om those of the later appearing mesonephros. The tubules of the pronephros arise in all the gments from the fifth cervical to the third lumbar.

As the permanent kidney, or metanephros, is developed the mesonephros atrophies; portion of it, however, is retained in the male, and forms the excretory apparatus of e testis. The Wolffian duct becomes the canal of the epididymis and the ductus ferens of the adult (see p. 1334). In the female, when the permanent kidney is rmed, the mesonephros and its duct undergo atrophy to a greater extent than in the ale, and they are only represented in the adult by the vestigial structures present in e broad ligament of the uterus (see pp. 1315 and 1316).

THE URETER AND PERMANENT KIDNEY.

The ureter arises as a tubular diverticulum from the Wolffian duct close to the point here the latter joins the cloaca (Figs. 1039 and 1042). This diverticulum is seen first uring the fourth week, and grows from behind forwards, dorsal to the body cavity. Even

Rectum

Wolffian duct

its very early condition the portion of he outgrowth which lies nearest to the Volffian duct, and from which the adult reter is developed, is more slender than The distal part, which becomes branched, and grows out to form the pelvis and calyces of the ureter. From the calyces Bladdernumerous collecting tubules grow out into the developing kidney and acquire connexions with the glandular or uriniferous tubules of the kidney. The uriniferous tubules of the kidney arise independently of the ureter in a backward prolongation of the tissue which, further forward, gives origin to the tubules of the mesonephros. The tissue in which the permanent kidney tubules arise lies behind the third lumbar segment. The blind distal end of each tubule soon dilates to form a capsule which, becoming invaginated on itself, encloses a tuft of capillary blood-vessels. The renal corpuscles arising in this manner are found in the human kidney, as early as the eighth week.

•

Neural tube

cloaca is dividing into rectal and urino - genital subdivisions. The ureter is arising as a bud from the Wolffian duct. (Drawn from a model constructed by Prof. F. Keibel.)

As regards their origin in the embryo we distinguish between the collecting tubules and their branches, and the uriniferous secretory tubules of the kidney. The former arise from the calyces of the ureter, and hence are derived from the Wolffian duct; the latter are formed in mesoderm, known as the metanephric cell mass, which is continuous at its anterior end with the tissue from which the mesonephros is derived. junctional tubules of the adult lie in the region where these developmentally distinct portions of the kidney unite.

The short

As the ureter increases in length, it becomes separated from the Wolffian duct, and acquires a distinct opening into the anterior part of the cloaca nearer the head of the embryo than that of the Wolffian duct. This part of the cloaca receiving the ureters becomes the bladder. The kidney is at first a distinctly lobulated body, and shows at birth, and sometimes even in the adult, distinct traces of its original subdivision into lobule.

The metanephric cell mass, in which the uriniferous tubules arise, lies at first on the medial side of the bud-like outgrowth, which represents the ureter; at a later time it comes to lie dorsally. As the ureter grows towards the head end of the embryo the cell mass which gives rise to the uriniferous tubules follows it; hence the metanephric tissue ceases to lie to the caudal end of the mesonephros. As the ureter divides to form the calyces, the metanephric cell mass becomes broken up into cap-like portions, one for each branch of the ureter, and later one for each of the collecting tubes which grow out from the calyces.

The formation of uriniferous tubules within the nephrogenic cell mass is continued until a few days after birth.

THE BLADDER.

The main portion of the bladder is formed from the superior (cephalic) part of the anterior subdivision of the cloaca. This at an early time becomes flattened dorso-ventrally, and produced laterally into two horn-like projections in the region where the Wolffian ducts open (Fig. 1043). Inferiorly it becomes constricted to form the uro-genital canal. Little by little the lower ends of the Wolffian ducts open out and are absorbed into the wall of the developing bladder, and Pelvis of kidney soon it comes about that the ureters, which originally were outgrowths of the Wolffian ducts, open directly into the bladder. The openings of the ureters become shifted latterally, but the final position of the openings of the Wolffian ducts is close to the median plane in the upper prostatic part of the urethra. The bladder has therefore a double origin-its main portion is derived from the entodermal cloaca; its smaller basal part arises from the opened-out lower ends of the Wolffian ducts. The latter portion approximately corresponds to the trigonum vesicæ of the adult, and must be regarded as having its source from the mesoderm. The extreme

Cloaca

Symphysis pubis.

