquely transverse direction from the upper or cephalic end of the genital cord (future uterus) to the Mullerian fimnel or fimbriate opening of the Fallopian tube. By the fourth month the transverse position is more marked, and since the ovary is originally on the medial side of the urogenital ridge, it remains on that side, and accordingly is situated on what is known in human anatomy as the dorsal side of the ligament.


The development of the round ligament and of the ligament of the ovary have been but little studied by modem methods ; Mihalkovics, 86.1, 418, and G. Wieger, 86.1, have shown that they are parts of the same cord of tissues, and that by the assumption by the ovary of its transverse position this cord of tissue is subdivided into the two ligaments by becoming bent at the summit of the uterus. The primitive ligament is usually regarded as the homologue of the gubemaculum of the male.


Epididymis and Epoophoron. — It is desirable to treat this organ, which is known under diflFerent names in the two sexes, as a single organ, not, as is often done, as a distinct organ in each sex. In both sexes there is a small nimiber of permanently preserved and considerably modified Wolfiian tubules from the anterior part of the urogenital ridge, which remain permanently connecteil with the cephalic or upper end of Wolffian duct. The organ thus formed l)econies in both sexes ver^' closely associated with, indeed we might better say incorporated in, the sexual gland. In the female the organ is rudimentary and has been variously named ; as it was first accurately described by Rosenmuller, 02.1, it has been widely known as the ' organ of Rosenmuller ;" Kobelt, who demonstrated, 47.1, that it was a remnant of the primitive kidney, introduced the term "parovarium." Waldeyer has proposed, 70.1, 142, "epoophoron" to be comparable with the epididymis, with which he recognized the parovariiun to be homologous. In the male the organ has great functional importance, for its tubules serve to convey the s])ermatozoa from the seminiferous tubules to the Wolffian duct, and accordingly it is in the male that the full development of the organ is attained.


A. Epididymis. — In the male human embryo of the third month there are found from ten to twenty tubules in the anterior part of the Wolffian bodv, which have become connected with sexual cords or future seminiferous tubules of the testis, and have retained also their connection with the Wolffian duct. These tubules constitute the epididymis, and the portion of the Wolffian duct which follows immediately below them, by becoming very much convoluted gives rise to the so-called head of the epididymis. At three months (Kolliker, "Grundriss," 2te Aufl., 420) traces of glomeruli can be still found in the primitive kidney, and the epithelial tubules anastomose with one another in the region between the Wolffian body proper and the testis proper. These anastomoses constitute the rete Halleri^ while the Wolffian tubules become the vasa efferentia of the adult. According to Kolliker, Z.c, the vasa become convoluted during the fourth and fifth month, and thereby develop the so-called coni vasculosi.


The early development of the epididymis is known chiefly through Braun's observations, 77.4, 149, on reptile embryos. Solid outgrowths appear early from the walls of the Malpighian corpuscles of the Wolffian body, and these penetrate toward the testis as cords, which Hubsequently acquire a lumen. The primitive connection is between the tubules of the testis and the mesonephric glomeruli — a disposition which is permanent in some of the amphibia (see J. W. Spengel), but in all amniota the glomeruli disappear. C. K. Hofmann (Bronn's " Thierreich," VI., III. Abth., p. 2002) asserts, in opposition to Braun, that the glomeruli persist in Lacerta agilis at least one year after hatching. In mammals, Mihalkovics, 85. 1, 4?2, found the outgrowths from the glomeruli in cat, dog, and rabbit embryos of 5-6 cm., but the Malpighian corpuscles disappear early during embryonic life.


B. Epoophoron (or organ of Rosenmilller,) — Beyond tracing out the general history far enough to establish the homology with the epididymis fVValdeyer, 70.1, 142), little has been done to elucidate the development of the organ in the embryo. It has been already pointed out that the medullary cords of the ovarj^ are presumably parts of the epoophoron. The epoophoron is formed from Serhaps ten to fifteen Wolffian tubules, and the outgrowths from the lalpighian corpuscles remain, in part at least, solid cellular cords ; the Malpighian corpuscles of the organ disappear very early in the human embryo (? third month). F. Toumeux, 88.3, has described the epoophoron in various mammals and in the human species at birth and in the adult, and has shown that its structure entirely confirms its homology with the epididymis.

Paradidym^is and Paroophoron.— By these names is designated, in males and females respectively, the organ constituted by the persistent tubules of the posterior part of the Wolffian body. The organ was first described in the male by Giraldes, 61.1, under the name of the '* corps innomine," and was first described in the female by Waldeyer, 70. 1, 142. The persistent rudimentary^ mesonephros of the human embryo has a yellowish color; the tubules are wide, their cells pale with indistinct nuclei, and have not only no connection with the sexual gland, but have lost their original connection with the Wolffian duct. The position of the organ in the male and female human embrvo of about three months has been figured by Waldeyer. The interesting post-foetal changes have been made the subject of an excellent piiper by Czemy, 89. 1.


Genital Cord. — That the posterior (lower or caudal) ends of the tvvo urogenital ridges unite into a single median mass, the genital cord, has l)een pointed out above, p. 401. The genital cord is a structure peculiarly characteristic of the placental mammalia, being found only in them and in certain marsupials. It does not occur in monotromes or Sauropsida. The genital cord and its significance were first recognized by Thiersch, 52.1. The fullest history of the cord yet published is that given by Mibalkovics, 86.1, 324-347, upon which this section is based.


The pelvic portions of the two urogenital ridges unite so as to form a transverse partition (rabbit embryo of about 14 mm., pig embryo of about 30 mm.). This partition ia the genital cord {Genttalstrang) of Thiersch. It stretches across between the rectum, which is on the dorsal side, and the allantois on the ventral side, compare Fig. 271; it ia thick and the mesenchyma of which it is chiefly composed is a dense tissue. At the time the two ridges unite, their pelvic ends contain only the Wolflian ducts, hence the Miillerian ducts, as they develop, grow into the already formed genital cord, and in the female (human embryo of 3 cm.) begin to unite almost immediately after they appear in the cord. The formation and position of the partition is well illustrated by Mihalkovics, 86.1, Figs. 114 and 115. After the genital cord is once formed, it is drawn more and more into the pelvis, and as the coelom extends farther into the pelvis on the dorsal than on the ventral side of the cord, we obtain in sections the picture reproducd in Fig. 271, which shows the typical relations of the genital cord in the indifferent stage; the cord consists chiefly of a very dense mesenchj-ma, and is


Suite sharply bounded, except against the allantois, and it contains iree longitudinal epithelial tubes, of which the nieilian rejtresents the united Miillerian ducts, the two lateral thi> Wolffiau ducts, fi'.d.


