Book - A Laboratory Manual and Text-book of Embryology 8

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Prentiss CW. and Arey LB. A laboratory manual and text-book of embryology. (1918) W.B. Saunders Company, Philadelphia and London.

Human Embryology 1918: The Germ Cells | Germ Layers | Chick Embryos | Fetal Membranes | Pig Embryos | Dissecting Pig Embryos | Entodermal Canal | Urogenital System | Vascular System | Histogenesis | Skeleton and Muscles | Central Nervous System | Peripheral Nervous System | Embryology History
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Pages where the terms "Historic Textbook" and "Historic Embryology" appear on this site, and sections within pages where this disclaimer appears, indicate that the content and scientific understanding are specific to the time of publication. This means that while some scientific descriptions are still accurate, the terminology and interpretation of the developmental mechanisms reflect the understanding at the time of original publication and those of the preceding periods, these terms and interpretations may not reflect our current scientific understanding.     (More? Embryology History | Historic Embryology Papers)


Chapter VIII. The Development of the Urogenital System

The excretory and reproductive systems are intimately associated in development. Both arise from the mesoderm of the intermediate cell mass (nephrotome), which unites the primitive segments with the lateral somatic and splanchnic mesoderm (p. 52; Fig. 205).

Vertebrates possess excretory organs of three distinct types. The pronephros is the fimctional kidney of amphioxus and certain lampreys, but appears only in inmiature fishes and amphibians, being replaced by the tnesonepkros. The embryos of amniotes (reptiles, birds, and manmials) possess first a pronephros, and then a mesonephros, whereas the permanent kidney is a new organ, the metanepkros. Whether these glands represent modifications of an originally continuous organ, or whether they are three distinct structures, is undecided, but however this may be, the pro-, meso-, and metanephroi of anmiotes develop successively in the order named, both as regards time and place.

The Pronephros

The pronephros, when functional, consists of paired, segmentally arranged tubules, one end of each tubule opening into the coelom, the other into a longitudinal pronephric duct which drains into the cloaca (Fig. 204 ^4). Near the nephrostome (the opening into the coelom) knots of arteries project into the coelom, forming glomeruli. Fluid from the coelom and glomeruli and excreta from the cells of the tubules are carried by ciliary movement into the pronephric ducts.

The human pronephros is vestigial. It consists of about seven pairs of rudimentary pronephric tubules, formed as dorsal sprouts from the nephrotomes (Fig. 205) in each segment, from the seventh to the fourteenth, and perhaps from more cranial segments as well. The nodules hollow out and open into the coelom. Dorsally and laterally, the tubules of each side bend backward and unite to form a longitudinal collecting duct (Fig. 204 B, A), The tubules first formed in the seventh segment begin to degenerate before those of the fourteenth segment have developed. Caudal to the fourteenth segment no pronephric tubules are developed, but the free end of the collecting duct, by a process of terminal growth, extends caudad beneath the ectoderm and lateral to the nephrogenic cord, until it reaches, and perforates, the lateral wall of the cloaca. Thus are formed the paired primary excretory {pronephric) duels. The pronephric tubules begin to appear in embryos of 1.7 mm., with nine or ten primitive segments (Felix, in Keibel and Mall, vol. 2) ; in 2.5 mm. embryos (23 segments) all the tubules have developed and the primary excretory duct is nearly complete. In 4.25 mm. embryos the duct has reached the wall of the cloaca and soon after fuses with it. The pronephric tubules soon degenerate, but the primary excretory ducts persist and become the dticts of the mesonephroi, or mid-kidneys.


Fig. 204. — Diagrams showing the development of the pronephric duct and pronephric tubules (modified from Felix). A shows a Ifttet stage tiian B.

Fig. 205. — Transverse section of a 2.4 mm. human embryo showing the intermediate cell d nephrotome (Kollmann).

The Mesonephros

The mesonephros, like the pronephros, consists essentially of a series of tubules, each of which at one end is related to a knot of blood vessels and at the other end opens into the primary excretory duct. Besides possessing an internal glomerulus alone they differ from the pronephric tubules in that the nephrotomes are transitory, never opening into the mesonephric chamber. The mesonephric tubules arise just caudal to the pronephros and from the same general source, that is, the nephrotomes. Only a few of the more cranial tubules, however, are formed from distinct intermediate cell masses, for caudal to the tenth pair of segments this mesoderm constitutes unsegmented, paired nephrogenic cords. These may extend caudally as far as the twenty-eighth segment. The primary excretory ducts lie lateral to the nephrogenic cords.

When the developing mesonephric tubules begin to expand there is not room for them in the dorsal body wall and as a result this bulges ventrally into the coelom. Thus there is produced on either side of the dorsal mesentery a longitudinal urogenital fold, which may extend from the sixth cervical to the third lumbar segment (Fig. 220). Later, this ridge is divided into a lateral mesonephric fold and into a median genital fold, the anlage of the genital gland.

Differentiation of the Tubules. — The nephrogenic cord in 2.5 mm. embryos first divides into spherical masses of cells, the anlages of the mesonephric tubules. Four of these may be formed in a single segment. Appearing first in the 13th, 14th and 15th segments, the anlages of the tubules differentiate both cranially and caudally. In 5.3 mm. embryos the cephalic limit is reached in the sixth cervical segment, and thereafter degeneration begins at the cephalic end. Hence the more cranial tubules overlap those of the pronephros. In 7 mm. embryos the caudal limit is reached in the third lumbar segment.

The spherical anlages of the tubules differentiate in a cranio-caudal direction (Fig. 206). First, vesicles with lumina are formed (4.25 mm.). Next, the vesicles elongate laterally, unite with the primary excretory ducts, and become S-shaped (4.9 mm.). The free, vesicular end of the tubule enlarges, becomes thin walled and into this wall grows a knot of arteries to form the glomerulus (embtyos of 5 to 7 mm.). The tubule, at first solid, hollows out and is lined with a low columnar epithelium. The outer wall of the vesicle about the glomerulus is Bowman's capsule, the two constituting a renal corpuscle of the mesonephros (Fig. 206 D). In the human embryo the tubules do not branch or coil as in pig embryos, consequently the mesonephros is relatively smaller. At 10 mm. about 35 tubules are present in each mesonephros and the glomeruli are conspicuous (Fig. 207). Each tubule shows a distal secretory portion and a proximal collecting part which connects with the duct (Fig. 208). The glomeruli form a single median column, the tubules are dorsal and the duct is lateral in position. Ventro-lateral branches from the aorta supply the glomeruli, while the posterior cardinal veins, dorsal in position, break up into a network of sinusoids about the tubules (see Chapter IX).


Fig. 206. — Diagrams showing the differentia- Fig. 207. — Diagtam showing the anlages tion of ihe mesonephric tubules (modified after of the urinary organs in about 10 nun. human Felix). L., lateral; Af., median. embryos as seen from the left side (based on reconstnictions by Keibel and Felix).


The primary excretory duct, or mesonephric duct, is solid in 4,25 mm. embryos. A lumen is formed at 7 mm., lAider opposite the openings of the tubules. The duct is important, as the ureteric anlage of the permanent kidney grows out from its caudal end, while the duct itself is transformed into the chief genital duct of the male, and its derivatives. The mesonephros is probably a functional excretory organ in human embryos even though its tubules degenerate before the metanephros becomes functional (Bremer, Amer. Jour. Anat., vol. 19, 1916). Degeneration proceeds rapidly in embryos between 10 and 20 mm. long, beginning cranially. New tubules are formed at the same time caudally. In all, 83 pairs of tubules arise, of which only 26 pairs persist at 21 mm., and these are usually broken at the angle between the collecting and secretory regions. They are divided into an upper group and a lower group. The collecting portions of the upper group, numbering 5 to 12, unite with the rete tubules of the testis or ovary. In the male they form the e^erent ductules of the epididymis. In the female they constitute the epodphoron. Of the lower group a few tubules 'persist in the male, as the paradidymis with its canalicvlus aberrans. In the female they form the paroophoron.


Fig. 208. — Reconstruction of the


The Metanephros

The essential parts of the permanent kidney are the renal corpuscles (glomerulus with Bowman's capsule), secretory tubules, and collecting tubules. The collecting tubules open into expansions of the duct, the pelvis and calyces. The duct itself is the ureter, which opens into the bladder. Like the mesonephros, the


Mtbie orij^in. The ureter, pelvis, calyces, and coDecting Mi;:rMLi*f m' :kt mesottePhric duct. The secretoi)- tuboks and the

'.Ik MiI«^tCT> ul' the meUnephros in a human cmbr^-o of about 9 mm. iv'i»t tHmr Ami OnUr nmr constitute the neidiragenic tissue of the

. vv>.i>u»i.i*.>«-uvt.tifft-rentiutud from the isolated caudal end of .uiU liHxN tu^v i sitmlar origin as the mesonephric tubules.


nation appears, dorsal and somewhat median in position (Fig. 216 B, C). The bud grows at first dorsally, then cranialiy. Its distal end expands and forms the primitive pelvis. Its proximal elongated portion is the ureter. The anlage grows into the lower end of the nephrogenic cord (Fig. 209), which, in 4.6 mm. embryos, is separated from the cranial end of the cord at the twenty-seventh segment. The nephrogenic tissue forms a cap about the primitive pelvis, and, as the pelvis grows cranialiy, is carried along with it. In embryos of 9 to 13 mm. the pelvis, having advanced cephalad through three segments, attains a position in the retroperitoneal tissue dorsal to the mesonephros and opposite the second lumbar segment. Thereafter, the kidney enlarges both cranialiy and caudally without shifting its position. The ureter elongates as the embryo grows in length. The cranial growth of the kidney takes place dorsal to the suprarenal gland (Fig. 232).


