Human Embryology and Morphology 23

From Embryology

Keith, A. Human Embryology And Morphology (1921) Longmans, Green & Co.:New York.

Human Embryology and Morphology: 1 Early Ovum and Embryo | 2 Connection between Foetus and Uterus | 3 Primitive Streak Notochord and Somites | 4 Age Changes | 5 Spinal Column and Back | 6 Body Segmentation | 7 Spinal Cord | 8 Mid- and Hind-Brains | 9 Fore-Brain | 10 Fore-Brain Cerebral Vesicles | 11 Cranium | 12 Face | 13 Teeth and Mastication | 14 Nasal and Olfactory | 15 Sense OF Sight | 16 Hearing | 17 Pharynx and Neck | 18 Tongue, Thyroid and Pharynx | 19 Organs of Digestion | 20 Circulatory System | 21 Circulatory System (continued) | 22 Respiratory System | 23 Urogenital System | 24 Urogenital System (Continued) | 25 Body Wall and Pelvic Floor | 26 Limb Buds | 27 Limbs | 28 Skin and Appendages | Figures

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Chapter XXIII. Urogenital System

Evolutionary Stages

The association of the genital with the urinary system has to be sought for in the ancestry from which vertebrate animals arose, for even in the lowest vertebrates they are already associated. The evidence of embryology makes it certain that man has been evolved from a type in which each segment of the body was provided with its own excretory tubule or kidney. The parts of an excretory or nephric tubule are diagrammatically represented in Fig. 380, A. Into its dilated head or beginning projects a vascular body — a glomerulus — similar to the glomeruli of the kidney ; at its commencement the tubule is also connected with the peritoneal cavity by an open funnel-shaped structure — ^the peritoneal funnel. By this communication ova or spermatozoa, which are shed from the genital glands, may escape from the peritoneal cavity and enter the excretory tubules, and thus pass outside the body. We shall see that the openings by which ova still escape in women and the passages by which semen leaves the testicle in men, are derived from the funnel elements of the nepliric tubules. The essential part of the excretory organ is the epithelial-lined wall of the tubule itself. The secretion of the tubules is conveyed to a common collecting duct — the nephric duct — which ends in the cloaca.


Fig. 380. Composition and Origin of Nephric Tubules.

A, Diagram of an Isolated Nephric Tubule. (After Semon.) B, Showing the manner in which the Intermediate Cell Mass (a, b, c) gives origin to the Nephric Tubule (a), Peritoneal Funnel (b) and the Nephrocele (c). C, The isolation of these parts from the Somite and their union to form a system. D, The Origin of a Glomerulus in the Wall of the Nephrocele (c). (After Felix.)

An inspection of Fig. 380 {B, C, D) will show how the various parts of the nephric tubule just named arise from the wall of the intermediate part of the coelom. We have already seen (p. 41) how the mesoderm on each side of the embryo becomes demarcated transversely into body segments or somites, and also longitudinally into the paraxial mass, the intermediate cell mass and the parietal laminae, and how extensions of the coelom are included in each of these longitudinal divisions. From Fig. it will be seen that a nephric tubule arises by an evagination of the outer wall of the intermediate part of the coelom, while the glomerular chamber or nephrocele (c) and the peritoneal funnel, are produced from the coelomic passage which originally connected the peritoneal cavity with the cavity of a somite (Fig. 380). Thus the nephridial and genital systems must be regarded as modified parts of the wall of the original coelomic cavity.^

Succession of Renal Systems

In the evolution of the higher vertebrates there has been a succession of three renal systems, the third being, the present functional system — the kidneys or metanephros . All of them — pronephros, mesonephros or Wolffian body and metanephros, are compounded of the same system of nephridial elements just as the milk and permanent teeth are successive manifestations of the same dental system. In the human embryo of the 6th week all three systems may be seen ; while the pronephric system, which is developed in the last four or five cervical segments and first two or three thoracic, is then undergoing retrogression, the metanephric in the hinder lumbar segments is only appearing ; in this week the mesonephric system is approaching the height of its development, extending from the 5th cervical to the 3rd lumbar segment.


Fig. 381. Schematic Section to show the Specialization of the Dorsal Part of the Coelom into Nephric Tubules, Peritoneal Funnels and Glomeruli. On one side the tubule is connected with the peritoneal cavity by an open funnel while the Glomerulus is intraperitoneal as is usual in pronephric tubules ; on the other side they are buried in the Wolffian or intermediate ridge.