Wolffian Duct. — The cephalic end of the duct remains, as we have seen, in connection with the anterior Wolffian tubules as the duct of the epididymis of the male, of the eixxiphoron of the female. In the male it also forms the adult spermiduct and the vesiculra seminales, in the female the rudimentary duet of Gartner. It is important to note that in both sexes the Wolflfian duct contributes to the formation of the utero- vaginal canal (fused Mullerian ducts) according to the observations of Mihalkovics, 85.1, and Toumeux, 87. 2, upon the male, and of Van Ackeren, 89. 1, upon the female.


A. Spermiduct of the Male. — Since the demonstration, by Johannes Miiller, that the Wolffian duct becomes the spermiduct, little has been done upon the history of the male canal. Thiersch in 1852 drew attention to the imion of the caudal parts of the urogenital fold as the genital cord, see above. This cord exists temporarily in the embryo of man, and while it lasts the two spermiducts run through it, together with the two Mullerian ducts, which partially abort later. This stage is described and figured by Kolliker (" Entwickelungsgeschichte," 1879, p. 985 and Fig. 598). Later the genital cord divides, and its dense tissue forms a thick wall around each epithelial Wolffian duct.


The vesicula seminalis arises as a lateral evagination of the Wolffian duct. At five months the evagination is a simple sac about 1 mm. long, and is situated entirely within the genital cord. The evagination passes at first out horizontally, and then bends upward (Mihalkovics, 85.1, 379).


B. Gartner's Canals of the Female, so named after their discoverer, are epithelial tubes which are sometimes found in the walls of the uterus, and even of the vagina, one on each side. Their significance is said to have been first recognized by Jacobson in 1830 nnd to have been clearly demonstrated by Kobelt in 1847. It is known that the Wolffian ducts always run through the genital cord, Fig. 271, and can be usually seen in cross sections of the uterine ]X)rti()n of the genital cord of the female human embryo of four to five months, and occiisionally in older specimens, and even in the adult. On the disappearance of the Wolffian duct see Van Ackeren, 89.1, 34. In the foetus the Wolffian ducts open into the vagina during the fourth month; their ends dilate, and the dilated cavities fuse with the lumen of the vagina.


Mullerian Duct

The history of this duct is the reverse of that of the Wolffian duct, since it becomes rudimentary in the male, and the functional sexual duct in the female.

A. Male

Hydatid of Morgagni and Uterus Masculinus. — In the male the middle part of the Mullerian duct usually aborts, leaving the upper part with its open funnel close to the testis and the lower part within the genital cord. The upper part gives rise to the hydatid of Morgaqiii as maintained by Kobelt, 47.1, and later by Waldeyer, 70.1, 127. This explanation of the origin of the hydatid has by no means been put beyond question by strict obsc^rvations — but we need no additional evidence to set aside the notion of Fleischl and Krause that the hydatid is the homologue of the ovary ( !) . The lower part of the Mullerian duct is contained within the genital cord, where it unites with its fellow to form a single median canal between the two Wolffian ducts — compare K31likor, '* Entwickelungsgeschichte," 1871), Fig. 598, m. This ( corresponds to the cavity of the uterus and vagina in the It varies greatly in its degree of development in individual usually persists iu the adult as a small sac, known as the vterus masculinus or veaicula prostatica. According to E. H. Weber, ttie veaicula, which is rudimentary in man, ia well developed in various mammals. For references to the literature of the subject, and an account of the organ in the rabbit, see V. von Mihalkovics, 86. 1, 3G4-378. The Wolffian duct contributes to the formation of the uterus masculinus, as it does to the formation of the vagina iu the female, see p. 606.


B. Female

Fallopian Tube, Uterus, and Vagina. — These are developed from the Mullerian ducts, but it is to be remembered that, strictly speaking, the epithelial Millleriiin ducts produce only the epithelial lining of the adult tubti, uterus, and vaKinn, and that the connective tissue, which forma the thickest i)art of the walls in the adult, is developed from the mesenchjina of the urogenital fold. Fallopian Tube. — The fullest account is that given bv Mihalkovics, 86.1, 304-306. The tube is develoi>ed from that part of the Mullerian duct which runs along the Wolffian body and is not includetl in the genital cord. The epithelium becomes much thinner except in the funnel, where it retains its cylindrical character. Later — in chicks about the eighth to tenth day — the inesenchyma l)egiiis to condense around the duct, thus initiating the development of the connective - tissue coats of the shortly after the mcsenchyma wall begins to develop the Mullerian funnel becomes larger, and its surface thrown into folds — the / ;" - • atilagea of the fimbria. As the Wulffian body atrophies and changes into the transverse broad

F, of a ligament. the Fallopian tube apTnwiti Human Embi^o of four MonthiL pears morc and more at the edge i^IS'Q'iSbri-tr; 2^d wolnta."d™t;l^*^ of the urogenital fold, and changes its primitive longitudinal course to a transverse one — the primitive course Ix^ing retained until the end of the tltird month. After the third month the tnbo elongates fcister than the broiid ligament and con8e<iuently assumes a sinuous course. By the sixth or seventh month, the definite transverse ixwition is attainetl. By the fourth month, Fig. at 3, the folds at the ovarial end of the tul>e, F. T, are well developed, but the thick dense niesencliymal coat is not yet divided into mu-^cular and lulventitial layers; at this time the small Wolffian duct, M'.il, still i)ersists, though later it usually disappears.