Primary collecting tubules grow out from the primitive pelvis in 10 mm. embryos. Of the first two, one is cranial, the other caudal in position, and between these there are usually two others (Fig. 210 B, C). From an enlargement, the ampulla, at the end of each primary tubule grow out two, three.or four secondary tubules. These in turn give rise to tertiary tubules (Fig. 210 D) and the process is repeated until the fifth month of fetal life, when it is estimated that twelve generations of tubules have been developed. The pelvis and both primary and secondary tubules enlarge during development. The first two primary tubules become the major calyces, and the secondary tubules opening into them form the minor calyces (Fig. 211). The tubules of the third and fourth orders are taken up into the walls of the enlarged secondary tubules so that the tubules of the fifth order, 20 to 30 in number, open into the minor calyces as papillary ducts. The remaining orders of tubules constitute the collecting tubules which form the greater part of the medulla of the adult kidney.

When the four to six primary tubules develop, the nephrogenic cap about the primitive pelvis is subdivided and its four to six parts cover the end of each primary tubule. As new orders of tubules arise, each mass of nephrogenic tissue increases in amount and is again subdivided until finally it forms a peripheral layer about the ends of the branches tributary to a primary tubule. The converging branches of such a tubular "tree" constitute a primary renal unit, or pyramid, with its base at the periphery of the kidney and its apex projecting into the pelvis. The apices of the pyramids are termed renal papillee and through them the larger collecting ducts open. The nephrogenic tissue forms the cortex of the kidney, and each subdivision of it, covering the tubules of a pyramid peripherally, is marked off on the surface of the organ by grooves or depressions. The human fetal kidney is thus distinctly lobated, the lobations persisting until after birth, a condition which is permanent in reptiles, birds, and some mammals (whale, bear, ox). The primary pyramids are subdivided into several secondary and tertiary pyramids. Between the pyramids the cortex of nephrogenic tissue dips down to the pelvis, forming the r«(a/co/wm«J (of Berlin). The collecting tubules, on the other hand, extend out into the cortex as the cortical rays-or pars radiaia of the cortex. In these rays, and in the medulla of the kidney, the collecting tubules run parallel and converge to the papilla.

Fig. 2I0.

Fig. 2I1. ces vid their branches from the metaDcphros of a li human embryo (Huber). X SO.



Fig. 2I2. SemidiagrajnniaUc figures of the anlage and difierentiation of renal veucles and early developmental stages of uriniferous tubules of mammals. 1 and 2, Anlage and successive stages in the differentiation of renal vesicles, as seen in sagittal sections; 3, section and outer fonn of tubular anlage before union with collecting tubule at the bceinning of S-shaped stage: 4 and 5. successive stages in the development of the tubules. Itowman's capsule, and glomerulus begiiuiinR with a tubular anlage showing a well-developed S shape (Huber).

Differentiation of the Nephrogenic Tissue

In stages from 13 to 19 mm., the nephrogenic tissue about the ends of the collecting tubules condenses into spherical masses which lie in the angles between the buds of new collecting tubules and their parent stems (Fig. 212). One such metanephric sphere is formed for each new tubule. The spheres are converted into veacles with eccentrically placed lumina. The vesicle elongates, its thicker outer wall forming an S-shaped tubule which unites with a collecting tubule, its thin inner wall becoming the capsule (Bowman's) of a renal corpuscle. The uriniferous tubules of the adult kidney have a dehnite and peculiar structure and arrangement (Fig. 213 ^4). Beginning with a renal corpuscle, each tubule forms a proximal convoluted portion, a U-skaped loop (of Henle) with descending and ascending limbs, a connecting piece, which lies close to the renal corpuscle, and a distal convoluted portion continuous with the collecting tubule. These parts are derived from the S-sha[>ed anlage, which is composed of a lower, middle, and upper limb. The middle limb, somewhat U-shaped, bulges into the concavity of Bowman's capsule (Fig. 213 B). By differentiation the lower portion of the lower limb becomes Bowman's capsule, ingrowing arteries forming the glomerulus (Fig. 213 B, O- The uf^r part of the same limb by enlargement, elongation, and coiling becomes the proximal convoluted tubule. The neighboring portion of the middle limb forms the primitive loop (of Stoerck); the base of the middle limb gives rise to the connecting piece, and the rest of it, with the upper limb of the S, forms the distal convoluted tubule (intennediate piece of Felix). The primitive loop of Stoerck includes both the ascending and descending limbs of Henle's loop and a portion of the proximal convoluted tubule. Henle*s loop is differentiated during the fourth fetal month (Toldt) and extends from the pars radiata of the cortex into the medulla (Fig. 214). The concavity of Bowman's capsule, into which grow the arterial loops of the glomerulus, is at first shallow. Eventually the walls of the capsule grow about and enclose the vascular knot, except at the point where the arteries enter and emerge (Fig. 212, 4 and 5). Renal corpuscles are first fully formed in 28 to 30 mm. embryos. The new corpuscles are formed peripherally from persisting nephrogenic tissue until the tenth day after birth, hence in the adult the oldest corpuscles are those next the medulla. Reconstructions of the various stages in the development of the uriniferous tubules are shown in Fig. 215.


Fic. 213. — Diagrams showing the difFerentiation of Ihe various parts of the uriniferous tubules o( the metanephros (based on the reconstructions of Huber and Stoerck): A, From an adull human kidney; B, C, from human embryos.


Fic. 214. — Diagram showing the relation oF Bowman's capsule and the unniferous tubules to the collecting tubules of the melanephros (Huber). c. Collecting tubules; t, end branches of collecting tubules; r, renal corpuscles; m, neck; pc, proximal convuluted tubule; di, descending limb of Henle's loop, I; al, ascending limb of Henle's loop; dc, distal convoluted tubule; j, junctional tubule.

Fig. 215. — —Several stages metanephri] n the development of the unniferous tubules and glomeruli of the human i of the seventh month (reconstructions bv Huber). X 160.


Renal Arteries. — One or more of the mesonephric arteries is transformed into the renal artery of the metanephros (Broman, 1906). As any one of the mesonephric arteries may thus form the renal artery, and as they anastomose, the variation of the renal vessels both as to position and number is accounted for. Bremer (Amer. Jour. Anat., vol. 18, 1915) derives the renal arteries not from the mesonephric vessels but from a periaortic plexus of multiple aortic origin.

Anomalies. — If the uriniferous tubules fail to unite with the collecting tubules, cystic degeneration may take place. The cystic kidneys of pathology may thus be produced. The nephrogenic tissue of the paired kidney anlages may fuse, resulting in the union of their cortex ("horse-shoe kidney")- Double or triple ureters and cleft ureters are sometimes present.


Differentiation of Cloaca, Bladder, Urethra and Urogenital Sinus

In embryos of 1.4 mm. the cloaca, a caudal expansion of the hind-gut, is in contact ventrally with the ectoderm, and ectoderm and entoderm together form the cloacal membrane (Fig. 216 ^4). Ventro-cranially the cloaca gives off the allantoic stalk. At a somewhat later stage, the cloaca receives laterally the mesonephric ducts and caudally is prolonged as the tail-gut (Fig. 216 5).


In embryos of 5 mm. the ureteric anlages of the metanephroi are present as buds of the mesonephric ducts (Fig. 216 C, D), Next, the saddle-like partition between the intestine and allantois grows caudally, dividing the cloaca into a dorsal rectum and ventral, primitive urogenilal sinus. The division is complete in embryos of 11 to 15 mm., and at the same time the partition, fusing with the cloacal membrane, divides it into the anal membrane of the gut and the urogenital membrane. At 11 mm., according to Felix, the primitive urogenital sinus by elongation and inu>AthiUni is differentiated into two regions: (1) a dorsal vesico-urethra) anlafj*' wlii* h receives the allantois and mesonephric duct, and is connected by the constricted portion with (2) the phallic portion of the urogenital anus (Figs. 217 and 218). The latter extends into the phallus of both sexes and forms a greater part of the urethra (Fig. 219). The vesico-urethral anlagc enlarges and forms the bladder and a portion of the urethra. In 7 mm. embryos the proximal ends of the mesonephric ducts arc funnel shaped, and at 10 mm., with the enlargement of the bladder, these emis arc taken up into its wall until the ureters and mesonephric ducts acquire separate openings. The ureters, having previously shifted their openings into the mesonephric ducts from a dorsal to lateral position, now open into the vesico-urethral anlage lateral to the mesonephric ducts. The lateral walls of the bladder anlage grow more rapidly than its dorso-median urethral wall, hence the ureters are carried cranially and laterally upon the wall of the bladder, while the mesonephric ducts, now the male ducts, open close together on a hillock, Milller's tubercle, into the dorsal wall of the urethra (Fig. 219). The fate of the phallic portion of the urogenital sinus is described on p. 226 in connection with the external genitalia.