^ See W. Felix, Keibel and MalVs Manual of Human Embryology, vol. 2, 1912 ; Eliz. A. Fraser, Journ. Anat. 1920, vol. 54, p. 287 ; Gynneth Buchanan and Eliz. Fraser, ibid. 1919, vol. 53, pp. 35, 97 ; F. T. Lewis, Amer. Journ, Anat. 1919, vol. 26, p. 423 ; J. L. Bremer, ibid. 1916, vol. 19, p. 179,

It is clear that pronephros, mesonephros and metanephros are parts of the same linear series of organs. All are made up of nephric tubules opening into a common excretory (Wolffian) duct. While in the cervical region the tubules are simple and retain their segmental arrangement, in the dorsal and lumbar region they multiply in number and complexity ; in the sacral region they become exceedingly numerous and massed round a diverticulum from the Wolffian duct — which forms the primitive ureter. In the second month of human development the Wolffian body is at the height of its development ; in the 3rd month the permanent kidney assumes its predominant position, and its predecessor — the Wolffian body • — is converted into a mere appendage of the genital system.


Fig. 382. Condition of the Nepliric or Renal System in a Human Embryo of 4 weeks. (After Ingalls.)

The Wolffian Body or Mesonephros (Fig. 382). — In lower vertebrates (Fishes and Amphibians) the Wolffian body is the functional kidney ; in higher vertebrates (Reptiles, Birds, and Mammals) it is merely a temporary or embryonic structure, the renal function being taken over by the permanent kidney. Apparently the permanent kidney (metanephros) arose by a hypertrophy and separation of the hindermost segment of the Wolffian body. The presence of the mesonephros in the human embryo and in the embryonic stages of the three great classes of higher vertebrates, with the presence of many curious stages in the development of their genito-urinary system, can be explained only by the fact that these higher forms are descended from ancestors of the lower.

In Fig. 383 is given a diagrammatic representation of the tubular composition of the Wolffian Body of the frog, which in many points corresponds to the same structure in the human embryo. Each body is made up of a main duct and a series of tubules. In the frog, as in the human embryo, the hind-gut ends in a dilatation, the cloaca. In the cloaca open the rectum, allantois or bladder, and the two Wolffian ducts — right and left. In the frog, the Wolffian bodies lie on each side of the spine, their anterior ends reaching forwards to the region of the heart. Each duct is joined by numerous convoluted tubules — the Wolffian or Nephric tubules. Each tubule is furnished with a glomerulus at its blind extremity, and in most features agrees with a secretory tubule — such as is seen in the permanent kidney. These tubules secrete the urine ; the Wolffian duct conveys the urine from the tubules to the cloaca. The anterior tubules, however, lose their secretory function and become associated with the genital gland. In the male frog they convey the spermatozoa to the Wolffian duct, which thus carries both urine and spermatozoa. In the female, the genital Wolffian tubules are connected with the ovary but are quite functionless (Fig. 383).


Fig. 383. Scheme of the Wolffian Body of the right side.

The Wolffian Body in the Human Embryo

By the middle of the second month of foetal life, the Wolffian body is well developed ; by the end of that month it is undergoing a process of atrophy, except those parts connected with the genital organs. Originally extending from the 5th cervical to the 3rd lumbar segment, by the 8th week it is confined to the region of the lower three thoracic and upper three lumbar vertebrae where it projects at the dorsal attachment of the mesentery (Fig. 384). Its anterior end lies at the pleuro-peritoneal orifice of the diaphragm. To its inner side, in the lower dorsal region, lies the genital ridge. The genital and the Wolffian bodies have each its own mesentery, but these two mesenteries have a common attachment — the common urogenital mesentery (Fig. 384). On section the Wolffian ridge is seen to be made up of convoluted tubules terminating at their blind extremities in glomeruli. The tubules open into the Wolffian duct, just as in the frog ; the duct is situated on the lateral margin of the ridge, dorsal to the Miillerian duct. It runs backwards in this ridge and turns into the pelvis to end with the Miillerian duct (also situated in the Wolffian ridge) in the cloaca of the hind-gut (Fig. 382). The whole arrangement is similar to that seen in the frog. Further, as in the frog, certain of the more anterior or genital tubules are connected with the genital glands, and are not, as the posterior are, secretory in nature. If the testes were functional at this time — which they are not — the spermatozoa and urine of the Wolffian body would pass to the cloaca by the Wolfl&an duct.

Origin of the Wolffian Duct and Tubules

The tubules which compose the Wolffian body are developed in the intermediate cell mass, in the manner already described (p. 359). The intermediate cell mass is divided from before backwards into segments ; two or three tubules arise in each segment. The tubules, although of the nature depicted in Fig. 380, appear in the course of human development as minute vesicles in the intermediate cell mass ; these vesicles become tubular ; one end opens into the Wolffian duct ; at the other a glomerulus is developed (see Fig. 381). The duct is developed in the outer part of the intermediate cell mass. Its anterior or cervical part appears early in the 4th week as a solid rod of cells formed by the union of the terminal ends of the pronephric tubules. By the end of the 4th week the caudal end of the pronephric duct has reached the cloaca and thus the pronephric duct forms the basis of the Wolffian duct — the duct into which the tubules of the Wolffian body open. At first the hinder or pelvic ends of the Wolffian bodies are separate, but in the 8th week they become approximated and fuse to form the genital cord. The genital cord contains the terminal parts of the Wolffian and Miillerian ducts. The Miillerian ducts being situated nearest to the middle line, fuse to form the uterus.