Septum recto-urethrale

FIG. 1043.-TAIL END OF HUMAN EMBRYO ABOUT 33 DAYS OLD. The cloaca is becoming separated into rectal and uro-genital portions by the formation of the septum recto-urethrale. The ureter has acquired a separate opening into the anterior division of the cloaca. (Drawn from a model constructed by Prof. F. Keibel.)

Urino-genital canal
Clitoris

cephalic end of the anterior part of the cloaca tapers gradually, and beyond the umbilicus is continuous with the allantois. This part of the cloaca loses its lumen about the fifth week, and from it is derived the fibrous cord of the urachus or median umbilical ligament, which in the adult reaches from the bladder apex to the umbilicus.

The cavity of the urachus is sometimes not lost so early, and in rare cases it has been found persisting in the child or adult as a pervious channel extending from the apex of the bladder to the umbilicus. Here it may open on the surface of the body.

THE MALE URETHRA.

The first part of the male urethra has an origin similar to that of the basal part of

Notochord

Ureter

FIG. 1044.-TAIL END OF FEMALE HUMAN EMBRYO ABOUT 9 WEEKS OLD.

The rectum has acquired an opening and the entoderm of the uro-genital canal is continued into the genital eminence (clitoris). (Drawn from a model constructed by Prof. F. Keibel.)

e bladder, and is derived from the ends of the Wolffian ducts (see above, p. 1332). e remaining portion, beyond the openings of the ductus deferentes of the adult, is rived from the uro-genital canal, or caudal subdivision of the anterior part of the paca. The uro-genital canal is early subdivided into a pelvic part lying within the ture pelvis minor and a penile part which occupies the region in which the corpus vernosum urethra is developed. The latter part of the uro-genital canal becomes led with closely and irregularly packed cells, which later, breaking down, re-establish e canal and give origin to a slit-like opening in the region in front of the anus. The nal for some time opens at a rhomboidal fossa situated in the groove at the base of the ans. In the glans penis a septum of densely packed cells passes forwards from this ssa and is known as the urethral septum. At a later stage these cells also break own and form a groove, the lips of which unite and enclose the terminal portion of the ethra. It is doubtful if any of the male urethra owes its origin to the ectoderm, but ere is some evidence to show that the urethral septum is to be regarded as ectodermal, which case the part of the canal which traverses the glans must have a like origin.

THE FEMALE URETHRA.

In the female the part of the urethra near the internal urethral orifice is developed from he inferior ends of the Wolffian ducts and has an origin similar to that of the basal portion f the bladder. The inferior part of the passage is derived from the uro-genital canal. When he uro-genital canal opens on the surface it is continued forwards as a sulcus on the genital eminence, as in the male sex. The margins of the slit-like opening do not unite, out form the labia minora of the adult, and the sulcus which appears on the glans litoridis is closed without forming a canal. At first the fused caudal ends of the Müllerian lucts open into the uro-genital canal, but later a downgrowth, which is at first solid, establishes a connexion between these ducts and the surface immediately in front of the rectum and behind the opening of the uro-genital canal. This new connexion becomes the vagina, and the uro-genital canal the urethra. By some embryologists it is believed that a shortening and spreading out of the inferior portion of the uro-genital canal, to form a part of the uro-genital cleft of the adult, is responsible for bringing the opening of the fused Müllerian ducts to the surface. If this latter view is accepted, the female urethra corresponds to the part of the male passage which lies above the opening of the utriculus prostaticus.

THE SEXUAL GLANDS.