Fig. 216. — Pout stages showing the differentiation of the cloaca into the rectum, urethra a bladder (after reconstructions by Pohlman). X about 50. A, from a human embryo of 3.5 mi B, at about 4 mm.; C, at 5 mm.; D, at 7 nun.

Fig. 217. — Reconstructions from a 12 mm. human embryo showing the partial subdiviuou of the cloaca into recttmi and urogenital sinus (after Pohlman). X 65.


Fig. 218. — Reconstruction of the caudal portion of an 11 5 mm human embiyo sbowing the differenti«tion of the rectum, bladder and urethra (after Keibel s model). X 25.


Fic. 219.— Reconstruction of the caudal end of a 29 mm. human embr>-o showjnfi the complete sepMation of tliereclmn and urogenital sinus and the rcUtions of the uroRenital ducts (after Keibel's model). X 15.


The apex of the bladder, continuous with the allantoic stalk at the umbilicus, is known as the urachus. Usually the epithelium of the urachus degenerates, but portions may persist and produce cysts. In some cases it forms after birth a patent tube opening at the umbilicus. Its connective tissue layers always persist as the fibrous lig, umbilicale medium.

The transitional epithelium of the bladder appears at 60 mm. (C H). The outer longitudinal layer of smooth muscle develops in 22 mm. embryos, and, in 26 mm. embryos, the circular muscle appears. The inner longitudinal muscle layer is found at 55 mm. (C H) and the sphincter vesicae in fetuses of 90 mm. (C H).

Anomaly. — A conspicuous malformation is that of a persistent cloaca, due to the failure of the rectum and urogenital sinus to separate.

The Genital Glands and Ducts

Indifferent Stage

In origin and early development, the ovary and testis are identical. The urogenital fold (p. 197) is the anlage of both the mesonephros and the genital gland (Figs. 122 and 220). At first two-layered, its epithelium in embryos of 5 mm. thickens over the ventro-median surface of the fold, becomes manylayered, and bulges into the coelom ventrally, producing the longitudinal genital fold. The genital fold thus lies mesial and parallel to the mesonephric fold. Large primitive sex cells are found in 2.5 mm. embryos in the entoderm of the future intestinal tract (Fuss). At 3.5 nmi. they migrate into the dorsal mesenteric epithelium and thence into the epithelium of the genital fold. At 10 to 12 mm. the genital epithelium shows no sexual differentiation (Fig. 221). There is a superficial epithelial layer and an fffner epithelial mass of somewhat open structure.

Owing to the great development of the suprarenal glands and metanephroi, the cranial portions of the urogenital folds, at first parallel and dose together, are displaced laterally. This produces a double bend in each fold which, in 20 mm. embryos, shows a cranial longitudinal portion, a transverse middle portion between the bends, and a longitudinal caudal portion. In the last named segment, the mesonephric ducts course to the cloaca and here the right and left folds fuse, producing the gettUal cord (fig. 232). As the genital glands increase in size they become constricted from the mesonephric fold by lateral and mesial grooves until the originally broad base of the genital fold is converted into a stalk (Figs. 225 to 227). This stalk-like attachment extends lengthwise and forms in the male the mesorchium, in the female the mesovarium. The urogenital fold is, at the same time, constricted from the dorsal body wall until it is attached only by a narrow mesentery which eventually forms either the ligamentum testis or lig. ovarii.


Fig. 220. — Ventral view of the urogenital folds in a human embryo of 9 mm, [Kollmann).


The Indifferent Stage of the Genital Ducts. — ^The mesonephric ducts, with the degeneration of the mesonephroi, become the male genital ducts. In both

ajF -';■ -^- Atilage of Mullerian dud

Fig. 222. — Tnuisvene sectioni througb the anlage of the right MulleiUn duct fmn a 10 mm. human embryo. X 250. ^.ihoiring theKTOovein tbeurofiRdtalqHtbeliuin; A, threeiectioDscaiidad showing the tubular anlage of the duct.

sexes there also develop a pair of female ducts (of MlUler). In embryos of 10 mm. the MUUerian ducts develop as ventro-lateral thickenings of the urogenital epithelium at the level of the third thoradc segment and near the cranial ends of the mesonephroi. Next, a ventro-lateral groove appears in the epitheliiun of the mesonephric fold (Fig. 222 A). Caudally, the dorsal and ventral lips of the groove dose and form a tube which separates from, and lies beneath, the epithelium (Fig. 222 B). Cranially, the tube remains open as the funnel-shaped ostium abdominale of the MUllerian duct. The solid end of the tube grows caudalward beneath the epithelium, lateral to the mesonephric or male ducts (Figs. 223 to 225). Eventually, by way of the genital cord, the MOllerian ducts reach the median dorsal wall of the urogenilal sintts and open into it (Figs. 2 19 and 238 A) . Their further development into uterine tubes, uterus, and vagina is described on page 219. Embryos not longer than 12 mm, are thus characterized by the possession of indifferent genital glands and of both male and female genital ducts. There is as yet no sexual difFerentiation. The development and position of the MUllerian ducts is well shown in ventral dissections of pig embryos (Figs. 223 and 224) ; the mesonephroi of the pig are of enormous size. In the lowest vertebrates the Miillerian duct arises by a longitudinal splitting of the mesonephric duct.


Fig. 223.— Ventml dissection of nn 18 ni

lliL-antagcsof the M lUlerian ducts. X 7.

Fig. 224. — Veotral dissection of a 24 mm. fAg embiyo showing the anUges of the MuUeriui ducts t.t ft kter stage of deveUqiment thin in Fig. 223. X (>.


jM 'T ^

MesotKpkric tubiilf —^ ^f*i\

Fin. 225,— Transverse section thr

and mesoDephros of a 20 n

Differentiation of the Testis

In the male embryos of 13 mm. the genital glands show two characters which mark them as testes: (1) the occurrence of branched, anastomosing cords of cells, the Uslis cords: (2) the occurrence between epithelium and testis cords of a layer of tissue, the anlage of the tunica alhuginea (Fig. 225). According to Felix, the testis cords are developed suddenly from the loose, inner epithelial mass by a condensation of its cells. The cords converge and grow smaller towards the mesorchium, where they form the dense, epithelial anlage of the rete leslis. Two or three layers of loosely arranged cells between the testis cords and the epithelium constitute the anlage of the tunica albugiiiea. According to Allen (Amer. Jour. Anat., vol, 3, 1904), the testis cords of the rabbit and pig are formed as active ingrowths of cellular cords from the epithelium.


The testis cords soon become rounded and are marked off by connective tissue sheaths from the intermediate cords, columns of undifferentiated tissue which lie between them (Fig. 226), Toward the rete testis the sheaths of the testis cords unite to form the anlage of the mediastinum testis: The testis cords are composed chiefly of indi^erent cells with a few larger genital cells. The cells gradually arrange themselves radially about the inside of the connective tissue sheath as a many-layered epithelium, in which, during the seventh month, a lumen appears. The lumina appear in the peripheral ends of the testis cords, and, extending toward the rete testis, meet lumina which have formed there. Thus the solid cords of both are converted into tubules. The distal portions of the testis tubules anastomose and form the tubuli contorti. Their proximal portions remain straight as the tubuli recti. The rete testis becomes a network of small tubules which finally unite with the collecting tubules of the mesonephros (see p. 218).


Fig. 226.— Section through the testis of a. 100 mm. bumaa fetus. X 44.


The primitive genital celb of the testis cords form the spermatogonia of the spermatic tubules and from these at puberty are developed the later generations of spermatogonia (p. 14). The indifferent cells of the tubules become the sustentacular cells (of Sertoli) of the adult testis. Primitive genital cells of the intermediate cords are transformed into large pale cells, which, after puberty, are numerous in the interstitial connective tissue and hence are caCed interstitial cells. The intermediate cords themselves are resorbed, but the connective tissue sheaths of the tubules unite to form septula which extend from the mediastinum testis to the lunica albuginea. The latter becomes a relatively thick layer in the adult testis and is so called because of its whitish appearance.

Anomalies. — The testis may be congenitally absent; the glands may be fused; or they may fail to descend into the scrotum (cryptorchism). Duplication of the testis is

The Differentiation of the Ovary

The primitive ovary, like the testis, consists of an inner epithelial mass bounded by the parent peritoneal epithelium. The ovarian characters appear much more slowly than in the testis. In fetuses of 50 to 80 mm. (C H) the inner epithelial mass, composed of indifferent cells and primitive genital cells, becomes less dense centrally and bulges into the mesovarium (Fig. 227). There may be distinguished a dense, outer cortex beneath

Fic. 227.— Section of an human fetus

the epithelium, a clearer medullary zone contairang large genital cells, and a dense, cellular anlage in the mesovanum the pnmitive rele ovartt, which is the homologue of the rete testis. No efnikeltal cords and no tuntca albuginea are developed at this stage, as in the testis. Later, three important changes take place: (1) There is an ingrowth of connective tissue and blood vessels from the hilus, resulting in the formation of a mediaslinum and of septula. (2) Most of the cells derived from the inner epithelial mass are transformed into young ova, the process extending from the rete ovarii peripherally (Fig. 227). (3) In fetuses of from 80 to 180 mm. {C R) length the ovary grows rapidly, owing to the formation of a new peripheral zone of cells, perhaps derived in part from the peritoneal epithelium. At the end of this period the septulEe line the epithelium with a fibrous sheath, the anlage of the tunica albuginea. Hereafter, although folds of the epithelium are formed, these do not penetrate beyond the tunica albi^;inea, and aU cells derived from this source subsequently degenerate. This new peripheral zone, according to Felix, is always a single cellular mass in man, cords or "PflUger's tubes" never growing in from the epithelium. , Generally it has been believed that the primary follicles are derived from the subdivision of such cwds.