Fig. 384. Diagrammatic Section to show the Position of the Wolffian and Genital Ridges on the Dorsal Wall of the Abdomen.

The Pronephros

Pronephric differ from mesonepliric tubules in retaining open peritoneal funnels attached to them (Fig. 383) and in having their glomeruli occasionally situated within the peritoneal cavity (Fig. 381). They reach their highest development in anterior segments of the human embryo during the 4th week and then disappear, leaving no trace. Their duct becomes the Wolffian duct, and if a remnant did persist we should seek for it at the commencement of this duct.

The Fate of the Wolffian Body (mesonephros) and Pronephros

1. In the Female

In Fig. 385 are shown the various remnants of the embryonic renal formations which may persist in the adult female. The Miillerian duct, the upper part of which becomes the Fallopian tube, is situated in the Wolffian ridge (Fig. 384). Hence when the ovary and tube migrate to the pelvis, the Wolffian mesentery, which comes to form the mesosalpinx, is also drawn within the pelvis, and with it all the Wolffian remnants. A hydatid attached to the mesosalpinx (part of the broad ligament) at the fimbriated extremity of the Fallopian tube (Fig. 385) represents the most anterior (cephalic) part of the Wolffian formation. The Wolffian duct (Fig. 385) runs towards the body of the uterus in the mesosalpinx ; it reaches the side of the uterus, but from that point onwards it has disappeared by the commencement of the 3rd month. Occasionally, however, remnants of the lower or distal part of the duct persist. They lie in the roof of the vagina. The point of termination of the duct is sometimes represented on the trigone of the vulval cleft a little distance from the side of the opening of the urethra. Only the upper part of the duct (mesosalpingeal part) persists in women. The uterine and vaginal segments, when they persist, get the name of duct of Gartner. The genital tubules, those attached to or connected with the ovary, persist and form the epoophoron. Organ of Rosenmiiller, or parovarium. The renal Wolffian tubules — those which acted as renal structures in the embryo — also persist, sometimes unconnected with the duct. They lie between the ovary and uterus and form the paroophoron. Fluid may collect in their cavities, and thus those vestiges may form cysts, but, as we shall see (p. 366), the Wolffian remnants which are the usual source of cystic formations in the female lie along the ovarian fimbria and are derived from the elements which give rise to the rete testis in the male.


Fig. 385. Remnants of the Wolffian Body in the Female (see also Fig. 387).

2. In the Male

In the male (Fig. 386) the Wolffian duct forms : (1) The tube of the epididymis, which is coiled up in the globus major, body and globus minor of the epididymis ;


Fig. 386. Remnant of the Wolffian Body in the Male (see also Fig. 387).

(2) The vas deferens and common ejaculatory duct. The duct opens at each side of the uterus masculinus in the prostatic urethra, a site corresponding to the vestibule of the vagina in the female ; (3) The vesiculae seminales arise from the Wolffian ducts as tubular diverticula at the end of the 3rd month ; the terminal part of the duct also becomes dilated to form an ampulla.

The stalked hydatid frequently seen on the upper extremity of the testicle corresponds to the hydatid at the fimbriated extremity of the Fallopian tube in the female, and is of similar origin (Figs. 385 and 386).

The genital tubules of the Wolffian body become the vasa efierentia and coni vasculosi.

1 J. L. Bremer, Amer. Journ. Anat. 1911, vol. 11, p. 393 (Dev. of Vasa Efferentia) ; Otto Petersen, Anat. Hefte, 1907, vol. 34, p. 239 (Dev. of Vesiculae Seminales) ; E. M, Watson, Amer, Journ, Anat, 1918, vol, 24, p, 395,

The renal tubules of the Wolffian body form :

  1. The vasa aberrantia found in the globus minor ;
  2. The paradidymis or organ of Giraldes situated in the cord above the globus major but not always present. The vas aberrans represents an elongated Wolffian tubule, which has effected a communication with the Wolffian duct, but not with the genital gland. The tubules of the paradidymis represent blind tubules, which retain the embryonic cystic form. All these tubules, both genital and renal of the Wolffian body, are situated originally in the mesentery of the Wolffian body (Fig. 384).

Thus it will be seen that while in the male the Wolffian tubules and duct become part of the genital system, in the female they become functionless and only of pathological importance. Their presence in the female is due to their being inherited from the male, just as the breasts in the male persist because of their utility in the female.


