John Clement Heisler.

A text-book of embryology for students of medicine online

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genital ridge. By the sixth
or seventh week, the ger-
minal epithelium consists of
several strata of cells, groups
of which begin to penetrate
the underlying mesodermic
tissue in the form of cord-
like processes (Fig. 112,
e, sch). The indifferent
mesodermic tissue at the
same time increases in quan-
tity, in turn penetrating
between the groups of ad-
vancing cells, so that what
takes place might be de-
scribed as a mutual inter-
growth. The presence of
the growing connective tis-
sue accentuates the grouping of the cells into cylindrical
masses. These latter are the sexual cords or egg-columns
(Pfliiger's egg-tubes). They contain two special kinds of
cells, the large sexual cells or primitive ova (Fig. 112, ite),
and the smaller but more numerous mesothelial cells.
The connection of the sexual cords with the germinal epi-
thelium is much more obvious in this case than in the
case of the developing testicle, and the primitive sexual cells
are much more abundant. The egg-columns, surrounded by
young connective tissue, constitute the nucleus of the cortical
part of the future ovary. This mass is later sharply marked
off from the free or peritoneal aspect of the gland, the region
of the germinal epithelium, by a zone of proliferating meso-
dermic cells which become the tunica albuginea of the ovary.
An important change now takes place in the egg-columns ;
the primitive ova, or large sexual cells, increase in size, their
nuclei becoming especially well developed, while the small

Fig. 111. — Internal organs of a female
fetus of about fourteen weeks (Waldeyer) :
o, ovary ; e, epoophoron or parovarium ;
w', Wolffian duct; m, Miillerian duct; w,
lower part of the Wolffian body.



mesothelial cells become smaller and less conspicuous. Sev-
eral of the large cells fuse into a single mass of protoplasm,
■ while one of the nuclei outstrips the others in growth and,
with the surrounding zone of protoplasm, becomes the ovum.
Each egg-column is now broken up into several groups of
cells by the penetration of connective tissue, each group (Fig.
112, e, sch') containing a single ovum but many of the smaller'

Fig. 112.— Part of sagittal section of an ovary of a child just born (after Wal-
deyer). Highly magnified: ke, germinal epithelium; e, sch, Pfliiger's egg-tubes;
u€, primitive ova lying on the germinal epithelium ; e, sch', long Pfliiger's tubes, in
process of being converted into follicles ; ei, b, egg-balls (nests), likewise in process
of being resolved into follicles ; /, youngest follicle already isolated : gg, blood-
vessels. In the tubes and egg-nests the primordial eggs are distinguishable from
the smaller epithelial cells, the future follicular epithelium.

cells. These groups are the young G-raafian follicles of the
ovary (/). The enveloping zone of connective tissue becomes
the theca of the follicle, while the single large cell constitutes
the ovum, and the smaller cells are the membrana granulosa.
At first the granulosa cells surround the ovum as a single
layer of flattened cells which gradually assume the columnar
type and become so numerous as to form many layers. They
secrete a fluid, the liquor folliculi, which crowds the ovum to
one side of the follicle where it is enveloped by a special
group of granulosa-cells, the discus proligerus (Fig, 113).



The question of the origin of the follicular cells is still an
unsettled one, though it seems probable that they are derived
from the cells of the egg-columns, and Minot believes that
they are probably descended from the primitive ova.

The formation of new Graafian follicles, and consequently
of ova, begins in the deeper part of the ovary and advances
toward the surface. The production of ova and follicles is

Fig. 113.— Section of human ovary, including cortex : a, germinal epithelium
of free surface ; 6, tunica albuginea ; c, peripheral stroma containing immature
Graafian follicles (d) ; e, well-advanced follicle from whose wall membrana granu-
losa has partially separated ; /, cavity of liquor foUiculi ; g, ovum surrounded by
cell-mass constituting discus proligerus (Piersol).

limited to the fetal stage and to the early part of post-natal
life, their formation not occurring, according to Waldeyer,
after the second year.