Coincident with the origin of a new zone of cells at the periphery of the ovary goes the degeneration of young ova in the medulla. By the ingrowth into thif


Fig. 228.— Ovaiy of five-moDths' fetus, showing primordial follicles fDe Lee).

region of connective tissue septa, the ova are separated into dusters or cords, the genital cells of which all degenerate, leaving in the medulla only a stroma of connective tissue. Late in fetal life indifferent cells, by surrounding the young ova of the cortex, produce primordial follicles (Fig. 229 A). During the first year after birth the primitive follicles are transformed into the vesicular {Graafian) JoUicles. By cell division the follicle cells form a zone many layers deep about the young ovum (Fig. 229 B). Next a cavity appears in the sphere of follicle cells, enlarges, and produces a vesicle filled with fluid (Figs. 3 and 230). The ovum is now located eccentrically and the follicle cells directly surrounding it constitute the cumulus oSphorus (egg-bearing hillock). About the stratum granulosum formed by the original follicle cells there is diflferentiated from the stroma of the ovary the theca folliculi. This is composed of an inner vascular tunica interna and of an outer fibrous tunica externa.


Fig,. 229. — PrimordEal ova and early stages in the devekipment of the Graafian follicle (De Lee).

Fig. 230. — Graafian follicle and ovum from the ovary of a fifteen year-old girl. X 30.


Fully formed Graafian follicles are found in the ovaries during the second year and they may even be present before birth. Ovulation may occur at this time, but usually these precociously formed follicles degenerate with their contained ova. Thus, although thousands of ova are produced in the ovary, only a comparatively few are set free ready for fertilization during the sexually active life of the female, from puberty to the climacteric period or menopause. The relation of ovulation to menstruation has been discussed on p. 87.

The Corpus Luteum

After ovulation, a blood clot, the corpus hemorrhagicum, forms within the empty follicle. The follicle cells of the stratum granulosum proliferate, enlarge, and produce a yellow pigment (R. Meyer, Arch. Gynakol., Bd. 91, 1911). The whole structure, composed of lutein cells and connective tissue strands, is termed the corpus luteum or yellow body. The blood clot is resorbed and replaced by fibrous scar tissue white in color and known as the corpus albicans. If pregnancy does not intervene the corpus luteum spurium reaches its greatest development within two weeks and then degenerates. In cases of pregnancy the corpus luteum verum continues its growth until, at the thirteenth week, it reaches a maximal diameter of 15 to 30 mm. At birth it is still a prominent structure in the ovary and it is believed to produce an internal secretion, for if the corpus luteum is removed the ovum fails to attach itself to the wall of the uterus, or if already embedded, development ceases (Fraenkel). An influence in retarding ovulation and stimulating the mammary gland function has also been shown experimentally (L. Loeb; O'Donoghue).

The Rete Ovarii

The cells of the rete ovarii remain compact, distinct, and continuous only with the stroma of the medulla, the tnedtUlary cords. The anlage is differentiated into a network of solid cords in 60 mm. (C H) fetuses and these connect with the collecting tubules of the mesonephros. Some time before birth lumina appear in the cords, transforming them temporarily into tubules homologous with those of the rete testis.

Comparison of the Testis and Ovary

It is clear that the superficial epithelium after forming the inner epithelial mass takes no further part in the differentiation of the testis and only a small part, if any, in that of the ovary. The testis cords, rete testis, and tunica albuginea are formed early from the inner epithelial mass, which determines their form. The inner epithelial mass of the ovary develops slowly and its passive cells are separated and surrounded by actively ingrowing connective tissue. The primordial follicles when developed are not the homologues of the testis cords, and the tunica albuginea appears late. The rete ovarii is the homologue of the rete testis, but remains a rudimentary structure.

uecting tubules

ise to the rete

.uid unite with

In the !iioua. and the latter . They convey iuct, which thus ^ancy the ductuli connective tissue H the male genital cranial end persists and, as the ductus

its opening into the ich is evaginated the


a<cf Ji columnar cells which form . .^.*^»iiviS'. -Tc surrounding mesenchyma

^iv*4> Ji mesonephric collecting

^ 4.*» ^v'.MHidl structures (Fig. 238 C).

,:x vestigial paradidymis. The

x^jTtS'^wt at 30 mm. The middle

^^^^ ^tjc ^xcsfe!^ as the appendix testis;

A ,s?«c4t in the median dorsal wall

-^^

. k

-3^ >^^9tid ot the female and is called

^ w*N A :^B»entar>- structure, yet some w -— ^-^ ;v«isiuu[ group of mesonephric

THE GENITAL GLANDS AND DUCTS— UTERUS AND VAGINA lig

collecting tubules which forms a rudimentary structure, the r/kN>>Aaroii (.Fig. 238 B). In its cords limiina appear, the epithelial cells become cilia teil, iind smooth muscle tissue is developed corresponding to that of the ef^ididymis. The caudal group of mesonephric tubules constitute the iHMrodipkoron. Usually the greater part of the male genital ducts atrophy in the female, the process beginning at 30 mm. Thus the tubules of the epoophoron are left without an outlet. Portions of the mesonephric ducts persist as Carlner^s amah.

These may extend as vestigial structures from the epodphoron to the lateral walb of the vagina, passing through the broad ligament and the wall of the uterus. They 0|>en into the vagina close to the free border of the hymen (R. Meyer). The canals are nirt^ly pn*!»ent throughout their entire length and are absent in two-thirds to three-quarters of the ca»eH examined. It is an interesting fact that in male and female embryos the ducts of the op|Xisite sex begin to degenerate at the same stage, 30 mm.

The Uterine Tubes, Uterus and Vagina

The Mlillerian, or female ducts, after taking their origin as described on p. 210, grow caudally. following the course of the mesonephric ducts (Fig. 224). At first lateral in position, the Miillerian ducts cross the mesonephric ducts and enter the genital cord median to them (Fig. 238 -4). In embryos of 20 to 30 mm. their caudal ends are dorsal to the urogenital sinus, extending as far as the Mlillerian tubercle, a projection into the median dorsal wall of the vesico-urethral anlage formed by the earlier entrance of the mesonephric ducts (Fig. 219). This tubercle marks also the position of the future hymen. In fetuses of 70 mm. (C H) the Mlillerian ducts break through the wall of the urethra and open into its cavity. Before this takes place the caudal .ends of the MUUerian ducts, which are pressed close together between the mesonephric ducts in the genital cord, fuse, and in both male and female embryos of 20 to 30 mm. give rise to the unpaired anlage of the uterus and vagina (Figs. 219 and 231 A). The paired cranial portions of the Miillerian ducts become the uterine tubes. During development the ostial ends of the uterine tubes undergo a true descensus from the third thoracic to the fourth lumbar vertebra.

The utero-vaginal anlage of the male remains rudimentary. The uterine portion of the anlage degenerates with the paired portions of the MUllerian ducts. The vaginal portion remains as the vagina masculina (prostatic utricle), and the extreme cranial end of each MUllerian duct persists as the appendix testis.

As pointed out by Toumeaux and especially by Felix, the term uterus mascufintif as applied to the remains of the utero-vaginal anlage is a misnomer, for the vaginal portioo of the anlage persists and its uterine portion degenerates.


Uterus and Vagina

Since the Mullerian ducts develop in the urogenital folds, they make two bends m their course {Fig. 231 A) corresponding to those of the folds (p. 208). Each consists of a cranial longitudinal pwrtion, a middle transverse portion, and a caudal longitudinal portion which is fused with its fellow to form the utero-vaginal anlage. At the angle between the cranial and middle portions is attached the inguinal fold, the future round ligament of the uterus (Figs. 232 and 233). The mesenchyma condenses about the uterovaginal anlage and the middle transverse portion of the Miillerian ducts, forming a thick, sharply defined layer, from which is differentiated the muscle and connective tissue of the uterus and vagina (Fig. 231 B). As development proceeds, the cranial wall between the transverse portions of the Miillerian ducts bulges outward so that its original cranial concavity becomes convex (Fig. 231 B). The middle transverse portions of the ducts are thus taken up into the wall of the uterus forming its fundus, while the narrow cervix of the uterus and the vagina arise from the utero-vaginal anlage. Through the differentiation of its mescnchymatous wall, the uterus is first brought into relation with the round ligament.


