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the later appearing mesonephros. The tubules of the pronephros arise in all the the fifth cervical to the third lumbar.
rmanent kidney, or metanephros, is developed the mesonephros atrophies; it, however, is retained in the male, and forms the excretory apparatus of Che Wolffian duct becomes the canal of the epididymis and the ductus the adult (see p. 1334). In the female, when the permanent kidney is mesonephros and its duct undergo atrophy to a greater extent than in the ey are only represented in the adult by the vestigial structures present in ament of the uterus (see pp. 1315 and 1316).
THE URETER AND PERMANENT KIDNEY.
ter arises as a tubular diverticulum from the Wolffian duct close to the point atter joins the cloaca (Figs. 1039 and 1042). This diverticulum is seen first fourth week, and grows from behind forwards, dorsal to the body cavity. Even early condition the portion of
Rectum wth which lies nearest to the
Wolffian duet Kidney bud
Notochord uct, and from which the adult
Neural tube eveloped, is more slender than part, which becomes branched,
out to form the pelvis and the ureter. From the calyces Bladdercollecting tubules grow out into oping kidney and acquire con- Cloaca ith the glandular or uriniferous f the kidney. The uriniferous of the kidney arise independently eter in a backward prolongation sue which, further forward, gives
the tubules of the mesonephros. ne in which the permanent kidney arise lies behind the third lumbar
Post anal gut - The blind distal end of each soon dilates to form a capsule Fig. 1042.- TaiL END OF A HUMAN EMBRYO ABOUT becoming invaginated on itself,
25 DAYS OLD. a tuft of capillary blood vessels. The cloaca is dividing into rectal and urino-genital al corpuscles arising in this manner subdivisions. The ureter is arising as a bud from and in the human kidney as
the Wolffian duct. (Drawn from a model con
structed by Prof. F. Keibel.) - the eighth week.
egards their origin in the embryo we distinguish between the collecting tubules
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-ventralls
, and produced laterally into two horn-like projections in the region where the Wolffia:
ducts open (Fig. 1043). Inferioris Allantois
it becomes constricted to form the uro-genital canal. Little by litt:
the lower ends of the Wolffian ducts Large intestine
open out and are absorbed into the Wolffian duct wall of the developing bladder, and Pelvis of kidney soon it comes about that the ureters
which originally were outgrowth of the Wolffian ducts, open directly into the bladder. The opening of the ureters become shifted latterally, but the final position of the
openings of the Wolffian ducts in Cloaca
close to the median plane in the upper prostatic part of the urethra.
The bladder has therefore a double Ureter
origin its main portion is derived Septum recto-urethrale
from the entodermal cloaca ; its
smaller basal part arises from the Fig. 1043.-TAIL END OF HUMAN EMBRYO ABOUT 33 Days OLD.
opened-out lower ends of the Wolffian The cloaca is becoming separated into rectal and uro-genital ducts. The latter portion approxi
The ureter has acquired a separate opening into the anterior mately corresponds to the trigonum division of the cloaca. (Drawn from a model constructed vesicæ of the adult, and must by Prof. F. Keibel.)
be regarded as having its source
from the mesoderm. The extreme 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.
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.)
is derived from the ends of the Wolffian ducts (see above, p. 1332). ortion, beyond the openings of the ductus deferentes of the adult, is ; uro-genital canal, or caudal subdivision of the anterior part of the »-genital canal is early subdivided into a pelvic part lying within the nor and a penile part which occupies the region in which the corpus three is developed. The latter part of the uro-genital canal becomes ly and irregularly packed cells, which later, breaking down, re-establish ive origin to a slit-like opening in the region in front of the anus. The time opens at a rhomboidal fossa situated in the groove at the base of the glans penis a septum of densely packed cells passes forwards from this mown as the urethral septum. At a later stage these cells also break 1 a groove, the lips of which unite and enclose the terminal portion of the 3 doubtful if any of the male urethra owes its origin to the ectoderm, but evidence to show that the urethral septum is to be regarded as ectodermal, the part of the canal which traverses the glans must have a like origin.
THE FEMALE URETHRA. male the part of the urethra near the internal urethral orifice is developed from ends of the Wolffian ducts and has an origin similar to that of the basal portion er. The inferior part of the passage is derived from the uro-genital canal. When ital canal opens on the surface it is continued forwards as a sulcus on the nence, as in the male sex. The margins of the slit-like opening do not unite, the labia minora of the adult, and the sulcus which appears on the glans 3 closed without forming a canal. At first the fused caudal ends of the Müllerian n into the uro-genital canal, but later a downgrowth, which is at first solid, s a connexion between these ducts and the surface immediately in front of the id behind the opening of the uro-genital canal. This new connexion becomes the nd the uro-genital canal the urethra. By some embryologists it is believed ortening and spreading out of the inferior portion of the uro-genital canal, to form
the uro-genital cleft of the adult, is responsible for bringing the opening of the üllerian ducts to the surface. If this latter view is accepted, the female urethra nds to the part of the male passage which lies above the opening of the utriculus
THE SEXUAL GLANDS. xual Glands.- In the development of the sexual glands, male and female, a ntiated thickened portion of the peritoneal epithelium is first recognised. This lised epithelium, which has received the name of germinal epithelium, is situated to nedial side of the mesonephros and of the Wolffan and Müllerian ducts. Here it sa longitudinally disposed ridge or elevation called the genital ridge. The germinal helium is not strictly limited to this ridge, but extends to some extent beyond its ts. The genital ridge is soon found to have numerous epithelial cells embedded in its nective-tissue stroma which appear to originate, in both sexes, by a proliferation from
deep surface of the germinal epithelium covering the ridge. From these epithelial Is the seminiferous tubules of the male, and vesicular follicles with their contained ova the female are developed. The tissue which gives rise to the genital ridge occurs in I the body segments from the sixth thoracic to the second sacral, but the cephalic end the ridge atrophies before the germinal epithelium can be recognised in the more caudal egments, and only about one-fourth of the ridge gives origin to the permanent sexual land. The part of the genital ridge which persists appears to correspond to three or our 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 beconies 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
ses of the para
of the Wol
The seminal ve:
the main tub
used portio Dacts in the Pe
secondarily with the ductuli efferentes which are derived from the tubules of the meso terens from the nephros, 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 nưmerous than the primitive sperm cells of the male, and form a very characteristic feature of the developing ovary. Āt first they lie isolated, but later-about tinal groove the fifth week—they become surrounded by other smaller cells having a like origin from
the germinal epithelium. Each primitive fetus deferens
mes shifted, b follicles (p. 1318). The proliferation of cells
- 1. canal, has from the surfaoe epithelium goes on until
. The fin the seventh month, but it is extremely 2 tse urogenita Fig. 1045.- THE URINO-GENITAL PASSAGES AT THE
doubtful if any new ova arise in the later The vaginal INDIFFERENT STAGE OF DEVELOPMENT. months of gestation or after birth.