What has been said above refers to the development of the
cortex of the ovary. The medulla is produced by the growth
toward the egg-columns of cord-like processes, the medullary
cords, from the epithelial walls of the Malpighian corpuscles
of the primitive kidney or Wolffian body, the cords becoming
surrounded by connective tissue and forming a network. The
fetal medullary cords are represented in both the cortex and
the medulla of the mature ovary by the groups of interstitial
cells disposed between the bundles of the stroma-tissue.


The Oviducts, the Uterus, the Vagina. — The system of pass-
age-ways that constitute the outlets for the ova and the
means of nourishing tliem and evacuating the product of
gestation from the body in the event of impregnation —
namely, the Fallopian tubes, the uterus, and the vagina — result
from the metamorphosis of the ducts of Miiller. These ducts,
as stated above, lie along the dorsal aspect of the body-cavity,
separated from it by the parietal peritoneum, and parallel
with the primitive spinal column (Plate VII.). The probable
method of their formation has been pointed out (p. 220).
Near the lowe'r (caudal) end of the body they approach each
other, and finally unite about the second month to form a
single duct for the rest of their extent (Plate VIL, Fig. 4). The
upper, ununited parts of the ducts become the Fallopian tubes
or oviducts, while the lower portions, now fused into one,
become the uterus and the vagina. The upper end of each
single duct expands trumpet-like to form the fimbriated ex-
tremity of the Fallopian tube.

Until the fifth month there is no distinction between the
vagina and the uterus, the two being represented by a
single sac-like structure. The development of a circular
ridge in the wall of the sac marks the division between the
two organs, the part above the ridge acquiring thick
muscular walls, while the part below it, the future vagina,
remains thin-walled and more capacious. In the third
month the uterus is bifid at its upper extremity, a condition
which is permanent in some animals and occasionally in the
human subject.^

The Wolffian duct, which, in the male, becomes metamor-
phosed into a part of the epididymis and the vas deferens,
remains undeveloped in the female, producing merely atrophic
or vestigial structures (Plate VIL, Fig. 4). The upper series of
Wolffian tubules, the remnant of the pronephros, frequently

^ The formation of the uterus aud of the vagina by the coalescence of two
parallel tubes affords an explanation of the uterus bicornis or bifid uterus and
of the condition of double uterus sometimes met with, as also of the ppesence
of a median septum in the vac/ina, since by the failure of union of the two
tubes in greater or less degree one or other of these anomalies would result.


persists, as in the male, in the form of a small pedunculated sac,
the stalked hydatid or hydatid of Morgagni. When present, it
is to be found in the broad ligament, in the neighborhood of
the outer extremity of the ovary. The middle or sexual
series of the Wolffian tubules with the adjacent part of the
Wolffian duct, which, in the male type, develop into the
epididymis, become in the female, an atrophic structure
known as the epoophoron or parovarium, or organ of Eosen-
miiller (Fig. 111). This structure, which is almost con-
stantly found between the layers of the broad ligament in
close proximity to the ovary, consists of a larger horizontal
tube representing a segment of the Wolffian duct, and of
shorter vertical tubes joining this at a right angle and rep-
resenting the transverse Wolffian tubules. The lower set
of small Wolffian tubules, those which, in the male become
the paradidymis, give rise in this case to a similar atrophic
body, the paroophoron. This is also situated in the broad
ligament, usually to the inner side of the ovary. The
Wolffian duct, with the exception of that portion of it that
assists in the formation of the parovarium, usually entirely
disappears. Occasionally, however, it persists as a small
canal traversing the broad ligament close to the uterus and
passing on the dorsal side of the upper part of the vagina to
be lost upon the wall of the latter or, more rarely, to open
near the urinary meatus. When thus persistent, it is known
as the duct of Gartner.