Fig. 2.11.— Diagrams showing the development of the uterus and vagina (modified after Felix)


At 80 mm. (C R) the mucosa and muscularis maybe distinguished. The first circular muscle fibers appear in 180 mm. (C R) fetuses, the other muscle layere develop later. The epithelium of the uterine tubes and the tubal portion of the uterus (fundus) remains simple, with cylindrical or cuboidal cells. The tubular fundus glands of the uterus may not appear until near puberty. The vagina is at first without a lumen. From the third to the sixth months of fetal life dorsal and ventral outgrowths of the epithelium form the fornixes of the vagina. The vaginal lumen appears in fetuses of 150 to 200 mm. (C R), arising from the degeneration of the central epithelial cells. The fomices hollow out and form the boundary line between the cervii uteri and the vagina. The epithelial cells of the former become stratified and cylindrical, those of the vagina are of the stratified squamous type (38 mm.


The Hymen

At the point where the utero-vaginal anlage breaks through the wall of the urogenital sinus there is present the tubercle of Miiller, which marks the lower limits of the vagina. The tubercle is compressed into a disk lined internally by the vaginal epithelium, externally by the epithelium of the urogenital sinus. These layers with the mesenchyma between them constitute the hymen, which thus guards the opening into the vagina. A circular aperture in the hymen is for a time closed by a knob of epithelial cells, but later when the hymen becomes funnel-shaped the opening is compressed laterally to form a sagittal slit, the ostium vagina, Miiller's tubercle persists in the male as the coUiculus seminaliSy from the summit of which leads off the prostatic utricle.

The Growth of the Uterus

The uterus grows but slowly until near puberty, being about the same length (27 mm.) at birth as in a girl of nine years. Just before and after puberty growth is more rapid, a length of 72 mm. being attained at 18 years. This is nearly the maximal length of the virginal uterus.

Anomalies. — Owing to the complicated processes leading to their formation, many cases of abnormal uterus and vagina occur. A complete classification is given by Felix (Keibel and Mall, vol. 2.) The more common anomalies are (1) complete duplication of the uterus and vagina due to the failure of the Miillerian ducts to fuse; (2) uterus bicornis, due to the incomplete fusion of the ducts. Combined with these defects the lumen of the uterus and vagina may fail, partly or completely, to develop and the vaginal canal may not open to the exterior. (3) The body of the uterus may remain flat (uterus planifundis) or may fail to grow to normal size (uterus fetalis and infantalis). (4) Congenital absence of one or both uterine tubes or of the uterus or vagina rarely occurs, but may be associated with hermaphroditism of the external genitalia.

The Ligaments of the Internal Genitalia. — Female, — ^The loose mesenchyma of the genital cord gives rise laterally to the broad ligaments of tJie uterus in females. A portion of the primitive genital fold unites the caudal end of the ovary to the genital cord. This acquires connective tissue and smooth muscle fibers and forms the proper ligament of the ovary. Since the uterus develops in the genital cord the ligament of the ovary extends to the posterior surface of the uterine wall. In the male the homologue of the proper ligament of the ovary is the ligament of the testis.

In both sexes the inguinal fold extends from the urogenital fold to the crista inguinalisy located on the inside of the ventral abdominal wall, a point which marks the future entrance of the inguinal canal. The inguinal fold thus forms a bridge in 14 mm. embryos between the urogenital fold (in the middle portion of which the uterus develops in the female) and the abdominal wall at the entrance of the inguinal canal (Fig. 232). In the inguinal crest is differentiated the conical anlage of the chorda gubernactdi, which later becomes a fibrous cord. The abdominal muscles develop around it, forming a tube, the inguinai canal, and the external oblique muscle leaves a foramen, through which the chorda connects with a second cord termed in the male the Ug. scroti, in the female the lig. labiaJe. 'I"he chorda gubernaculi and the Ug. lahiale together constitute the round Hgamcnl of the uterus (Fig. 233), as they form a continuous cord extending from the urogenital fold to the base of the genital tubercle. With the development of the uterus in the urogenital fold, the round liganient becomes attached to its ventral surface,

The ligamenlum testis, like the lig. ovarii, develops in the geniu! fold anti extends from the caudal end of the testis to the mesonephric fold at a point opposite the attachment of the inguinal fold. The inguinal fold, as we have seen, is continuous with the inguinal crest and the chorda gubernaculi. A cord develops in the mesonephric fold and connects the ligamentura testis with the chorda gubernaculi, for in the male the uterus does not intervene between these two. The chorda gubernaculi is continued to the integument of the scrotum by way of the ligamentum scroti. Thus there is formed a continuous cord, the gubernactilum testis, extending from the caudal end of the testis through the inguinal canal to the scrotal integument. The gubemaculum is composed of the ligamentum testis, of a mesonephric cord, of the chorda gubernaculi, and of the Hg. scroti, and is the homologue of the ovarian ligament plus the round ligament of the uterus, between which the uterus intervenes (Fig. 233).


Fig. 232, — Ventral dissection of a human embryo of 23 mm. showing the urogenital organs. The light Euprarenal gland has been removed to show the metanephros.


The Descent of the Testis and Ovary

The original position of the testis and ovary is changed during the later stages of development. At first they are elongate structures, extending in the abdominal cavity from the diaphragm caudally towards the pelvis (Fig. 220). Since their caudal ends continue to grow and enlarge while their cranial portions are atrophying there is a wave-like shifting of the glands caudad. An actual internal descent, however, does not occur. When the process of growth and degeneration is completed the caudal ends of the testis lie at the boundary Une between the abdomen and pelvis, whereas the ovaries are located in the pelvis itself, a position which they retain. Owing to the rotation of the ovary about its middle point as an axis it takes up a transverse position. It also rotates nearly 180° about the Mullerian duct as an axis and thus comes to lie caudal to the uterine tube.


Fic, 233,— Ventral dissection of a female human embryo of 34 mm. The urogenital organs are dissected out and the left suprarenal gland has been removed.

The testis normally leaves the abdominal cavity and descends into the scrotum. As described above, there is early developed between the testis and the integument of the scrotum a fibrous cord, the gubernaculum testis. Owing to changes in the position of the ventral abdominal wall and umbilical arteries, changes connected with the return of the intestinal coils into the ccelom, there are formed in each side of the abdominal wall sac-like pockets, the anlages of the vaginal sacs. Close to each saccus vaginalis lies the caudal end of a testis, while extending into the scrotum outside the peritoneum is the gubernaculum testis. The saccus vaginalis later invaginates into the scrotum over the pubic bone.


Whether due to the active shortening of the gubernaculum testis or to its retarded growth, the descent of the testis into the vaginal sac begins during the seventh month of fetal life, and by the eighth month, or at least before birth, the testis is usually located in the scrotum (Fig. 234). It must be remembered that the testis and gubernaculum are covered by the peritoneum before the descent begins, consequently the testis follows the gubernaculum along the inguinal canal dorsal to the peritoneum, and, when it reaches the scrotum, is invaginated into the saccus vaginalis, but does not lie in the coelomic extension. The gubernaculum is said to degenerate during the descent of the testis or immediately after. Abnormally, the testis may remain in the abdomen, a condition known as cryptorchism (concealed testis) and associated with sterility in man. In some mammals (bat, whale, and elephant) it is the normal condition.


Shortly after birth the inguinal canal, connecting the saccus vaginalis with the abdominal cavity, becomes solid and its epithelium is resorbed. The now isolated vaginal sac becomes . the tunica vaginalis of the testis. Its visceral layer is closely applied to the testis and its parietal layer forms the lining of the scrotal sac. The ductus deferens and the spermatic vessels and nerves are of course carried down into the scrotum with the testis and epididymis. They are surrounded by connective tissue, and, with the spermatic vessels, constitute the spermatic cord. Owing to the descent of the testis, the ductus deferens is looped over the ureter in the abdomen (Fig. 238 C). In some cases the inguinal canals remain open so that the testis may slip back into the abdominal cavity. Such conditions may lead to inguinal hernia of the intestine. Open inguinal canals occur normally in rodents.

In the female, shallow peritoneal pockets, the diverticula of Nucky correspond to the vaginal sacs of the male. Rarely a more or less complete descent of the ovary into the labium majus occurs.


Fig. 234. — Descent of the testis (Cunningham), a.c.j Abdominal cavity; />.r., vaginal sac; i.f testis; s.y scrotum; /.»., tunica vaginalis; x., rudiment of vaginal sac.


The External Genitalia

Indifferent Stage

The external genitaUa of both sexes are identical until the beginning of the third month of development, when the indifferent anlages become moulded into sexually distinct organs. There develops early in the midline of the ventral body wall, between the tail and umbilical cord, the cloacal tubercle. Upon this appears a knob-like structure, the phallus, and the two together constitute the genital eminence (Fig, 220). Cranially about the phallus the cloacal tubercle forms a crescent-shaped genital tubercle, which later gives rise to the right and left genital swellings. The phallus grows rapidly carrying with it the phallic portion of the urogenital sinus (Fig. 219). At the end of the phallus the epithelium of the sinus forms a soUd urethral plate. Along the anal surface of the phallus, in the midline, the wall of the urogenital sinus breaks through to the exterior and forms the slit-like primitive urogenital opening (Fig. 235), In embryos of 21 to 26 mm., at the end of the phallus, the glans is marked off from the base by a circular groove, the coronary sulcus (Figs. 232 and 235 B).