Uterus and Vagina. — As stated al>ove, p. 50?, the genital coni contains four ducts, compare Fig. I : the two laterally placed Wolffian ducts, and the two Mullerian ducts, which lie nearer the meilian line and more dorsally. In man the genital ciird is the anlage of lx)th the uterus and the vagina; within the <'i)rd the two Miillerian ducts unite in the meiliau line, forming a single caual; the cephalad portion of this canal becomes dilated into the uterine cavity, and its epithelium becomes the lining of the uterus; the caudad portion develops into the vagina; the mesodermic tissue of the cord is converted into the muscular and connective tissue layers of the adult passages; finally the WolflBan ducts atrophy, ususally completely, but they sometimes persist to a greater or less extent as rudiments, known as Gartner's canals, which lie on one or both sides in the walls of the uterus.


Our knowledge of the development of the uterus and vagina is based upon numerous investigations. The fusion of the Miillerian ducts was known to Johannes Miiller, 30.1, in 1830, but he failed to ascertain that the fusion protluced not only the uterus, but also the vagina ; the latter was regarded for some time as a derivative of the urogenital sinus. This error was corrected by Bischoflf in his manual of "Embryology." Important advances were made by Thiersch, 52. 1, and by Leuckart in his important article " Zeugung," in Wagner's " Handworterbuch," 1853. We pass to the modem period of investigations based chiefly on the microscopic study of sections. L. Fiirst described in 1867 the fusion of the Miillerian ducts very accurately. H. Dohrn's researches (" Schriften Ges. Nat.wiss.," Marburg, 18G9, No. 3, also Bd. IX., 1871, p. 255) confirmed Fiirst 's observations and did much to elucidate the history of the uterus. Finally may be mentioned the very admirable monographs of Tourneux and Legay, 84.1, of Mihalkovics, 86.1, 332 and 347, and W. Nagel, 91.2, which have been my chief guides in the preparation of the following account, and to which I refer the reader for a fuller index of the literature. Interesting additional details have been recorded by Van Ackeren, 89. 1.


The genital cord extends by the fourth month from the insertion of the Hunterian or round ligaments to the urogenital sinus. The Miiller 's ducts fuse in the median line between these two points, except at the upper end ; that is to say, the ducts diverge, after the complete fusion, a little below the roiind ligaments and these divergent portions become the horns of the uterus. The fusion commences at the end of the eighth week about two-thirds of the* way down from the cephalad end of the cord to the urogenital sinus, and progresses from that point both upward and downward, but the upper two-thirds are united before the lower extremities. The process is completed according to Fiirst by the end of the third month. In the pig, mouse, and rabbit, the fusion commences at the same relative point, but in the sheep it begins higher up. The single canal thus pro<iuced is known as the genital canals or better as the uterovaginal canaL A failure of the lower ends to fuse leaves two openings (double or bi perforate hymen).


W. Nagel, 91.1, has pointed out that the genital cord becomes bent very early in the human embryo, so as to divide the cord into an upi^r or uterine limb, which is inclined ventralward over the bladder, and lower or vaginal 4imb, which runs longitudinally between the bladder and rectum. At the end of the third month, the simple epithelium lining the cavity of the canal changes its character in its lower third, becoming there a stratified pavement epithelium, which passes over very gradually into the cylindrical epithelium of the upper portion. The change progresses upward, and as it advances, the demarcation between the two kinds of epithelium becomes sharper. By the eighth month the passage is abrupt and occurs at the middle of the canal. The stratified epithelium lines the vaginal limb, which occupies half the genital cord at birth. After birth the uterine limb enlarges more rapidly than the vaginal.

Vagina

During the fourth month the vaginal limb expands laterally and becomes flattened dorso-ventrally. Its two epithelial surfaces meet and grow together, closing the lumen of the vagina and forming an epithelial lamina, the cells of which now commence a rapid proliferation which thickens the vagina and forces down its lower end, thus forming the hymen because the ac^tual diameter of the vagina, where it is connected with the sinus, does not share in the general dilatation. The epithelial plata of the vagina has two features requiring special mention: 1. A short distance above the sinus it is T-shaped in transverse section ; the two side portions are probably remnants of the Wolffian ducts which unite with the vagina at this point. In this connection it is significant to observe that in the cow the persistent ducts of Gartner (Wolff) open into the vagina ; the question arises whether this connection is not general in the Placentalia. 2. At its upper end the lamina forms a cup-shaped outgrowth, which embraces the lower end of the uterus. Everywhere between the two points thus specialized the lamina is crescentic in section, the concavity facing the back. The anlages of the rugje of the vagina appear during the end of the fourth month as budding ridges on the outside of the lamina. Finally the ix?rmanent lumen of the vagina begins to appear during the sixth month and is formeil by the breaking down of the central cells of the lamina. This process penetrates the cup-shaped outgrowth just descrited, so that the lower end of the uterus protrudes into the vagina, in consequen(*e, be it remarked, of the vagina growing up around the extremity of the uterus. The stratified epithelium often extends a short distance inside the os uteri.

Uterus

The cavity of the uterine limb is always open, and its epithelium comjwsed of a single layer of cells, which diminish in height from 5(^/ji (third month) to 25// (eighth month). A short time before birth the epithelium of the cervix develops into l)eaker cells. This transformation has ])een well described bv Moricke, 82. 1. The cells increase in length and the nuclei move toward the base of the layer; the upper portion becomes clear and no Irmger st^iin with picrocamiine owing to the formation of mucus. Thest> cells secrete the mucous plug which fills the cervix at birth. As far as ascertained there are no cili«i in the foetal uterus. The development of the arbor vit^e of the ut<*nis commences at the end of the fourth month with the appearance of the main stems (rachis), which extend from a little above the future os nearly to the fundus. Their disposition is asymmetrical, the two stems of the posterior wall lying to the left, of the anterior wall to the right ; hence the ciivity c^f the uterus is somewhat S-shaped in section. The arl)or vitae is merely a set of folds of the uterine mucosa.


The mestxlerm of the genital cord diff(*rentiates ver\' slowly. The first noticeable change is the increased vascularity of the part next the epithelium; this vascular layer becomes the mucosa, and the tissue outside it the muscularis. The latter does not become distinct histologically pntil the close of the fifth month. The muscular fibres are very irregularly disposed; however, the trend of the inner ones is circular, of the outer longitudinal.


The glands of the uterus and vagina do not appear until after birth, except in the cervix uteri, the glands of which arise the middle of the fifth month (Van Ackeren, 89.1). Cadiat, 84.1, maintains that those of the corpus uterus arise during foetal life. This opinion I consider erroneous ; has not Cadiat mistaken folds of the arbor vitse for the anlages of glands?