The change of position of the ovaries is similar to, though
less marked than, that of the testes. The inguinal ligament in
the female (Plate VII.) extends from the primitive position
of the ovaries in the lumbar region of the abdominal cavity
to the groin, Avhere it passes through the abdominal wall,
traversing the inguinal canal, to terminate in the labium
majus. The upper part of this ligament, containing invol-
untary muscular substance, firmly unites witli the ovary. In
the third month the ovary descends to the lower part of the
abdominal cavity and is now connected, by the succeeding
portion of the inguinal ligament, with the uterus. This con-
nection may be a factor in the final change of position of the


ovary — that is, its descent into the true pelvis. The part of
the inguinal ligament that passes from the ovary to the ute-
rus is the permanent ligament of the ovary, while the remain-
ing portion, which passes from the uterus through the ingui-
nal canal to the labium majus of the vulva, is the round liga-
ment of the uterus. As the inguinal ligament perforates the
abdominal wall, a small diverticulum of peritoneum goes
with it. Normally this peritoneal pouch subsequently be-
comes obliterated. Occasionally, however, it persists and
then constitutes the canal of Nuck. Should the canal of
Nuck be present, the ovary may pass into or through it,
thus reaching the labium majus. A patulous canal of Nuck,
as in the case of a patulous funicular process of the tunica
vaginalis of the male, may permit the sudden occurrence of
an inguinal hernia in the female.

The account of the development of the external genital
organs will be deferred until after the consideration of the
formation of the urinary bladder and of that part of the
urethra that originates from the same embryonic structure.


As stated in Chapter V., the urinary bladder and a part of
the urethra are derived from the intra-embryonic portion of
the allantois. In the same chapter the allantois was described
as a sac which developed as a pouching-out of the ventral
wall of the gut-tract near its caudal end (Plate II.,
5 and 6). The sac protrudes from the still widely open
abdominal cavity, enters the extra-embryonic part of the
body-cavity, and reaches the inner surface of the false
amnion, with which structure it intimately unites to form
the true chorion (Plate III.). As the walls of the abdomen
gradually close, leaving only the umbilical aperture, it is,
necessarily, through this aperture that the allantois pro-

We have seen (p. 82) what becomes of the extra-abdom-
inal part of the allantois — in what degree it contributes to
the formation of the placenta and of the umbilical cord.


Obviously, with the severing of the umbilical cord after
birth, all this extra-embryonic part of the allantois disap-
pears, giving rise to no adult organ.

Its intra-embryonic portion consists of a tube extending
from the caudal end of the intestine to the umbilicus (Plate
II., 5 and 6). As early as the second month, the middle
segment of this tube dilates and assumes the form of a spindle-
shaped sac, which becomes the urinary bladder (Plate VII.).
The part of the tube connecting the summit of this sac with
the umbilicus remains small, gradually loses its lumen, and
constitutes in the adult the (usually) impervious cord known
as the urachus. Should the cavity of the urachus persist in
its entirety, and should there be at the same time an external
opening at the umbilicus, the condition would constitute an
umbilical urinary fistula. The proximal part of the allantois
— that is, the portion intervening between the bladder and
the intestine — is designated the sinus urogenitalis, while the
caudal end of the intestine, which is, in effect, a pouch in
which both the allantois and the intestine terminate, is known
as the cloaca (Fig. 84). The urogenital sinus receives the
terminations of both the Miillerian and the Wolffian ducts
(Plate VII.).

In the sixth week or slightly earlier, there appears upon the
surface of the body, in the region corresponding to the position
of the cloaca, a depression, the cloacal depression (Fig. 84),
which later, except in man and the higher mammals, meets
the cloaca, and thus establishes a communication between it
and the exterior. In the Amphibia, in reptiles, and in birds,
as also in the lowest mammals, the monotremes, the cloaca
is a permanent structure, and through it, in these groups of
animals, not only the fecal matters and the urine, but also
the genital products, the spermatozoa and the ova, are evacu-
ated from the body. In all mammals, however, with the
exception of the monotremes, the cloaca undergoes division
into a posterior part or anal canal and an anterior urogenital
aperture. This division is brought about by the growth of
three ridges or folds, of which one springs from each side
of the cloaca and one from the point of union of the uro-


genital sinus and the intestine. These folds coalesce about
the eighth ^ week to form a complete septum, which continues
to thicken antero-posteriorly up to the time of birth and
constitutes the perineum.