Fig. 387. Diagrams showing the fate of the Junctional Cords in the Ovary and in the Testis. A, ovary and Fallopian tube, showing the rudiments of the junctional tubules in the ovario-fimbriate margin of the broad ligament ; B, the origin of the junctional system ; C, the junctional system of the testis. M.D. Miillerian duct ; W.D. Wolffian duct ; W.T. Wolffian tubules.

Rete Testis

The junction between the genital tubules of the Wolffian body and the seminal tubules of the testes is effected by the development of a separate element to which the names of rete-cords or junctional cords have been given. In Fig. 387, B, is represented the origin of the junctional tubules, according to the account given by Dr. Allen. ^ The ovary and testis represent only the middle part of the original genital ridge ; the anterior and posterior parts atrophy and disappear by the end of the 2nd month. In the anterior vestigial part of the ridge solid cords representing peritoneal funnels (Fig. 380), grow into the mesentery of the Wolffian body, and from these cords, as shown in Fig. 387, B (where only two cords are represented), is formed the rete testis. The rete testis effects communications with the seminal tubules by means of outgrowths, which form the vasa recti, and also with the glomerular or blind extremities of the genital tubules of the Wolffian body (Fig. 387, C). In the female the junctional cords are formed ; vestiges usually remain. Frequently fimbriated hydatids (peritoneal funnels) are attached to them (Fig. 387, A). The majority of parovarian cysts ^ are formed from the junctional rudiments. Isolated vestiges may also be seen in the testicle between its upper pole and the globus major (Fig. 387, C). They also may give rise to cysts.

^ See Bennet M. Allen, Amer. Journ. Anal. 1906, vol. 5, p. 79.

The Kidney

Origin of the Permanent Kidney. — In Fishes and Amphibians the Wolfiian body alone acts as a kidney. In Reptiles, Birds and Mammals the permanent or hind kidney appears, and supplants the Wolffian kidney. Like the Wolffian body the kidney arises by the combination of two elements which are developed separately — a duct or collecting system, and a nephric or secretory system. The collecting system arises as an outgrowth from the hinder end of the Wolffian duct, and forms the ureter, the pelvis of the ureter and the collecting tubules, which compose the main part of the medullary pyramids of the kidney. The secretory part arises from the hinder end of the nephridial system — just behind the part which gives origin to the mesonephros ; it forms the cortex of the kidneys — the glomeruli, convoluted tubules and loops of Henle ; in short, the secretory substance of the kidney (Fig. 389). Already, at the beginning of the 5th week, the ureteric part of the kidney is apparent as a dilatation or slight evagination at the hinder end of the Wolffian duct, near the cloaca. The stage reached by the beginning of the 6th week is shown in Fig. 388, A ; the ureteric bud is stalked, the stalk representing the ureter and its dilated pelvic end the renal pelvis and collecting tubules. The nephrogenic tissue forms a cap on the pelvic dilatation. At this time the kidney lies under the 4:th. and 5tli lumbar segments. At a later stage in the 6th week (Fig. 388, B) the ureteric stalk has become elongated, the pelvic dilatation has given rise to primary evaginations representing the calyces of the kidneys ; round the evaginations is massed the nephrogenic cap. The kidney now lies dorsal to the Wolffian body and under the 2nd and 3rd lumbar segments.

^ For the pathological significance of this structure see Alban Doran, Journ. of Obstetrics and Oynaec. of Brit. Empire, Oct. 1910, vol. 18, p. me.


Fig. 388a. The Ureteric Bud and Nephrogenic Cap at the beginning of the 6th week.

Fig. 388b. The same parts later in the 6th week. C indicates the stage of Renal development reached in the 7th week.

2 G. C. Huber, Amer. Journ. Anat. 1905, vol. 4, Supplement (Dev. of Renal Tubules) ; F. T. Lewis, Amer. Journ. Anat. 1919, vol. 25, p. 423 ; A. F. Dixon, Journ. Anat. and Physiol. 1911, vol. 45, p. 117 (Supernumerary Kidney) ; E. Muthmann, Anat. Hefte, 1907, vol. 32, p. 577 (Horse-shoe Kidney).

Fig. 389. Illustrating the Development of the Renal Tissue. A, growing end of collecting tubule with bud of nephric tube attached to it ; B, first stage in the development of a nephric bud into a nephric tubule ; C, fully developed renal tubule ; the part formed from the ureteric bud is represented in outline and the part from the nephric tubule is shaded. (After Huber.)