The following table indicates the growth of the uterus and vagina :


Fcptiis from vertex to eiiciryx.


Supposed Age.


Caual, Length.


' Vagiua, Length.


Vtcrus.


7.5 cm

9.0



5. mm.

6.5 13.0 21. 39.


1 2.2 mm.

3.5 ! 6.5 ' 10. 16.


2. 8 mm. 3.0


12.5


6 5


16.0


11.


20


13.




Chilli


Eight days. Four months. Three years.


35. 50. 65.


10. 30. 40.


25.


Child

Child


20. 25.


A few words must be added concerning the comparative morphology of the uterus. The round or Hunter's ligaments mark in all mammals the division between the Fallopian and the uterine portion of Miiller's ducts. In man the whole of the uterine division is included in the genital cord and participates in the formation of the single median uterus.


====Hymen====The hymen is said to be the homologue of the verum montanum of the male urethra. It appears about the beginning of the fifth month as a transverse ridge situated on the ventral side of the vestibular end of the vagina, and projecting into the urogenital sinus (vestibulum) . At this time the vagina begins its dilatation, and as it widens it appears to force down the hymen, which is thereby rendered more protuberant. The hymen is a thin non -muscular fold covered on one surface by the epithelium of the sinus, and on the other by the epithelium of the vagina, the latter being much thicker than the former. The hymen grows rapidly after its first appearance. When, as may happen by an arrest of development, the lower ends of the Miillerian ducts do not fuse, the hymen presents two orifices leading into a single vagina (H. Dohm, 75.1; Tourneux and Legay, 84.1, 345). The development of the hymen has been studied by H. Dohrn, 75.1, 78.1, Tourneux et Legay, 84.1, Mihalkovics, 86.1, 340, and Van Ackeren, 89.1, 30.


Development of the Kidney.* — The true or permanent maniote kidney has no homologue, so far as known, in the amniota, the so-called kidneys of the latter teing Wolffian iKxlies (inesonephri). We are entirely unable at present to trace the probable evolution of the kidney, for the view advanc*ed by Semper, 75.2, that it is a modification of the hind end of the Wolffian body is negatived by the embryonic development of the kidney. Nor do we pOBsess any light as to the factors by which the development of the kidneyB is initiated in embryos. In short, we are compelled to confine ourselves to a bare narration of the actual development, as known at present.

  • For further detaihi see Ove Hauiburger, 90. 1.


The Renal anlage

The renal anlage consists of three i>arts, the epithelial evagination of the Wolffian duct, the condensed mesenchyma, and Braun'a cords, which appear in the order named.

The epithelial evagination appears on the dorsal tiide of tlio Wolffian duct near the opening of the duct into the intestin;d canal (cloaca) . The evagination appears in the chick at the end of the fourth day, in crocodiles of 13 mm., in Lacerta agilis about eight days after the eggs are laid, in the rabljit the eleventh day, iu slwp embryos of 8 mm., in human embryos of 5 mm. The evagination rapidly changes in character : first, bv elongating forward and by the enlargement of its cephalic end. Fig. 444, iV; seroiul, by acquiring (in the chick by the uixth day) a direct opening into the m-ogenital sinus or hind end of the intestinal canal. The enlarged blind entl is the anlage of the epithelial portions of the kidn?y, that is to say, of the lining of the pelvis and of the renal tubules ; the remainder of the evagination becomes a long nan-ow tube, which iiwiy be at tmce designated as the ureter, although it corresponds, of course, only to the epithelial lining of the a«lult ureter. The way in which the evagination grows is well illustrated in Fig. 444, B,I>,C.


The blind end of the renal evagination grows forwan! on the dorsad side of the Woltfiiui l>ody and continues this growth while it is developing into the kidney, so that the more advanowl the kidaey in its differentiation, the more of the Wolffian body is covered dorsally by it. The niosenchyma around the blind end very soon bcKTomes condenseil, but the condeiiMition, at least in croc-odiles, iwcnrs chiefly on the medial side of the- renal tube. the relations just deacrilx^l are well illustrated in Fig. 'i7:i. The condensed niesenohyma can 1)6 followed some di^bmce along the ureter and there gKwhudly bes CmriHiiii.- comes looser, and its concentric arrangement disj'ppcars (Wiedersi!!fS?r™r™nlS!I"v"h\'!'1;.^iiin"""n'^ heim, 90.3, 44(1). The primitive foniitii^kiciiii-y; Hirt,i:uniienii«iniew;ii<.-hyiiia. auhige of the kidncv, therefore comprises the dilattnl end of an epithelial tnlw and condensed inesencliyma. It is convenient to consider the history of the two seiwirately.


====Mesenchyma====The histogenesis of the mesenrliymal portions of the kidney is almost unknown. It seems to me iwirticularly desirable that the historj' of the blood-vessels shonld l)e asofrtninc«l. ftolgi, 88.1, 341, observed that in the fa'tal kidney the artorit'^ Hnbdivi<le and form an irregular network of capillaries in the peripheral portion of the OT^an, and when the glomerulus begins to form it contaioB a single looD of this network, and later from this primary loop secondar>' loops Dud forth until the circulation of the glomerulus is completed.


Fig. S7Z. — Croiw Si


It is important to note that the fibrous capsule is developed very early, before there are any glomeruli — for instance, it is present in the kidney of an embryo rabbit of fifteen days, and at sixteen days is figured as quite thick by Kolliker (" Entwickelungsgesch.," ISTSl, Fig. 581). The capsule is definitely present in human embryos'of 30-'25 mm. length and is formed of spindle-shaped anastomosing cells (W, JTagel, 88.3, SGT). My observations have led me to believe that the capsule is the essential mechanical condition for tho development of the glomeruli, compare below.