Sexual Glands.-In the development of the sexual glands, male and female, a differentiated thickened portion of the peritoneal epithelium is first recognised. This specialised epithelium, which has received the name of germinal epithelium, is situated to the medial side of the mesonephros and of the Wolffian and Müllerian ducts. Here it covers a longitudinally disposed ridge or elevation called the genital ridge. The germinal epithelium is not strictly limited to this ridge, but extends to some extent beyond its limits. The genital ridge is soon found to have numerous epithelial cells embedded in its connective-tissue stroma which appear to originate, in both sexes, by a proliferation from the deep surface of the germinal epithelium covering the ridge. From these epithelial cells the seminiferous tubules of the male, and vesicular follicles with their contained ova of the female are developed. The tissue which gives rise to the genital ridge occurs in all the body segments from the sixth thoracic to the second sacral, but the cephalic end of the ridge atrophies before the germinal epithelium can be recognised in the more caudal segments, and only about one-fourth of the ridge gives origin to the permanent sexual gland. The part of the genital ridge which persists appears to correspond to three or four segments in the region of the 4th or 5th lumbar to the 1st or 2nd sacral segments.

In the male, as early as the thirty-third day, the epithelial cells embedded in the stroma of the developing testis have become arranged into a network of anastomosing cords within which certain larger cells are seen to be irregularly scattered. These larger cells have received the name of primitive sperm cells, and are relatively few in number. They undergo frequent division, and in the later stages are not to be distinguished from the other cells of the cords. The cellular cords undergo direct transformation into the seminiferous tubules of the testis, the tubuli recti and the rete testis. At a very early stage the superficial part of the stroma of the developing testis becomes denser, and gives origin to the tunica albuginea. The tissue surrounding the cellular cords becomes converted into the septula testis and the mediastinum. A lumen can first be recognised in the seminiferous tubules in the seventh month. The rete testis becomes connected

secondarily with the ductuli efferentes which are derived from the tubules of the mesonephros, and thus the mesonephric or Wolffian duct becomes the passage for the secretion of the testis.

In the female large epithelial cells are found in the stroma of the developing ovary, beneath the germinal epithelium, as early as the thirty-third day. These primitive ova are much more numerous than the primitive sperm cells of the male, and form a very characteristic feature of the developing ovary. At first they lie isolated, but later-about the fifth week-they become surrounded by other smaller cells having a like origin from the germinal epithelium. Each primitive ovum surrounded by its cells becomes a primitive follicle, the further development of which has already been described (p. 1318). During the later stages the epithelium has the appearance of growing down into the stroma in the form of long branching cellular processes which break up into little nests of cells to form the future follicles (p. 1318). The proliferation of cells from the surface epithelium goes on until the seventh month, but it is extremely doubtful if any new ova arise in the later months of gestation or after birth.

FIG. 1045. THE URINO-GENITAL PASSAGES AT THE
INDIFFERENT STAGE OF DEVELOPMENT.

Ureter, green solid outline. Wolffian duct, green dotted outline. The origin of the vesicula seminalis is indicated. Müllerian ducts, orange. Rectum, bladder, and urino-genital canal, red.

THE GENERATIVE DUCTS.

Generative Ducts.-As has been already stated, the male ducts arise from the Wolffian, and the female from the Müllerian ducts of the embryo. Both sexes at first possess well-developed Wolffian and Müllerian ducts, which are arranged in a very definite manner. The Wolffian ducts, communicating directly with the tubules of the mesonephros, lie at first parallel to, and at a considerable distance from, one another. As they pass towards the caudal end of the embryo they approach one another, and each becomes enclosed in a fold of peritoneum called the plica urogenitalis. More caudally the ducts become closely approximated to each other, are embedded in a cord-like mass of connective tissue, to which the term genital cord is applied. They finally open into the anterior subdivision of the cloaca (Fig. 1042).

The Müllerian ducts, opening freely into the body cavity at their cephalic ends, lie to the lateral side of the Wolffian ducts. As they are traced caudally they cross the Wolffian ducts and enter the genital cord, within which they unite and form a canal, which occupies the median plane, and opens into the anterior subdivision of the cloaca, between the Wolffian ducts (Fig. 1042). The manner in which the ureters become separated from the Wolffian ducts has already been described.