Fig. 235. — Three stages in the development of the external genitalia in hutnaD cmbrjtM oC 24 to 34 mm. (after Toumeux in Heisler). Indifferent stage: 1, Phallus; 2, glans; 3, primitive urogenital opening; 4, genital tubercle or swelling; 5, anus; 6, cocc>i.


Female

A deep groove appears about the base of the phallus, separating it from the genital tubercle, which becomes a circular swelling (Fig. 235 C), From the swelling differentiates: (1) cranially, the mons pubis; (2) laterally, the right and left labia majora; (3) caudally, the posterior commissure of the labia majora (Fig. 236) . The glans of the phallus forms the glans cliloridis of the female. On the anal surface of the phallus, beginning at the coronary sulcus, the primitive urogenital opening closes distally, forming the urethral groove. Proximally it remains open, as the definitive urogenital opening near the base of the phallus. The lips of this groove and opening enlarge and become the labia minora. The cranial surface of the phallus forms a fold, the prepucium. which, however, is not the homologue of the male fore-skin. This in the female is represented by a ring-like rudiment at the base of the glans clitoridis.

Male

The phallus grows rapidly at its base so that the glans and primitive urogenital opening are carried some distance from the anus (Fig. 237). A cylindrical collar of the epithelium, incomplete on the anal side, grows down into the end of the glans, which becomes the glans penis. By the disappearance of the central cells of the epithelial downgrowth an outer cylindrical mantle, the prepuciiim or fore-skin, is formed about the spherical glans (cf. Fig. 158). Where the epithelial downgrowth is incomplete the glans and fore-skin remain connected, the [)ersisting connection being the frenulum prepucii.


Fig. 236 — Three stages in the dev'elopment of the female external genitalia (afta Toumeuj in Heisler). 1, Clitons; 2, j^lans clitoridis; 3, urogenital aperture on each side of which are the lalna minora (7); 4, labia majora; 5, anus; 7, labia minora.


Fig. 237.— Three stages in the development of the male external genitals (after Toumeux is Heisler). I. Penis; 2, glans; 3, urogenital groove; 4, genital swellings corresponding to latna majora of female; 5, anus; 7, scrotal area with perineo-scrotal raphe.

The urogenital sinus, as we have seen, extends out into the phallus and in the glans becomes the solid urethral plate. With the great elongation of the male phallus, the open portion of the urogenital sinus also is lengthened and forms the greater part of the penile urethra. In fetuses of 70 mm. (C R) the groove-like primitive urogenital opening, located in the male near the glans and distant from the anus, begins to close and thus forms a further portion of the urethra. The failure of this opening to close gives rise to an anomaly known as hypospadias. The lips of the urogenital opening, it will be remembered, correspond to the labia minora or nymphce of the female. Finally, at 100 mm. (C H ?), the solid urethral plate of the glans splits, forms a groove to the tip of the glans, and this groove in turn is closed, continuing the urethra to the definitive opening at the tip of the glans. Owing to the rapid elongation of the penis, there is formed between its base and the anus an unpaired area, termed by Felix the scrotal areay as it is the anlage of the scrotum. At 60 mm. (C H) this forms a median scrotal swelling, continuous laterally with the paired genital swellings which form the labia majora in the female. When the scrotal sac develops in the scrotal area, the dense tissue in the median line is compressed and forms the septum scroti. The attachment of this septum forms an external median depression. The testes descend into the vaginal sacs of the scrotum through the paired genital swellings, as described on p. 223, but the scrotum itself is an unpaired structure derived from the scrotal area. After the descent of the testes the genital swellings disappear (Fig. 237 C).


Comparing the male and female external genitalia, it is plain that the glans penis and glans clitoridis are homologous. The labia minora correspond to the phallic folds which close about the primitive urogenital opening and the anal surface of the penis. The greater part of the stem of the male phallus does not develop in the female. On the other hand, the genital swellings enlarge and become the mons pubis and labia majora of the female, while in the male they are only temporary structures. The scrotum does not develop in the female, being represented only by the posterior commissure of the labia majora.

Accessory Glands

The prostate gland develops in both sexes as entodermal outgrowths of the urogenital sinus (urethra) both above and below the entrance of the male ducts. The tubules arise in five distinct groups and total an average number of 63 (Lowsley, Amer. Jour. Anat., vol. 13, 1912). In the male the surrounding mesenchyme differentiates both white fibrous connective tissue and smooth muscle fibers into which the anlages of the prostate grow. In the female the tubules remain isolated. The prostatic anlages appear in male fetuses of 55 mm. (C H), chiefly as dorsal and lateral outgrowths. Two- thirds of the tubules are caudal to the openings of the male ducts. In the female the gland is rudimentary (paraurethral ducts), the maximal number of outgrowths being three.


The biilbo-urethrd glattds (of Cowper) arise in male embryos of 30 mm. (C R) as solid, paired epithelial buds from the entoderm of the urogenital sinus. The buds penetrate through the mesenchyme which forms the corpus cavernosum urethraN about which they enlarge. The glands branch, and, at 120 mm. (C R), the epithelium becomes glandular. The vestibular glands (of Bartholin) are the homologues in the female of the bulbo-urethral glands. They appear in embryos of 30 mm. (C R), grow until after puberty, and degenerate after the climacterium.


Homologies of Internal and External Genitalia

Male '

Indiferent Stage

Mesonephric collecting tubules. Cranial group. Caudal group.

Mesonephric duct.

Female

Ductuli elTcrentes.

Paradidymis. !

Epoophoron. Paroophoron

Ductus ei)ididymidis. Ductus deferens. Seminal vesicle. Kjaculatory duct.

Gartner's canal.

(!) Appendix testis. (2)

{?>) Utriculus i^rostaticus (W'lgina masculina).

Miillerian duct.

(1) Uterine tube.

(2) Uterus.

(3) Vagina.

Colliculus seminalis.

Miiller's tubercle. Urogenital sinus.

Hymen.

(1) Prostatic and membranous urethra.

(2) Prostate gland.

(3) Bulbo-urethral glands.

(1) Urethra and vestibule.

(2) Paraurethral ducts.

(3) Vestibular glands.

Glans penis.

Anal surface of f>enis.

(1) (2) (3) Scrotum.

Phallus. Glans. Lips of urethral groove.

Genital swellings.

Glans clitoridis. Labia minora.

(1) Mons pubis.

(2) Labia majora.

(3) Posterior commissure.

Hermaphroditism. — True hermaphroditism consists in the development and persistence of both testes and ovaries in the same individual. It is of rare occurrence in man (according to Pick, Arch. f. mikr. Anat., Bd. 84, 1914, four authentic cases only), is not uncommon in the lower vertebrates, and is the normal condition in many invertebrates (earth worms, snails, etc.). In cases of human hermaphroditism of this type the secondary sexual characters are usually intermediate between the male and female, tending now one way, now the other. The external genitalia show a small penis with hypospadias, cryptorchism, or small vaginal opening.

False /lermaphroditism is characterized by the presence of genital glands of one sex in an individual which exhibits more or less marked secondary characters and external genitalia of the opposite sex. In masculine hermaphroditism an individual possesses testicles, but the external genitals (by retarded development) and secondary sexual characters are like those of the female. In feminine hermaphroditism ovaries are present, but the other sexual characters {e. g., abnormally developed clitoris) are male. The cause of hermaphroditism is unknown.



Fig. 238. — Diagrams to show the development of male and female generative oi^ns from a common type (Allen Thomson).

A. Diagram of the primitive urogenital ol^ns in the embryo previous to sexual distinction. 3, Ureter; 4, urinary bladder, 5, urachus; ol, genital ridge from which either the o\ary or testicle is formed; H', left mesonephros; u', «■, right and left mesonephric ducts; m, m, right and left Mtillerian ducts which unite and course with the mesonephric ducts in g.c, the genital cord; iig, urogenital sinus; i, lower intestine; cl, cloaca; cp, phallus which becomes clitoris or penis; h, fold of integument from which the labia majora are formed.

B. Diagram of the female t}!^ of sexual ocgans. 0, Left ovary; fo, epoOphoron; II', scattered remains ol caudal mesonephric tubules (paroGphoron) ; d C, remains of the left mesonephric duct (canal of Gartner) represented by dotted lines; that of the right side is marked ic; /, abdominal opening of the left uterine tube; m, uterine tube of the right side; u, uterus; g, round ligament, corresiionding to gubemaculum in part: i, lower intestine; va, \'agina; h, situation of the hymen (MUller's tubercle); C, vestibular gland (of Bartholin), and immediately above it the urethra; cr, corpus cavemosum clitoridis; sc, corpus cavemosum of clitoris; n, n>Tnplia; /, labium; t, vulva,

C. Diagram of the male type of sexual organs. I, Testicle in the place of its original formation; e. caput epidid>'midLs; vd, ductus deferens; H', scattered remains of the mesonephros, constituting the organ of Giraldte, or tlie paradid>-mis; vh, vas aberrans; m, MUllerian duct, the upper part of which remains as the appendix testis, the louver part, represented by a dotted line descending (o the prostatic utricle, constitutes the occasionally existing comu and tube of the vagina masculina; |, gubemaculum; M, vrsicula seminalis; pr, prostate gland; C, bulbourethral gland of one side; cp, corpora cavernosa penis cut short; sp, corpus cavernosum urethra; i, scrotum; I', and the dotted lines above, indicate tbe path of descent of the testis and epidid>'mis from the abdomen into the scrotum whereby the ductus deferens, td, becomes lotqied over the ureter, 3.