Tubules and Malpiohian Corpuscules. — The tubules arise as branches of the blind end of the renal evagination and the blind enils of the branches form the so-called Malpighian corpuscles. The branching begins very early, compare Fig. 444, D,C, and gradually a distinction becomes recognizable between the enlarged end of the ureter, destined to form the pelvis and the tubules proper^a distinction which becomes more and more marked as develop ;; — ^^* T~tl_ , men t progresses. The branches are at fitb -^-^ "^ ~ t? "5 " :=^ short but wide, and form wide angles with •^'•uf^ one another; their walls are a rather high -^^^ cylinder epithelium. At an early period — in the rabbit by the fourteenth day — the branches reacli the capsule, which has moan while been differentiated from the surround ing mesenchyma. The capsule seems to prevent the further elongation of the branch , in its line of growth, and to force the end of the branch to curl over, thus by a simple ^^ mechanical condition causing the formation j^ ^ of the anlage of the Malpighian corpuscle tt.i This role of tho capsule has not been noticed hitherto, so far as I am aware. My atten tion was called to it by observing that in older kidneys (human ombrj-os of three four, and five months) the formation of the Malpighian corpuscles always goes on close against the capsule. Fig. -^74; one sees a straight collecting -tubule, which nms to tho capsule and there l)end8 over into the anlage of the convoluted tubule and Malpi- fki. arj.-Secri.m ..r a Kidnnr,


Human Kinlm'n of ahc

ghian corpuscle; the younger the corpuscle MomhM. Mino«c.)iiH.-tii«i._

the nearer is it to the capsule. To explain the difference in jKJsitiiiii, we must assume tuimie. that the corpuscles remain approximately where they arise and that the capsule enlai^s, and thereby gives opportunity fi>r new M)d])igliian cor]>uscles to l>e develoiMHl outside of the older ones — examination of the carefully drawn Fig. 2T(; will make the distribution of the corpuscles clear. The collecting tubules appear to all arise as branches, at first from the end of the ureter, after that from the collecting tubules already formed — the details of their developiiient have still to be ascertained; at first the branches devnricate at wide angles, but later they show the characteristic U-shaiied fork of the adult, compare Fig. a*G, col. The convoluted tubules and Mfilpii/liiun corpuscles develop according to Golgi, 89.1, as follows: The end of the tubules bend over, Fig. ST5, into an S-shape; in Golgi's diagrams each main tubule is represented as forming two convoluto<l tubules at once; whether this is the case is not quite clear from his text, but it is prolmbly true, I think, of the first-formed Malpighian corpuscles, but later each straight tubule forms, so far as I can oluierve, only one corpuscle and convoluted tubule. The different parts of the S-shaped tubule have each their fixed destiny. The end of the S (in the diagrams the lower part) receives the vascular loop, which gives rise to the bloodvessels of the future glomemlus, gl; the lower limb of the S, n. elongates enormously and forma the first division of the convolutetl tubule including the loop of Henle, S; the upper limb, b, of the S also elongates very much — though less than the lower limb — and is the anlage of the second division of the convoluted tubule; where the two limbs join the tubule i«»sses close to the Malpighian corpuscle and seems to be intimately attachei.1 to it. This attachntent is preserved, aci^urding to Golgi, in the adult kidney. During developmc'nt it acts as a fixed [Kiint, which parts the convoluteil tubule into two primary divisions, which, as is well known, are persistent. Honle's loop rapidly elongates in the direction puridlel to the straight <ir irollecting tubule and toward the medulla, its elongation [x'rhaiM exiilainiug why it increasi's in diameter less rapidlj- than the remaining parts of the tubules. The development of the corpuscles has I»een descrilM'd ipiite fully by Toldt, 74.1, and als<> by Kollikcr in his " EntwickelungiHp«cihichte, l.-^T'.i," H+'J, but it is to K- n<it«l that the S-shaped tubule is not merely the anlage of the Mal])ighian corpuscles. as supjxiseil by these authors, but also of the convohift-d tubule. The blind end alone forms the corpuscle ; the wall of this end is pushe*! in by the very formation of the S, and the end fissumcs somewhat the shape of a rubber ball with one side pusht'd in (Toldt), in the concavity of which a network of capilliiries npi»oars, Fig. -JTS, ifl. In older kidneys of the human embryo, the concave side is always tumeJ away from the straight collecting tubule with which the cor[)u8cle 8 counected F g b Tl e ep thel um upon the con ex Bide \a mucl thinner than hat of the con a e s de and as development progresses tl is d Sere ce beco nes more marked the space of the tubule is the cavity of the corpuscle ; the thin epithelium is the lining of Bowman's capsule; the thicker epithelium covers the glomerulus. The further differentiation depends chietiy upon the assumption of the spherical form and upon the growth of the glomerulus and its vessels. The original area, by which the vessels enter the glomerulus, remnins about the same, or perhaps even diminishes in size, but the Malpigbian corpuRcle grows, and hence the neck by which the vessels enter l)ecome8 relatively much sniHller. The corpuscles continue their growth for a long period, and are smaller in the fcetiM than in the adult, therefore they must continue to grow after birth.


Some authors liave maintained that there is an atrophy of some of the tubules of the foetal kidney, but I agree with Golgi, 89.1, in believing that of this there is no valid evidence,


I present figures of two typical sections of human fcetal kidneys, FigB. 276, 3T7. The first. Fig. 370, represents a radial section of a kidney at nlK>ut five months. The capsule, Cnp, is fibrous iiud thick. The separation of the cortical, C, and modullarj- zones, M, is given by the diHtribution of the Malpighian corpuscles, of which the youngest stagt's are found near, the oldest farthest from, the capsule ; between the two zones are situateil tlio main bloml-vessels, I'l; drawni dark in the figure; the mcilullarj" rays, H, are di.-jtiuct, but consist each of only a few tubules : the ci>nvolute<l tubules, cc, are ver^' pale and not all of thein are represented ; to render the figure clearer they are drawn without nuclei ; Henle's loops. J?, are finmd at all levels, and show, aa yet, no very distinctive histological features; the collecting tubules, coll^ are large and show the typical branching with great perfection. Especially characteristic of the foetal kidney is the large proportion of connective tissue, and the consequent wide separation of the tubides. The second section is through the medulla at right angles to the direction of the tubules. Fig. 277. Here the wide separation of the tubules by connective tissue is more apparent than in the previous figure. The collecting tubules are large and have a cj'linder epithelium with evenly placed nuclei; the Henle's tubules, J?, are much smaller, but vary greatly in size; as Golgi has pointed out, it is sometimes the ascending, sometimes the descending, limb which is small. Every collecting tubule is surrounded by a space, which at first I thought artificial, but as I find it in all specimens, including the freshest and best preserved, I conclude that it exists during life, and regard it as probably a lymph space.