It will be remembered that the ureters originally spring
from the terminal parts of the Wolffian or mesonephric ducts
(Fig. 104). Owing to alterations brought about by processes
of unequal growth, the orifices of the ureters subsequently
change their position so as to open into the urogenital sinus
(Fig. 107), and still later, by the further operation of the
same agency, they come to open into the bladder on its dor-
sal wall, thus gradually assuming their permanent relations
(PI. VII.). After the division of the cloaca the urogenital
sinus, as stated above, opens independently upon the surface
of the body. In the female it is transformed into a short
tube, the urethra, and an expanded terminal recess or fossa,
the vestibule of the vulva (PI. VII.). In the male it be-
comes the first or prostatic part of the urethra.

In the twelfth or thirteenth week, the future prostatic ure-
thra acquires very thick muscular walls, and the original
epithelial tube pouches out into the muscular tissue in the
form of little sacs, the lining cells of which assume the char-
acters of secreting epithelium. In this way is produced the
aggregation of muscular and glandular tissue known as the
prostate gland. This is a well-developed structure by the
fourth or fifth month (Tourneux). The recess in the floor
of this part of the urethra, the sinus pocularis or uterus mas-
culinus, has been previously referred to as the homologue of
the uterus, being the persistent caudal extremities of the
ducts of Miiller (Plate VII.).


In the early stages of the development of the external
genital organs no sexual distinctions are apparent.

Reference has been made to the cloacal depression as a
superficial fossa which makes its appearance at the caudal end
of the body of the embryo in the sixth week (Fig. 84). At
^ Fourteenth week, according to Minot.



about the same period an encircling elevation, the genital ridge
(Fig. 114:, A, 4), is seen to surround this depression. Within
the genital ridge, at the anterior part of the cloacal fossa, a
small tubercle, the genital eminence, appears at the same time.

Fig. 114.— Four successive stages of development of the external genital organs
(indifferent type) of the human fetus of 24 to 34 mm. (0.95 to 1.35 inch) (Tourneux) :
1, genital eminence or tubercle ; 2, glans ; 3, genital groove ; 4, genital ridge ; b,
cloacal depression ; 6, coccygeal eminence.

On the under aspect of the genital eminence there is soon
distinguishable the genital groove (Fig. 114, 3), which appears
as if a continuation of the fissure-like cloacal depression (5),
and the groove very shortly becomes flanked by two ridges,
the genital folds, one on each side.

The genital eminence becomes the penis or the clitoris,
according to the sex of the fetus. It very early acquires a
knob-like extremity (2) which is the beginning of the glans
penis or of the glans clitoridis, as the case may be. Further
development of the glans is brought about by the appearance
of a partially encircling groove which serves to differentiate
it from the body of the organ.

At this stage of development, the rudimentary organs, as



described above, are precisely alike in the two sexes. Early
in the third month — about the ninth week — sexual distinc-
tions begin to become manifest. Since the female organs
exhibit the less degree of deviation from the early indifferent
form, they will be first considered.

The External Genital Organs of the Female. — The sexually
indifferent genital eminence which, as we have seen, presents


•'IS f



. i


I ■

Fig. 115. — Four successive stages of development of the external genital organs
of the human female fetus (Tourneux) : 1, clitoris ; 2, glans clitoridis ; 3, urino-
genital fissure; 4, labia majora; 5, anus; 6, coccygeal eminence; 7, labia minora.

even by the end of the second month a rudimentary glans and
an indication of a prepuce, elongates somewhat and becomes
the clitoris. The genital folds bounding the genital groove
on the under surface of the genital eminence (Figs. 114 and