The separation of the ureter from the Wolffian duct has commenced. In Fig. 388, C a still later stage is shown. Tubules now begin to form in the nephrogenic cap and collecting tubules to bud out from the pelvic bud. Collecting tubules arise by the division and redivision of the growing end of the pelvic outgrowths. In the third month the process of outgrowth from the ureteric bud continues ; the growing end of each bud divides and redivides, and in this manner the collecting tubules of the pyramids are formed (Fig. 389, C). In Fig. 389, A, the growing extremity of such a collecting duct is represented. Near one of its terminal buds is represented one of the numerous tubule-rudiments, formed from the nephrogenic tissue surrounding the ureteric outgroA^ths. Like a Wolffian tubule, it appears in a vesicular form. At one extremity it establishes a communication with the collecting tubule ; at the other a glomerulus develops (Fig. 389, B). The tubule elongates, becomes convoluted, and quickly assumes the adult form represented in Fig. 389, C. Glomeruli appear at the commencement of the 3rd month ; a capsule becomes difierentiated for the kidney from the surrounding mesodermal tissue in the 3rd month. The kidneys have by then reached their final position — extending from the 11th thoracic to the 4th lumbar segment. Up to the time of birth, tubular and glomerular formation are seen in full activity within the subcapsular zone of the kidney. The deep tubules are the first to difierentiate. Soon after birth the formation of new elements ceases ; increase in size is then due to pure growth of the parts already formed. The collecting tubules, arising from each primary evagination of the ureteric bud become massed in pyramids ; the bases of the pyramids, clothed by nephrogenic caps, appear on the surface of the kidney and give it a lobulated structure. In the fissures between the lobules cortex is formed ; soon after birth, as new cortical tissue is laid down, the depressions between the lobules are filled up. In many mammals (ox, bear, seal) the renal substance remains broken up into numerous lobules.

The upper pole of the kidney reaches the 11th rib in the 5th month, and is then in juxtaposition with the adrenal, which is developed at the anterior end of the Wolffian body. At their first appearance the renal buds receive temporary branches from the common iliac artery and from the aorta, but when they come to lie on the dorsal aspect of the Wolffian body in the 7th week, the arterial network, supplying the tubules of that body, invade the nephrogenic tissue of the renal buds and thus the kidneys annex the series of Wolffian arteries— stretching from the 11th thoracic to the 4th lumbar. The definite arteries are derived from those of the 2nd lumbar segment but frequently more than one pair persists.

With the development of the lumbar and sacral regions of the body the ureter becomes elongated. The termination of the ureter becomes separated from the Wolffian duct early in the 7th week, by a process to be mentioned later.

As the kidney grows forwards its hilum at first looks towards the pubic region, and even when it has reached its final position and the poles become upper and lower, the hilum of the kidney still looks towards the ventral wall of the abdomen. In the 4th and 5th month an anterior lip is formed to the hilum by the development of cortical tissue, and the hilum then assumes its normal form and position. The anterior lip is usually absent from horseshoe kidneys, an abnormality which arises from a fusion of the right and left nephrogenic buds. Such kidneys are usually supplied with multiple renal arteries. In other cases the renal buds grow, not towards the loins but towards the sacral region, becoming developed in the pelvis and drawing their arteries from the sacral and iliac vessels. The ureteric bud may divide, and give rise to a forked ureter, or to double or even triple ureters. The nephrogenic element may remain single, or it also may become divided, giving rise to two kidneys on one side. Another common developmental error is the failure of the nephric tubules to effect a union with the collecting tubules. The nephric tubules then become dilated and cystic, giving rise to congenital cysts of the kidney.

The Mullerian Ducts

The Mullerian Ducts or Oviducts are present in almost all vertebrates, and convey the ova from the peritoneal cavity to the surface of the body. In fishes, amphibians, reptiles, birds and lower mammals (Marsupials) the ducts terminate in the cloaca. This is also the case in the embryonic stages of man and all higher mammals. The development of the duct in man is very simple (Fig. 390). The first part to be formed is the ostium abdominale which appears on the ventro-lateral aspect of the Wolffian ridge (Fig. 390) as a funnel-like invagination of the coelomic mesothelium. This invagination, which appears in the 6th week at the anterior end of the Wolffian ridge (under the 3rd thoracic segment) represents a modified peritoneal funnel (Fig. 380) or nephrostome (Fig. 390). From the apex of the funnel-like invagination of coelomic epithelium, a solid rod-like process of cells grows backwards on the Wolffian ridge, ventral to the Wolffian duct (Fig. 384) reaching the region of the cloaca in the 7th week. Although the ostium is developed thus, the fimbriae which surround it are not formed until the 3rd month, when they appear as outgrowths of the lining membrane of the tube. More than one ostium may be developed, representing neighbouring nephric funnels. As it passes backwards in the Wolffian ridge the Mullerian duct lies below and internal to the Wolffian duct and comes in contact with its neighbour of the opposite side in the pelvis (Fig. 392). The Miillerian duct is formed in the embryo later than the Wolffian duct ; its posterior growing end does not acqmre a lumen until late in the 3rd month when it opens on the dorsal wall of the urogenital sinus —a derivative of the cloaca (Fig. 391). The openings of the two Mullerian ducts are situated between the orifices of the Wolffian ducts. Mullerian ducts, although they only reach their full development in woman, are yet as completely and strongly formed in the male embryo as in the female.