Braun's Cords. — Max Braun, 77.4, 109-201, described cords of cells, which extend in very early stages of the embryos of lizards through the renal anlage. These cords differ but little from the rest of the mesenchyma, except in having their cells more closely crowded together, and that they can be traced to a direct connection with the mesothelium. This observation has since been confirmed — on chicks by A. Sedgwick, 80.1, on crocodile and turtle embryos by R. Wiedersheim, 90.3. The cords I must regard from Braun's ovra descriptions as merely the beginning of the condensed mesenchyma of the renal anlage. The three authors who have observed the cords regard them as the anlages of the convoluted tubules, though they bring no direct proof in support of this view, and since it has been positively demonstrated that the convoluted tubules arise from the collecting tubules, the view in question must be abandoned.


Shape. — The kidney is early marked out definitely by the development of its capsule, and in its first form is already "kidneyslia|>ed," and has a smooth surface. When the development of the Malpighian corpuscles begins, the surface of the kidney chianges, and at ten weeks (Burdach) is already divided into lobes, separated from one another by shallow but sharply defined depressions. The number of lobes is usually about eighteen in the human embryo, but Burdach (" Physiologie, " Bd. II., 1828) describes eight lobes at ten weeks. The lobato stage is found in all amniota and is j^rmanent in Sauropsida and cetficeans, but in most mammalia is confined to the foetal period. In man the lobes disappear soon after birth and the surface of the kidney again becomes smooth. Each lobe corresponds to the base of a Malpighian pyramid.


The appearance of the foetal kidney is also affected by its upper end l)eing covered by the relatively large hood fonned by the suprarenal capsule.


Human Kidney. — The following dates as to the development in man are taken cliiefly from Kolliker's '^Entwickelungsgeschichte," 1870, p. 052. In an embryo six or seven weeks old the kidney measured If mm., was flattened, bean-shaped, and overlaid the Wolffian Ixxly. In the eighth week, it measured 2.5 mm. long, and lay far behind the large supra-renal capsule, with which •it comes in contact during the third month. The lobules, as first fully described by Toldt, 74.1, appear during the second month and remain marked upon the external renal surface throughout foetal life. The Malpighian corpuscles begin to form toward the end of the second month, and continue forming until a few weeks after birth. The Henle's loops, as shown by Qolgi, 89.1, begin their development immediately after the corpuscles appear, and may be recognized in threemonths' embryos, as I have observed, but are not well developed until the fourth month.


Ureter. — Concerning the embryonic history of the ureter little is known. Kupffer, 66.1, 66.1, has shown that the stretch of the WolflSan duct between the original evagination and the urogenital Sinus elongates somewhat, but as development proceeds this part becomes included more and more in the sinus, with the result that the two- canals open separately. During these changes, the ureter becomes twisted so that its opening is situated in front of that of the Wolffian duct.


As to the histogenesis of the ureter I know of no observations.

Historical Note. — Remak, 60.1, was the first to describe correctly the development of the kidney ; he obser^ed the forward growth of the ureter from the cloaca, the enlargement of the end of the ureter, and the outgrowth from it to produce the collecting and convoluted tubules. Kupffer, 66.1, 66.1, showed that the ureter was an evagination of the Wolffian duct near the cloaca, and this has since been confirmed by numerous observations on all classes of amniota ; Kupffer added also the erroneous notion that the uriniferous tubules do not all arise as products of the ureter. Unfortunately Kupffer's error was upheld by Bomhaupt, 67.1, Colberg {CbL Med. Wiss,,lSCy'S). Goette,Thay8en, 78. 1, Braun, 77.4, Sedgwick, 80. 1, Balfour, Riedel, 74.1, and Emery, 88.1, and even Wiedersheim, 90.8. The authors since Braun have been largely influenced by theoretical considerations, especially by the wish to demonstrate that the true kidney is developed similarly to the WolflSan body (mesonephros), in other words that its secretory tubules are different in origin from its ducts. Remak's original view found few upholders, of whom Waldeyer, 70.1, 132, Toldt, 74.1, KoUiker ("Entwickelungsgeschichte," 1871)), and Golgi, 89.1, and W. Nagel, 89.3, 365, are certainly the most importimt. Golgi may be said to have put the matter beyond debate so far as mammals are concerned. Mv own observations enable me to affirm with confidence that the tubules arise as evaginations of the ureter, and that in man the convoluted tubules and Malpighian corpuscles arise as branches of the collecting tubules. The facts are so clear that it is difficult now to understand how the opinion could have been entertained that the convoluteil tubules arose from the blastema, and not as outgrowths of the collec*ting tubules.


Allantois and Bladder.— That jx^rtion of the allantoir. which lies within the IxkIv of the enibrvo, and extends from the anu.s to the umbilicus, becomesthe bbidder. It lias been mentioned already that the ureters very early se|)arate entirely in mammals from the Wolffian ducts and come to oi)en into the neck of the allantois. The dilatation of the embryonic jHirtion of the allantois to a fusiform vesicle begins in man during the seomd month; one end of the vesicle is connection with the anal end of the intestinal canal, while the other end tapers out and is prolonged as the so-called urachus, into which the cavity of the vesicle is prolonged, but at some time not yet definitely ascertained the cavity of uie urachus disappears, though seldom completely, for Luschka (Virchow's ^rcA., aXIII.) foimd remnants of it even in the adult. The urachus is transformed into the ligamentum vesicae medium (Kolliker, "Entwickelungsgesch.,** 1879, p. 953).


The main vesicle becomes the bladder. The entoderm of the allantois becomes the epithelium, and the mesenchyma becomes the connective tissue and muscular walls of the bladder. The histogenesis and changes in shape of the embryonic bladder have still to be investigated.


Recently Retterer, 90. 1, and Keibel, 91.1, have revived Rathke*s conception, 32.1, I., 57, that the bladder is an outgrowth of the cloaca, which becomes early divided into a dorsal or intestinal and ventral or allantoic division. The distinction seems to me arbitrary between this notion and the view adopted above, since the allantois is in any case a prolongation of the entodermal canal, and neither Retterer nor Keibel show that there is a true division of the cloaca.