FIG. 1046. THE URINO-GENITAL PASSAGES IN
THE MALE.

Ductus deferens, dotted green outline. Ureter, solid
green outline. Utriculus prostaticus, orange.
Bladder and pelvic part of urethra, red. Penile
portion of urethra, black.

Ducts in the Male. The seminiferous tubules of the testis become connected with the Wolffian duct through a fusion of certain tubules of the mesonephros with the rete testis. The connexion is definitely established in the third month. The number of tubules taking part varies considerably, but corresponds to the number of ductuli efferentes found in the adult. The connecting tubules becoming much convoluted, just as they join the Wolffian duct, form the lobuli of the epididymis. The canal of the epididymis is directly formed from the cephalic part of the Wolffian duct, and the ductus

deferens from the
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on of the Wolf
testis.

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In the hu
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can be trac ras as far as th Prostate.prostate arise ths from th al canal duri growths, whic branched ar

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erens from the more caudal portion. The ductuli aberrantes and the rudimentary ules of the paradidymis are to be looked upon as persistent tubules, of a more caudal tion of the Wolffian body, which have failed to become connected with the tubules of e testis.

The seminal vesicles are developed in the third month as evaginations which arise from e Wolffian ducts, near their caudal extremities. Each at first has the appearance of a ngitudinal groove in the wall of the ductus deferens, which closes over and becomes cut off om the main tube except at the point where, later, the duct of the seminal vesicle joins e ductus deferens.

The Müllerian ducts atrophy in the male embryo, but the appendices of the testis are estigial remains of their cephalic portions, while the utriculus prostaticus represents the audal fused portions which, in the embryo, occupy the genital cord.

Ducts in the Female. The Müllerian ducts in the female retain their openings into he body cavity, and their cephalic portions become the uterine tubes. Their fused caudal parts, which at first join the uro-genital canal, give rise to the uterus and vagina. The manner in which the original position of the opening of the Müllerian ducts becomes shifted, by the formation of a new passage or by the shortening of the urogenital canal, has already been mentioned (p. 1333). The final position of the opening is in the uro-genital cleft of the adult.

The vaginal portion of the fused Müllerian ducts is at first relatively very short, and at the point where it opens into the uro-genital canal a slight fold appears, which is the future hymen. The vagina increases rapidly in length as its opening moves downwards towards the uro-genital cleft. In the human embryo during the third month the closely applied Müllerian ducts, which higher up have fused to form the uterus, are represented at their inferior ends by a pair of rapidly elongating solid cellular cords, which at a later stage break down to form the vagina.

The Wolffian ducts and the mesonephros atrophy in the female, but traces of them are to be found in the ep-oöphoron and par-oöphoron of the adult. In the foetus the Wolffian duct can be traced along the side of the uterus as far as the upper end of the vagina.

Prostate. The glandular portion of

FEMALE.

the prostate arises as a series of solid out- FIG. 1047. THE URO-GENITAL PASSAGES IN growths from the epithelium of the urogenital canal during the third month. The outgrowths, which are at first simple, become branched and finally acquire a lumen. They are arranged in three groups-an upper and a lower dorsal, and a ventral group. The glands of the ventral group soon became reduced in number and often completely disappear; those of the upper dorsal group form the chief part of the gland.

The prostatic glands arise in both sexes, but in the female, where they are known as para-urethral glands, they are few in number and not densely packed as in the male. The muscular tissue of the prostate is derived from the muscular wall of the urethra.

The bulbo-urethral glands arise in the third month, and appear to be developed from the epithelium of the uro-genital canal.

The larger vestibular glands in the female arise as epithelial outgrowths in the same manner as the bulbo-urethral glands.

THE

Derivations of the Müllerian duct, orange. Ureter, green solid outline. The ep-oöphoron is indicated in green near the opening of Müllerian duct and near the ovary.

EXTERNAL GENITAL ORGANS.

The external genital organs are developed in the region of the ectodermal cloacal fossa, and those of the male and female cannot be distinguished from one another in the earlier stages. The fossa at first extends on the ventral aspect of the body almost