Fig. 390. Diagrammatic Section across the "Wolffian and Genital Ridges to show the Origin and Relations of the Miillerian Duct to the Duct and Tubules of the Wolffian Body. (After Pasteau.)

1 S. E. Wichmann, Anat. Hefte, 1912, vol. 45, p. 629. 2 a

By the beginning of the 3rd month the fimbriated tube has retreated to the level of the 2nd lumbar segment.

The Genital Cord.— During the 3rd month the Miillerian ducts show two distinct stages in their course :

(1) Lumbar, which lies in the Wolffian ridge and is suspended from the posterior abdominal wall by the Wolffian mesentery. This stage afterwards forms the Fallopian tube (Fig. 392).

(2) Pelvic, which is embedded in the genital cord. The posterior ends of the Wolffian ridges, with their contents, the Wolffian and Miillerian ducts, fuse in the pelvis during the 8th week, and thus form the genital cord.

Fig. 391. Diagram of the Genital Ducts at the commencement of the 3rd month of Foetal Life. Lateral view.

With their fusion the peritoneal space of the pelvis is separated into a deep posterior part — the pouch of Douglas and a shallow anterior depression — the utero-vesical (Fig. 391). The parts of the Miillerian ducts within the cord form the uterus and vagina. The ureter is also enclosed within the mesodermal tissue of the genital cord, but afterwards becomes separated from it.

The genital cord of the foetus at the beginning of the 3rd month shows the two Miillerian and two Wolffian ducts — in the male as well as in the female (Fig. 391). One of the first signs of sexual differentiation is to be observed in the genital cord. Whereas the genital cord in the male embryo is closely appHed to the bladder, so that there is no utero-vesical pouch, in the female the cord remains separated from the bladder by a deep peritoneal pocket.

The Round Ligament of the Uterus, which is apparent early in the 3rd month, is attached to the Miillerian duct on each side (Fig. 392). The point of attachment marks the junction of the uterine and Fallopian segments of the Miillerian ducts. The round ligament corresponds to the gubernaculum testis in the male and its development is similar. Both are developed in the following manner : Part of the Wolffian ridge is continued backwards as a peritoneal fold to the groin, this part forming the inguinal fold (Fig. 392). Within the inguinal fold, in the mesenteries of the Wolffian body and genital gland and in the subperitoneal tissue of the genital cord a stratum of non-striated muscular tissue is developed. The mesodermal tissue, in the lower end of the inguinal fold, begins to pierce the abdominal wall external to the deep epigastric artery in the 3rd month, the piercing force being obtained purely from the inertia of growth. The growing end, at first merely represented by fine strands of tissue, in later months increases in mass, and carries over it and in front of it, into the scrotum or labium ma jus, a process of the peritoneum and attenuated representatives of each stratum of the belly wall (Fig. 421). The inguinal canal, the round ligament of the uterus and the gubernaculum testis are thus formed by the extension of the substance of the inguinal fold. From the stratum of muscular tissue which is found everywhere under the pelvic peritoneum, particularly around the genital cord, are formed the round ligament of the ovary, the muscular tissue in the utero -rectal (utero -sacral) ligaments and in the broad ligaments, and also the outer muscular coat of the uterus.

Fig. 392. Diagram of the Miillerian Ducts at the commencement of the 3rd month. Ventral view.

Formation of Uterus and Vagina

The parts of the Miillerian ducts lying side by side in the genital cord (Fig. 392) begin to unite in the 3rd month, and by their fusion the uterus and vagina are formed. In all the members of the vertebrate series below and including the Monotremes, the Miillerian ducts remain separate and open in the cloaca (Fig. 393, A). The process of fusion begins with the formation of the genital cord in the 8th week and is continued throughout the 3rd month. The septum formed by the fused mesial walls (Fig. 394) disappears first below the region of the uterine cervix ; the process may be arrested at this stage — a stage shown by some adult marsupials. Next, the lower or cervical part of the uterine septum disappears ; the human uterus then (2| months) resembles that of higher mammals (carnivora, etc., Fig. 393, C). It may be arrested at this stage (uterus bicornis). Lastly the upper part of the septum disappears (3| months, Fig. 394). The fundus, which is the last part to be developed and is only found in the highest primates, is quite well marked in the child at birth.

The musculature ^ appears in the wall of the uterus, vagina and tubes during the 4th month, the inner or circular layer appearing before the

Fig. 393. Evolution of the Human Form of Uterus.