Urogenital Sinus. — I base this section upon Mihalkovics' monograph, 86.1, 307-324. As shown in Fig. 444, the allantois is the direct continuation of the intestinal canal, and the urogenital ducts open into the allantoic portion or the part of the canal on the ventral side of the future anus. After the anus is formed, there is a terminal portion, the so-called cloaca, into which both the intestinal canal proper and the allantoic canal open. The greater part of the allantois dilates into the bladder, but between the bladder and the cloaca the allantoic canal remains narrower ; it is into this narrower portion that the Miillerian ducts open; the stretch between the bladder proper and the opening of Muller's duct is the urethra in the strict sense, while the part below received from Johannes Miiller the name of sinus nrogemtalis. The female adult urethra corresponds to the urethra as here defined,* but the male urethra includes both the urethra and the sinus. This may be called the monotreme stage, and is characterized by there being merely a single or cloacal opening, through which the excrement, urine, and genital products are discharged ; the stage is the i)ermanent one in non-mammalian vertebrates and in monotremata. An important advance is made in placental mammals by the subdivision of the cloacal opening into the ventral urogenital opening and the dorsal anal opening, which takes place in the human embryo about the fourteenth week, and involves the complete separation of the urogenital sinus from the intestinal canal. In the male the closure of the raphe penis converts the sinus into the pn^longation of the urethra proper, as we may term the neck of the allantois or bladder above the oi)ening of the fused Miillerian ducts (uterus masculinus). In the female the sinus persists as the vestibulum into which the urethra and vagina both open. The separated urogenital and anal openings lie at first in a shallow fossa or recess, the raised edges of which represent the anlages of the external genitalia; see the following section.


Prostatic Gland

This gland is present during the fourth (Kolliker) or fifth month (Mihalkovics) as a series of branching evaginations of the epithelium of the lower end of the urogenital sinus, which expand into wide irregular cavities. The muscular tissue is develoi>ed much later f i*om the mesenchyma of the walls of the sinus (Kolliker, *'Entwickelungsges., istu, p. 10(H), and Mihalkovics, 86.1, 378). The evaginations make their first appearance, according to Tourneux, 89.1, '^57, about the twelfth or thirteenth week, and i^ersist in the female, though more or less in a rudimeutar>' condition (Tourneux, Sex. Biol., Paris, Jan., 188S).


Cowper's and Bartholin's Glands

These names are applied to the same glands in the male and female resixx*tively ; they arise as paired evaginations of the lower part of the urogcniital sinus. According to Van Ackeren, 89.1, 44, the glands of Bartholini U^gin their development in man toward tiie end of the fourth month; during the fifth month the branches (acini) increase in numl)er an*! are found septirated from one another, by considerable mc^senchymal tissue^' ; by the sixth month, as jJready described by R. (xeigel, 83. 1, they form considerable organs lxl.8 mm., of roundt^d fonn, but the left gland is a little smaller than the right; tli(» acini now lie close together.

III. External Genitalia

The main facts in the development of tln^ external genitalia, and the homologies l)etween the two sexes, were workeil out by Tii^denwinn — see his Anatomic der kopfiosen Misgeburten," Landshut, 181)3, p. 84. A very good description of the fa*tal jx^nis and clitoris is given by Job. Fr. Meckel in his "* Handbuch der menschlichen Anatomie," 1815-18-20, so tlmt Johannes Miiller in ls;5(», 30.1, could add but little. Some further details have lxH?n given by H. Rathke, 32.2, and by KiJliker. Ecker in his " Icones Physiologicse has given a series of figures, which have lxH?n ext(Misively copieil in text-b(X)ks, and have been repr(.)duced in the well-known and somewhat inaccurate wax mcnlels of Ziegler. In 18S8-8U appeared Tourneux's admirable monographs, 88.1, 89.1, ujx)!! which the following account is based. We have to consider the history, 1, of the genital tulx^rcle (jienis-clitoris) , 'i, of the genital labia (scrotumlabia majora). The external genitals are homologous in the two sexes, but in the male they are nu)re specialized than in the female; the condition in the adult female corresjXMids to that of the ftetal male.


Genital Tubercle. — The anal plate IxM'omes veiy nuich thickened until it constitutes — shee]) embryos l)]-**5 mm., ])ig embryos 14-*20 nun. — a thick plug of e[)ithelium, on the dorsal side of which ap])ears an external invagination. Fig. ".^Ts, the restihule auah.* of Tourneux, which gradually penetrates until it h»aves only a thin epithelial membrane to close the rectum, while the main plug {oondion clo(trnl) closes the urogenital sinus or piMlicle of tin* allantois, *S. n(/. The accumulation of mesenchyma on thi» ventral side of the e])ithelial plug, />/. of the cloaca is indicated by an (External prominence, which may Ix? already d(»siguated as tht^ genital tulnTcle, t. (/. As development progresses the genital tubercle rapidly lengthens and the epithelium upon its dorsal side is reduced from a great plug to a thin layer, and by the disappearance of the plug both rectum and urogenital sinus become open to the exterior ; th«; mass of tissue between the two openings is termed by Toumeux the repli or eperun perineal. The genital tubercle owes its origin to the thickening of the anal plate, it gives rise to the penis in the male, to the clitoris and nympliie in the female. The tubercle is quite prominent — measuring 1.5 mm, in length — in the human pmbr\o by the tenth week and is then found to have its end knob-like, indicating the future glans, and its dorsal or anal side with a shallow groove, which directly prolongs the channel of the urogenital sinus, but only as far as the knob of the glans. About the tenth week there appear two eminences alongside the genital tubercle and the urogenital opening, which -we may call the genital labia — comjmre Fig. 2>iO, lab. The labia are solid hillocks of mesenchyma with a covering of epithelium, see Fig. 2H1.

In the female they persist as the labia majora ami in the male as the scrotum.