The Uterus during Menstruation and Pregnancy

THE UTERUS DURING MENSTRUATION AND PREGNANCY: PLACENTA

AND DECIDUAL MEMBRANES

Two sets of important changes take place normally in the wall of the uterus. One of these is periodic between puberty and the fortieth to forty-fifth year and is the cause of menstruation (monthly flow). These periodic changes may also be regarded as preparatory to the second set of changes which take place if pregnancy occurs and give rise to the decidual membranes and placenta.

Menstruation

The periodic changes which accompany the phenomenon of menstruation form a cycle which occupies twenty-eight days. This period is divided into: (1) a phase of uterine congestion — six or seven days; (2) a phase of hemorrhage and epithelial desquamation — three to five days; (3) a phase of regeneration of the uterine mucosa — four to six days; (4) finally, an interval of rest or slight regeneration — twelve to sixteen days.


During the first phase, the uterine mucosa is thickened to two or three times its normal condition, both because of vascular congestion and on account of the actual increase of reticular tissue. The uterine glands become longer and their deeper portions especially are dilated and more convoluted because they are filled with secretion. From the enlarged veins and capillaries blood escapes into the reticular tissue and forms subepithelial masses. At the end of this stage the uterine mucosa shows a deep spongy layer and a superficial compact layer, these corresponding to similar layers in the decidual membranes of pregnancy.


During the second phase, that of menstruation proper, blood escapes into the uterine cavity between the epithelial cells of the mucosa and there is an active discharge of secretion from the uterine glands. The surface epithelium and a portion of the underlying tissue may or may not be desquamated. In some normal cases the surface epithelium and most of the compact layer may be expelled, aided by painful contractions of the uterus.


In the third stage, the mucosa becomes thin, with straight, narrow glands, between which are fusiform, closely packed stroma cells. Any surface epithelium which has been desquamated is regenerated from the epithelium of the glands and gradually the mucosa returns to a resting condition during which, however, there is a slow process of cell proliferation.


The premenstrual changes of the first phase are regarded as the most important part of the cycle, the uterine mucosa thereby being prepared for the reception of a fertilized ovum and for the development of the decidual membranes. Menstruation proper, as seen in the second phase, is the result of an over-ripe condition of the mucosa and has been regarded as the abortion of an unfertilized ovum.


The Implantation of the Ovum

The earliest known human ova are already completely embedded in the uterine mucosa. From the careful study of early human embryos by Bryce and Teacher, Peters, Herzog, and others, and from more complete observations on other mammals (e. g., guinea-pig), the course of events in man is tolerably certain.


Ovulation sets the ripe ovum free within the abdominal cavity, from whence the beating cilia on the fimbriae of the uterine tube sweep it into the tubal ampulla. There it may be fertilized and carried to the uterus by the cilia of the tubal epithelium. During this period of migration, which is estimated as occupying about eight days, the ovum loses its surrounding follicle cells and pellucid membrane and begins its development. Thus when it reaches the uterus, and is ready for implantation, it is an embryo with trophectoderm developed although the blastodermic vesicle is not more than 0.2 mm. in diameter (Graf Spee).


If ovulation precedes menstruation proper by ten or twelve days, as Ancel and Villemin maintain, then the embryo would reach the uterus during the premenstrual period. The congestion and loosening of the uterine tissue at this time would favor the implantation of the embryo and the glandular secretion would afford nutriment for its growth until implantation occurs. The first phase of menstruation according to this view, that of Grosser, prepares the uterine mucosa for the reception of the embryo. If pregnancy supervenes, it soon inhibits any further premenstrual changes so that menstruation does not occur.


The embryo penetrates the uterine mucosa as would a parasite, the trophectoderm supposedly producing an enzyme which digests away the maternal tissues until the embryo is entirely embedded (Fig. 239). During implantation, the trophectoderm also probably absorbs nutriment from the uterine mucosa for the use of the embryo. The process of implantation is supposed to occupy one day. At the point where the embryo enters the mucosa a fibrin clot soon appears and eventually the opening is completely closed (Fig. 239).


The Decidual Membranes (Figs. 240 and 241). — ^With the increase in size of the embryo and chorionic vesicle, the superficial layers of the maternal mucosa bulge into the cavity of the uterus and form the decidua capsularis (old term, decidua reflexa). The deep layer of the mucosa on the side of the embryo away from the uterine cavity forms the anlage of the future maternal placenta and is the decidua basalts (d. serotina). The mucosa lining the rest of the uterus is differentiated into the decidua vera (d. parietalis of Bonnet).


Differentiation of the Trophectoderm

The chorion is at first composed of an inner mesodermal layer and an outer epithelial layer, the trophectoderm (Fig. 74). From the trophectoderm there is developed an outer syncytial layer, the Iroplioderm (Fig. 239). This invades and destroys the maternal tissues. In it large vacuoles are formed either directly by the syncytial tissue (Bryce and Teacher) or by the blood escaping from the ruptured vessels under pressure (Peters), and thus blood lacuna are produced. The trophoderm thickens at intervals and forms on the surface of the chorion solid cords of cells, the primary villi (Fig. 239). The chorionic mesoderm grows out into these cords, which branch profusely and become secondary or true villi (Fig. 242). During the development of the villi, the blood lacuna; in the trophoderm around the villi expand, run together, and produce intervillous blood spaces which surround the vilK and bathe the epithelium with blood. The syncytial trophoderm, from being a spwngy network, is now reduced to a continuous layer covering the outer surfaces of the vilU and chorion. Branches of the umbilical vessels develop in the mesoderm of the chorion and villi. The mesodermal core of each villus and its branches is now covered by a two-layered epithehum, an inner ectodermal layer w'th distinctly outlined cubical cells, and an outer syncytial trophoderm layer (Fig. 242). The embryo is established. The blood vessels of the uterus open into the intervillous blood spaces and here the maternal blood circulates. The syncytial trophoderm covering the villi is bathed in the maternal blood. Its functions are three-fold: (1) like endotheliimi it prevents the coagulation of the maternal blood; (2) it allows transfusion between the blood of fetus and mother; and (3) it assimilates substances from the maternal blood and transfers them to that of the embryo.


Fig. 239. —Blood dot KIk- 2,W.^Section through a human embryo of .16 m diagrammalic after Pelers). am., Amniotic cavity; i.x., body stalk; «i:f., embryonic ectodenn; eni., entoderm; nics., mesoderm; y.s., yolk sac.



Fig. 240. — Section through a gravid uterus q( twelve to fourteen daj's (KollmuiD). epithelium also forms solid columns of cells which anchor the ends of certain villi to the maternal tissue. Islands, or nodes of epithelial cells, are attached to the villi or lie free in the decidua basalis and represent portions of the primitive trophectoderm. In the vessels of the chorionic villi the chorionic circulation of the


Fig. 241. — Diagrammatic section through a. pregnant uterus at the seventh or eighth week (after Allen Thomson). c,c, Openings of uterine tubes; c', cervix with mucvus |dug; dv, decidua vera or parietalis; dr, decidual capsularis; 4s, decidua basilis (serotina); ck, chorion with villi; the villi extaMUng into the decidua basalis are from the chorion frondo«um; am, amnion; u, umbilical coid; al, allantoia;


Fig. 242.~Diagram illustrating the second phase in the development of the chorionic villi sod placenta (after Peteis)


Chorion Ltevt and Frondosum

The villi at first cover the entire surface of the chorion. As the embryo grows more and more out into the uterine cavity the decidual capsularis and that portion of the chorion attached to it are compressed, and the circulation in the intervillous spaces of these structures is cut off (Figs. 241 and 243). Thus, beginning at the pole of the decidua caosularis, the villi in this portion of the chorion degenerate during the fourth week and form the chorion lave. The villi on that part of the chorion which is attached to the decidua basalis continue their development, and, persisting, form the chorion fro dosum. Thi^, with the decidua basalis of the uterus, constitutes the placenta. The embryo is attached first to the chorion frondosum by the body stalk (Figs. 77 B and 239), later by the umbilical cord (Fig. 241). Through the umbilical vein and arteries in the cord the placental circulation of the embryo takes place. The Deddua Vent.— During the first phase of menstruation the uterine mucosa begins to differentiate into a broad, superficial compact layer and into a narrower, deep spongy layer in which are found the dilated ends of the uterine glands. After pregnancy these two layers are still further differentiated in the wall of the decidua vera and d. basalis. The compact layer is much thicker than the spongy layer and in it are found numerous stroma cells, enlarged blood vessels, and decidual cells (Fig. 244). The decidual cells, frequently multinucleate, are derived from the stroma cells of the mucosa. They are large, being 50 fx in diameter, with clear cytoplasm and vesicular nuclei. Their function is in doubt. Glycogen has Ijeen found in them, but during the later months of pregnancy many of them degenerate.