A, form seen in lowest mammals, reptUes, amphibians, fishes, and in the 2nd month human foetus ; B, form of Miillerian ducts in rodents ; C, form in carnivora, etc., and in the 3rd month human foetus ; D, form found in man and higher primates.

outer or longitudinal. Sometime after birth additions are made to the musculature of the uterus, and the distinction between the two primary layers becomes obliterated. Glands begin to form in the uterine mucosa during the 4th month and at the same date the cervix becomes differentiated from the vagina. At this time, too, the ovarian extremity of the Fallopian tube becomes wide and trumpet-shaped ; the mucous membrane within it becomes plicated. The fimbriae are then formed by the extremities of the plicae growing out at the ostium abdominale. Secondary or accessory ostia may also be produced.

H. R. Clarke, Journ. Obstet. Gynae. 1911, vol. 20, p. 85.

By the 7th month (Fig. 402) the foetal uterus is divided into two parts, the cervix or lower segment and body or upper segment. The lower segment or cervix forms then two-thirds of the uterus ; its walls are thick and its upper part is lined by columnar non-ciliated epithelium, containing racemose mucous glands. Its mucous membrane is arranged in palmate folds. The upper or uterine segment composes only a third of the uterus. It is lined by columnar epithelium, which becomes ciliated at the end of foetal life. At puberty the body of the uterus, instead of being half the size of the foetal cervix, becomes larger than it. The cervix takes no part in menstruation nor in containing the foetus ; its true function is unknown. The external os is formed at the junction of the vaginal cords with the uterine segment of the Miillerian ducts ; it becomes demarcated at the end of the 4th month. For some time after birth the body of the uterus actually undergoes a reduction in size (Bayer) ; growth does not become marked until the 7th year.

Fig. 394. Showing the manner in which the Miillerian Ducts fuse to form the Uterus and Vagina.

Metamorphosis of the Vagina

About the middle of the 3rd month the lower ends of the Miillerian ducts of the human embryo undergo a remarkable metamorphosis, first fully described by Berry Hart and lately reinvestigated by F. Wood Jones.^ The epithelium lining the vaginal tracts of the Miillerian ducts proliferates, forming two cords of cells, while, at the same time, the mesodermal tissue in the lower end of the genital cord, which surrounds the terminal segments of the Miillerian and Wolffian ducts, undergoes a rapid growth, pushing downwards that part of the cloaca in which they end — the urogenital sinus (Fig. 395, 1., II.). The vaginal cords formed by the Miillerian linings (Fig, 396) proliferate into the tissue of the genital cord and fuse together, the vagina being formed by the breaking down of the epithelial core in the 4th month. Thus the orifice of the vagina, originally situated high in the urogenital sinus, is carried downwards until it opens in the vulval cleft. Atresia of the vagina results from a failure of the process of canaliculization. Septa in the vagina result from incomplete fusion of the two cords. Only the tip of the vaginal cords reach the urogenital sinus ; hence a partial septum — the hymen ^ — marks the opening of the vagina into the urogenital sinus. The extent to which the terminal septum breaks down varies widely ; hence the numerous forms assumed by the hymen.

1 Brit. Med, Journ. 1904, Dec, 17th ; Journ. Anat. 1914, vol. 48, p. 268.

Fig. 395. Diagrams showing the Termination of 'the]Vagina about the seventh weelc (I.) and about the thirteenth week (II.)- (After Wood Jones.) A, Mullerian ducts (vagina and uterus) ; B, urogenital sinus ; C, bladder ; D, rectum ; E, vagina represented by a cord of epithelium.

Fig. 396. Diagram Illustrating the manner in which the Vagina is formed by the Fusion of two solid Processes or Cords. (Wood Jones.)

^ For development of hymen: D. Berry Hart, Edin. Med. Journ. 1911, vol. 6, p. 577 ; F. J. Taussig, Amer. Journ, Anat. 1908, vol. 8, p. 89.

An explanation of this remarkable change may be found in the formation of a new vagina in lower marsupials which was discovered by J. P. Hill.^ In lower marsupials the vaginal segment of the Miillerian ducts are separable into two parts — upper, which lie side by side, and reach towards the cloaca (Fig. 397) ; lower, which form lateral loops before terminating in the cloaca. Hill found that the young were born by passing from the upper or median segments into the cloaca by the formation of a new passage (Fig. 397). In higher marsupials he found that the upper parts of the vaginal segments became fused to form a median vagina, and that the new passage to the cloaca was not temporary as in lower marsupials, but permanent. In monotremes, the Miillerian ducts have to serve only for the passage of unhatched ova, but with the evolution of gestation the ducts, which could convey ova, were unfitted for the transmission of young, and a new passage or median vagina was formed. The evidence is conclusive that there was a phase in human evolution when the Miillerian ducts terminated in a cloaca, and the metamorphosis which takes place in the lower ends of these ducts of the human embryo is evidently an abbreviated recapitulation of the formation of the median vagina of marsupials.