Penis. — In the male the genital tubercle continues elongating as follows: JnVSeoSS^lK^Sni^

Fa,tu«. . .40. M. 60. 105. mm. t},S^J^'^ii>£\^^t

Penis 2. 2,;i 3. 3.5 mm. IIn:/.^, foHiltal iulM>n;l^: f<l. ••niihe and its dorsal groove not only deepens x"atiiiam^A^Toun while it remains veiy narrow, but also closes, beginning at the base; the line of closure remains permanently marked by the raphe of the penis; the effect of the closure is to form an epithelial canal which prolongs the urogenital sinus (or urethra) into the penis; the epithelial canal separates from the epithelium covering the i>eniH, except just lx>low the glans, where the permanent oi)ening is established. During the third month there appears first an epithelial ridge upon the glans, as in Fig. 28(1; tliia ridge lies in the pnMongation of the gnxive; it soon disapjwars and the grtMjvc extends gradiially on to the glans. It is at this stage (end of the thini month) that the thick prepuce of the glans begins to develop, but the gniovc on the anal side prevents its forming a complete ring around the organ. The prepuce appears as a slight ridge which ovtrgrows the glans. the e]»itJieliHni of the inner Rui-face uniting, aw the fold extendi, with the epithelium of the glans — the two opitliclia fusing into one solid plate. Fig. 297, ep. Later the gi-oove Iiccomes closed to a canal. an<l the terminal opening of the canal is shut by the gmwth of the cpithelimn, Fig. 27!i, f(, which plugs up the oriHce. This fact is imi)ortant from its hearing on the 'luestioii of the origiu of the amniotic fluid, p. ;i4(l. The twi> epithelial layers of the j)rL'puce are separafeil by mesoderm. Tin" relations which now exist ran l>e Ix'ttcr explainetl by reference to Fig. -J7!i, which sli()ws the glans in longitudinal section; observe llie thickened epitheliinii, a, closing the orifice of the urethra; also

that the *!f)ithelial platr — and ron-^riaently tJie prepuw alsii — ertends

fiirtJii-r on thf; ventral sifl**. *';>, than '<n th.> (li.r.-jal. ami that thiiiigh

th<r glans. O, is vcrj- vH>t-ii ^^^ft^^ lar. the (.--irpus cavi:ru<>5Uii) is remarkahlf for the alR^euce of bl-xxi-veisselr-. Finally there may Ik.- noticed in the several where the celU arraii^'t-d more or the appearanivs have be«-n

thougrht bv Schwi-i^^ger-Seidel. to o mneote the the Tliii! clearage

doeti not take place after

The corpus

marked out a dense mes

the blood

^^^^^^^H ^^^HT capillaries more more, lieginniiig in the thinl

Flo. KV.— Uuwlluilinal Krclliui of tht; IVnla of a month ; hilt it 18 IKlt Until the

HunuU) Embrro of >>iriut flri- MouUul Miuut ( idl. ' ,■ ,i_ 1.1 ^ .1 -i

Ho-M. «.EnllhBlI«lplu(t; /^..iirepuce: ep,«pllhe. Sixth montli that the capill!?'u^/r '°'"*'"™ '^^' "■*'*"' 'ari.fl begin to show any

marke<l dilatation. The corpuH fti>mt(/ioHHin ilovoioim also chiefly by the expansion of capillaries, hut considerably later than the cavemosum. Retterer (Soc. Biol., Paris, IHKii, p. :t!i!i) stat*^ that in various niammals the cori)U9 cavernoHum isverydenrto imd fibrous lefore any bliXHl- vessels appear in it. C'MTORls ANiJ NvMl'H.K.— Thedevelopmentof the genital tubercle in the feinah; is in all n-sjiects similar to that of the male, but it does not \Him l>eyonil the Htagci in which there is an oiK'H urethral griMJve. The gbins and preiHue an' fonned, as in thu male, to constitute the clitoris, hut the bonlers of tlM- urethral gWMivc do not unite, nn they do in the male to form the stiilk of the pi>uis, but remain as elevattnl ridges known as the labia minoni or iiyinphic in the adult. During the third inoutli thediffen-nrti Ih^Iwm'u the male and female tulH'rcle iH^MTue.x more and more clearly marki"!, and

the distance iMitween the urogenital and anal j^{. _

openings increases. Bv the end of the thinl «i-<iim>ir nu, iiiTii|.iw: infc. innrilh, Fig. 280, the ghuis measunw alxnit I !ilJ'.lSZ'lS'™.i-«"in"""\i?«> mm., while the li|>s {luihiges nf the nyniidue) T-Himriix. ■'ll)■.«I.^•liarllll. of the urethral gnMivc measure alxiiit :s iiuu. in length: arinind the basi- of llie ghiiis can l>e seen the <i)mineneing fold of Ibc pri'imtiuiu.


and upon the glanB can be seen the median epithelial crest, which aubsequently disappears, the urethral groove extending on to the glaoB during the fourth month.

The groove persiBts, bo that in the adult the prepuce does not extend, as in the male, completely around the glans, but is cleft on the


Fro. 281.— SectloD



If MoDtli& (Hlnot CollectloQ No. W.


anal Bide. Toumeux, 88.1, 254, observed in two cases epithelial ingrowths from the epithelium of the groove of the glans; these ingrowths he regards as theanlages of the "glande clitoridienne" of Werthheimer {Jauni. de VAnai. ef Physiol., 1883), the homologua of the mucous glands of the sinus of Querin in the male. The mes


Flo. SKi,— External Oisui


enchj-ma of the glans persists at a stage corresponding approximately to that of uie homologous tissue in the male at eight months, but the corpora cavernosa develop aa in the male into true erectile tissiie. The accompanying Fig. '■iHI repnsenti a section of the clitoris and labia niiijores of a fa-tus of the fifth montli, the urethra extends into the glans, which is covered by the prepuce ; the glans is ahnost buried between the large labia majora.

Scrotum and Labia Majora. — There appear two prominences during the tenth week, one on each side of the genital tubercle. These prominences, which are merely hillocks, so to speak, of mesoderm covered by foetal epidermis, are the anlages of the male scrotum and the female labia majora. Their relations are well shown in Fig. 280. In both sexes the genital labia attain considerable size ; in the female the foetal type. Fig. 282, is but slightly modified, but in the male the two labia meet and unite during the fourth month between the base of the penis and the anus to form the scrotum ; the raphe marks in the adult the line of junction ; as stated above, p. 498, the vaginal processes grow into the scrotum and later the testis descends into it, p. 499.




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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



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