Fir,. 243. — Human ova: A, of three weeks; B, of sU weeks, showing formation of chorion l«ve by degeneration of the chorionic villi (De I*e),


Fig. 244.— Vertical througb the wall of the uterus alxiut seven monihii pregnant with the ni branes in situ (Schuper in Lewis and Stohr). X 30.

In the spongy layer of the mucosa occur the enlarged and tortuous uterine glands oj pregnancy (Fig. 244). During the first two months of pregnancy the long axes of the glands are perpendicular to the surface of the mucosa. Later, as the decidua is stretched and compressed owing to the growth of the fetus, the glands art' broadened and shortened and the cavities of the glands become elongated clefts parallel to each other and to the surface of the decidua. The gland cells become stretched and flattened until they resemble endothelial cells. At birth, or in case of late abortion, the plane of separation is in the spongy layer. Only the deep portions of the glands remain attached to the uterine wall, and, by the division of their cells, regenerate the epithelium of the uterus. v

The Decidua Capsularis

The capsularis, as we have seen, becomes compressed as the embryo grows (Fig. 241). To it is attached the chorum lave, the villi of which degenerate. During the fourth month the increased size of the fetus brings the capsularis into contact with the decidua vera with which it fuses, thereby obliterating the uterine cavity. Eventually it largely degenerates, completely so opposite the internal os uteri, where the chorionic villi are obliterated also. During pregnancy, the lumen of the cervix is closed by a plug formed by the secretion of the glands opening into the cervix uteri (Fig. 241).


The Placenta

The placenta is composed of the decidua basalis, constituting the maternal placenta, and of the chorion frondosum, the placenta fcEtalis. The area throughout which the villi of the chorion frondosum remain attached to the decidua basalis ts somewhat circular in form, so that at term the placenta is disc-shaped, about seven inches in diameter and one inch thick (Fig. 245). Near the middle of its felal surface is atUched the umbilical cord, and this surface is formed by the amnion, the mesoderm of which is closely applied to, and fused with, that of the chorion frondosum (Fig. 246).

Fin. 245.— Mature ptacenU. a. Entire organ, showlntt letai surface with memhranes attached to the periphery; b, a portion of attached surface showing cotyledons (Heisler).


Chorion Frondosum.—The villi of this portion of the chorion form profusely branched tree-like structures which lie in the intervillous spaces (Fig. 247). The ends of some of the villi are attached to the wall of the decidua basalis and are known as the anchoring villi. In the connective tissue core of each villus are commonly two arteries and two veins, branches of the umbilical vessels, cells like lymphocytes, and special cells of Hofbauer, the significance of which is not known. Lymphatics are also present. The epithelium of the villi, as we have seen, is at first composed of a layer of trophcctoderm with the outlines of its cuboidal cells sharply defined (Fig. 248 A). This layer (of Langhans) forms and is covered by a syncytium, the trophoderm. In the later months of pregnancy as the villi grow, the trophectodenn is used up in forming the syncytium, so that at term the trophoderm is the only continuous epithelial layer of the villi (Fig. 248 B). About the margin of the placenta the trophectotlerm persists as the closing ring, which is continuous with the epithelium of the chorion lEeve.


Fig. 246.— Sectii


Decidua Basalis

This, the maternal placenta, like the decidua vera is differentiated into a compact layer or basal plale, which forms the floor of the intervillous spaces, and into a deep spongy layer (Figs. 246 and 247). The first is the remains of the rotnpacl layer ol the uterine mucosa, formed during the premenstrual phase and partially destroyed by the implantation of the ovum. The second is the modi&ed spongy layer of the premenstrual period, and, though thinner, shows the same differentiation as docs this same layer in the decidua vera. TTie glandular spaces are less numerous in the spongy layer of the decidua basalis; Ijetween the spaces occur s\ncytial giant tells said to be derived from the trophoderm of the villi. It is in the plane of this spongj' layer that the separation of the placenta takes place at birth.


exhaust of blood


The basal plate, or compact layer of the decidua basalis, is composed of a connective tissue stroma containing decidual celb, canalized fibrin, and persisting portions of the epithelium of the villi. The canalized fibrin is believed to be formal both from the syncytial trophoderm of the villi and from the modified

CmtvidjI iditof lluhaullayv

m of cboriook villi: A. at thr fourth weti: fi. tT. at the Mid of

'Scibaper in Lckv and Stithrt.

iiSrir. f>: the maternal blood Tig. 242^ From the hnsal plalo, fc^Xa rxtmd into the iritcr\-il)ous spacer but do not unite with tlic chorion frondre^m Grosser in Kc*iSc'. anii Mall. vol. 1 :. Near term these constitute the scptti f^nh-fttUt which inci»mplctely divide the placenta into lobultfs, or totylnions vFijts. 245 nnd 247).


Fin. 24Q.— Scmidiaiiramniatic section of uterus, shoning relations of fetal and maternal placenta (AhlfeM). Driulua i.rMina, old Icrminolixiy for decidua basiilh; d. rtgexa. old terminology for d. (apsulnrii.


The maternal arteries and veins pass through the basal plate, taking a sinuous course and opening into the inter\'illous spaces (Fig. 247). Near their entrance they course obliquely and lose all but their endothelial layers. The original openings of the vessels into the intervillous spaces were formed during the implantation of the ovum, when their walls were eroded by the invading trophoderm of the villi. As the placenta increases in size the vessels grow larger. The ends of the villi are frequently sucked into the veins and interfere with the placental circulation. At the periphery of the placenta is an enlarged intervillous space, which varies in extent and never more than partly surrounds the placenta. This space is the marginal sinus through which blood is carried away from the placenta by the maternal veins. The blood of the mother and fetus does not mix, although the epithelial cells of the villi are instrumental in transferring nutritive substances to the blood of the fetus and in eliminating excreta from the fetal circulation into the maternal blood stream of the intervillous spaces.


The Relation of the Fetus to the Placenta and the Sepaxation of the Decidual Membranes at Birth

The relation of the embryo to the fetal membranes has been described on p. 71. During the first months of pregnancy the embryo floats in the cavity of the amnion, attached to the placenta by the umbilical cord (Fig. 241). Later, as we have seen, the amnion fuses more or less completely to the chorion frondosum and laeve. The decidua capsularis largely disappears or is fused to the decidua vera. Before birth, the placenta is concave on its amniotic surface, its curvature corresponding to that of the uterus (Fig. 249). At term, the duration of which is taken as ten lunar months, the muscular contractions of the uterus, termed "pains," bring about a dilation of the cervix uteri, the rupture of the amnion and chorion laeve, and cause the extrusion of the child. With the rupture of the membranes the amniotic liquor is expelled, the fetal membranes remaining attached to the decidual membranes. The pains of labor begin the detachment of the decidual membranes, the plane of their separation lying in the spongy layer of the decidua basalis and decidua vera, where there are only thinwalled partitions between the enlarged glands. Following the birth of the child, the tension of the umbilical cord and the "after pains'^ which diminish the size of the uterus, normally complete the separation of the decidual membranes from the wall of the uterus. The uterine contractions serve also to diminish the size of the ruptured placental vessels and prevent e«tensive hemorrhage. From the persisting portions of the spongy layer and from the epithelium of the glands the tunica propria, glands, and epithelium of the uterine mucosa are regenerated.


The decidual membranes and the structures attached to them when expelled constitute the "after-birth." The placenta usually is everted so that its amniotic surface is convex, its maternal surface concave. It is composed of the amnion, chorion frondosum, viIli with intcrxdllous spaces incompletely divided by the septa into cotyledons and includes on the nuternal side the basal plate and a portion of the spongy layer of the dccidua basalis. The amnion is usually attached to the chorion, but the membranes may rupture in such a way that the child is born enveloped in the amnion, the part covering the head being known popularly as the "caul." Near the center of the placenta is attached the umbilical cord, and at its margins the placenta is continuous with the dccidua vera and the remains of the chorion lieve and decidua capsularis.


The Position of the Placenta in Utero and its Variations

The position of the placenta is determined by the point at which embryo is implanted. In most cases it is situated on either the dorsal or ventral wail of the uterus. Occasionally it is lateral in position and very rarely (1 in 1600 cases) it is located near the cervix and covers the internal os uteri, constituting a placenla pravui. A partially or wholly duplicated placenta or accessory {succenluriate) placentas may be formed from persistent patches of villi on the chorion Ijeve. Cases have been observed in which from three to seven subdivisions of the placenta occurred.



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Prentiss CW. and Arey LB. A laboratory manual and text-book of embryology. (1918) W.B. Saunders Company, Philadelphia and London.

Human Embryology 1918: The Germ Cells | Germ Layers | Chick Embryos | Fetal Membranes | Pig Embryos | Dissecting Pig Embryos | Entodermal Canal | Urogenital System | Vascular System | Histogenesis | Skeleton and Muscles | Central Nervous System | Peripheral Nervous System | Embryology History
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