man ducts Ureter, green solid outline. Wolffian duct, green dotted
... and at the outline. The origin of the vesicula seminalis is
THE GENERATIVE DUCTS.
so genital indicated. Müllerian ducts, orange. Rectum,
a is the bladder, and urino-genital canal, red.
Generative Ducts. - As has been
sees rapid already stated, the male ducts arise from
is downwa the Wolffian, and the female from the Müllerian ducts of the embryo. Both sexes at
In the first possess well-developed Wolffian and Müllerian ducts, which are arranged in a very
month definite manner. The Wolffian ducts, communicating directly with the tubules of the
to which mesonephros, lie at first parallel to, and at a considerable distance from, one another.
ateris, ar 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
by in the approximated to each other, are embedded in a cord-like mass of connective tissue, to which the term genital cord is applied.
can be 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 Fig. 1046.-Tue URINO-GENITAL PASSAGES IN separated from the Wolffian ducts has
THE MALE. already been described.
Ductus deferens, dotted green outline. Ureter, solid Ducts in the Male. The seminiferous
Utriculus prostaticus, orange tubules of the testis become connected Bladder and pelvic part of urethra, red. Penile with the Wolffian duct through a fusion of
portion of urethra, black. 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
is bs a pair o dar cords, whi na to form the The Wolffias
be found in I of the ad
i as far
Prostate - prostate Srbs from mai canal oths, branche e are a et and a
gla far tissue The bulbo I the epic The larg
n the more caudal portion. The ductuli aberrantes and the rudimentary e paradidymis are to be looked upon as persistent tubules, of a more caudal le Wolffian body, which have failed to become connected with the tubules of
nal vesicles are developed in the third month as evaginations which arise from
ducts, near their caudal extremities. Each at first has the appearance of a groove in the wall of the ductus deferens, which closes over and becomes cut off ain tube except at the point where, later, the duct of the seminal vesicle joins deferens. llerian ducts atrophy in the male embryo, but the appendices of the testis are mains of their cephalic portions, while the utriculus prostaticus represents the d portions which, in the embryo, occupy the genital cord. in the Female.—The Müllerian ducts in the female retain their openings into cavity, and their cephalic portions become the uterine tubes. Their fused rts, which at first join the uro-genital canal, give rise to the uterus, and The manner in which the original position of the opening of the Müllerian ducts hifted, by the formation of a new passage or by the shortening of the uroinal, has already been mentioned
The final position of the opening uro-genital cleft of the adult. vaginal portion of the fused i ducts is at first relatively very d at the point where it opens into genital canal a slight fold appears, s the future hymen. The vagina s rapidly in length as its opening downwards towards the uro-genital In the human embryo during the nonth the closely applied Müllerian which higher up have fused to form rus, are represented at their inferior y a pair of rapidly elongating solid r cords, which at a later stage break to form the vagina. ne Wolffian ducts and the mesonephros ay in the female, but traces of them are Found in the ep-oöphoron and par-oöphof the adult. In the fætus the Wolffian can be traced along the side of the is as far as the upper end of the vagina. Prostate.---The glandular portion of prostate arises as a series of solid out. Fig. 1047. --TAE URO-GENITAL Passages in ths from the epithelium of the uro
FEMALE. tal canal during the third month. The Derivations of the Müllerian duct, orange. Ureter, rowths, which are at first simple, be
green solid outline. The ep-oöphoron is indicated
in green near the opening of Müllerian duct and e branched and finally acquire a lumen.
near the ovary. =y are arranged in three groupsman Der and a lower dorsal, and a ventral group. The glands of the ventral group soon ame reduced in number and often completely disappear; those of the upper dorsal up form the chief part of the gland.
The prostatic glands arise in both sexes, but in the female, where they are known as na-urethral glands, they are few in number and not densely packed as in the male. The iscular 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 rom the epithelium of the uro-genital canal.
The larger vestibular glands in the female arise as epithelial outgrowths in the ime manner as the bulbo-urethral glands.
EXTERNAL GENITAL ORGANS. The external genital organs are developed in the region of the ectodermal cloacal Rossa, 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