Fig. 397. Diagram showing the arrangement of the Miillerian Duct in a Marsupial Mammal and the manner in which a New Vagina is formed for the Passage of the Young at Birth. (F. Wood Jones after J. P. Hill.)

The Mullerian Ducts in the Male

In the male foetus of the 3rd month the Miillerian ducts are undergoing atrophy, the distinction between the testis and ovary being quite marked by that time, and the process of sexual diiierentiation already to be seen on close examination. All that remain of the Miillerian ducts in the adult male are their fused terminal or vaginal segments forming the sinus pocularis or uterus masculinus in the prostate (Figs. 386, 398). Its depth is commonly about 3 or 4 mm., but occasionally such a form as is represented in Fig. 399 occurs and shows the real nature of the sinus pocularis. The vagina and uterus can be recognized in such cases (Primrose). The fimbriated ends of the MiiUerian ducts persist as the sessile hydatids on the testicle (Fig. 386). The intermediate part of the tube disappears in the 3rd month and its site becomes greatly stretched during the descent of the testicle. A remnant of its upper end can be found in the sharp anterior border of the epididymis until quite a late period in foetal life.^ The mesosalpinx shrinks and completely disappears in the anterior border of the epididymis.

1 Proc. Linnean Soc. New South Wales, 1899, March 29th, p. 42 ; 1900, Aug. 29th, p. 519.

Fig. 398. A Section of the Prostate showing the Remnants of the Lower Ends of the MiiUerian Ducts in the Male.

The Urogenital Sinus or Canal

The Miillerian ducts open into the cloaca of the embryo side by side, between and below the openings of the Wolffian ducts (Fig. 391). The passage which serves as a common channel for bladder, Miillerian and Wolffian ducts is the urogenital sinus (Fig. 400, A, B). In the female foetus at the 3rd month it is still well marked. In all mammals except man this passage-like sinus is retained. By the beginning of the 4th month in the female foetus (Fig. 400, B) it will be seen that the urogenital sinus has become shortened and opened out to form the floor of the pudendal or vulval cleft from the glans clitoris in front to the fossa navicularis behind, and thus the end of the Miillerian ducts (vagina) and urethra come to have separate openings. The metamorphosis in the genital cord which leads to the formation of the vagina plays a large part in the transformation (Fig. 395). In the male (Fig. 401) the early foetal form is retained, and the urogenital sinus becomes that part of the urethra between the sinus pocularis and the fossa navicularis in the glans penis. The female urethra corresponds to the prostatic part above the opening of the sinus pocularis of the male urethra (Figs. 400, 401).

Fig. 399. A Section of a Prostate showing an unusually developed Uterus Masculinus. (After Primrose.)

1 J. H, ^a.tson,' Journal of Anat. 1902, vol, 36, p. 147.

Fig. 400. Section showing the Urogenital Sinus. A, in the 3rd month female human foetus ; B, in the 5th month female human foetus ; a, the vesico-vaginal septum.

The Hymen is formed at the junction of the vagina with the urogenital sinus, being covered on its outer surface by epithelium derived from the urogenital sinus, and on its deep surface by epithelium of the vaginal cord. Usually at one point on the hymen, but occasionally at several,

^ See references, p. 374, the epithelial coverings fuse and break down, leading to one or more perforations. On the other hand, the vaginal cords may not reach the urogenital sinus, the hymen being then imperforate. In reptiles, as in the human embryo, the part of the urogenital sinus into which the Miillerian and Wolffian ducts open, forms the trigone of the bladder (see p. 381). In such animals the hymen prevents the reflux of urine into the Miillerian ducts.

Fig. 401. Section showing the Urogenital Sinus in the Male Foetus. a indicates the part corresponding to the vesico-vaginal septum of the female, occupied by the 3rd lobe of the prostate.

Fig. 402. A Section to show the condition of the Vagina and Uterus at the 7th month of Foetal Life.

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Human Embryology and Morphology: 1 Early Ovum and Embryo | 2 Connection between Foetus and Uterus | 3 Primitive Streak Notochord and Somites | 4 Age Changes | 5 Spinal Column and Back | 6 Body Segmentation | 7 Spinal Cord | 8 Mid- and Hind-Brains | 9 Fore-Brain | 10 Fore-Brain Cerebral Vesicles | 11 Cranium | 12 Face | 13 Teeth and Mastication | 14 Nasal and Olfactory | 15 Sense OF Sight | 16 Hearing | 17 Pharynx and Neck | 18 Tongue, Thyroid and Pharynx | 19 Organs of Digestion | 20 Circulatory System | 21 Circulatory System (continued) | 22 Respiratory System | 23 Urogenital System | 24 Urogenital System (Continued) | 25 Body Wall and Pelvic Floor | 26 Limb Buds | 27 Limbs | 28 Skin and Appendages | Figures