115, A) never unite with each other as they do in the male,
but become prolonged in the direction of the future anus and
constitute, by the fourth month, the lateral boundaries of the
orifice of the urogenital sinus, or, in other words, of the ves-
tibule of the vagina (Fig. 115, 3). These folds, continuous
over the dorsum of the clitoris with its rudimentary prepuce,
are the nymphae or labia minora (Fig. 115, D, 7) of the fully-
formed state. The masses of erectile tissue in close
relation with each labium minus, the pars intermedialis
and the bulbus vestibuli, are the homologues respectively
of a lateral half of the male corpus spongiosum and its
bulb. The genital ridge, which, from the first, encircles
the genital eminence and the cloacal depression, and, con-
sequently, the later clitoris and the aperture of the sinus
urogenitalis, increases greatly in thickness. The part of it
situated on the ventral side of the clitoris becomes the mens
veneris, while the lateral parts of the ridge become the labia
majora of the vulva. The several parts of the female geni-
talia develop to such a degree during the fourth month that
their sexual characters at this time are well marked.

The reader is again reminded that in the stage when the
cloaca is present, the Miillerian ducts terminate in the sinus
urogenitalis (Plate VIL). As previously stated, the sinus uro-
genitalis becomes the female urethra, its terminal portion
expanding into the vestibulum vaginae. The openings of the
Miillerian ducts fall within this latter vestibular region of
the sinus. The lower portion of the two ducts by this time,
however, have fused to form the uterus and the vagina, and
hence is established the permanent relationship of these parts
— that is, the opening of the vagina and the urethra by sepa-
rate orifices into the vestibule.

The formation of the hjnnen begins in the fifth month as a
little crescentic fold at the posterior margin of the aperture
of the vagina.

The glands of Bartholin develop as evaginations of the
wall of the vestibular region of the urogenital sinus.

The Male External Genitals. — The male external organs
represent a farther stage of development than the corre-



spoil ding female parts. The genital eminence elongates rather
rapidly, its length in the tenth week being 1.5 mm. The
knob-like extremity becomes better marked and constitutes
the glans penis, while the integumentary fold that partially
encircles the latter assumes more distinctive character as the
prepuce. This fold gradually advances over the glans and
adheres to it, the adhesion persisting until, or shortly after,
birth. All of the rudimentary penis, exclusive of the glans
and of the genital folds, becomes the corpora cavernosa of
the adult organ. The characteristic structure of the corpora



Fig. 116.— Four successive stages of development of the human male fetus
(Tourneux) : 1 , penis ; 2, glans ; 3, genital groove ; 4, scrotum ; 5, anus ; 6, coccygeal
eminence; 7, perineoscrotal raphe.

cavernosa is foreshadowed as early as the third month by the
appearance in the penis of capillary blood-vessels, which, in
the sixth month, undergo marked dilatation.


The groove on the under surface of the penis becomes
deeper, and the genital folds, which bound the groove laterally,
increase in size. This groove extends from the orifice of the
urogenital sinus to the glans penis. The genital folds, which
in the female, remain distinct and become the nymphse, unite
with each other in the male and convert the groove into a
canal, which latter is practically an extension of the uro-
genital sinus along the entire length of the penis to the
glans. The canal thus formed is the anterior part of the
male urethra, or, in other words, it includes all of the urethra
except its prostatic portion, which represents the urogenital
sinus. The orifice of this newly-formed canal, situated in
the glans, is the meatus urinarius. Failure of union of the
genital folds, either wholly or in part, results in total or in
partial deficiency of the floor of the urethra, this anomaly
being known as hypospadias. If the defective closure in-
volves only the glans, the condition is denominated glandular

The genital folds form not only the sides and the floor of
the penile urethra, but by an extension of their growth, also
its roof, thus completely surrounding it. Upon the acquisi-
tion, by the now united genital folds, of blood-vessels and
cavernous spaces, they become the corpus spongiosum of the
penis, and thus is established the permanent or adult relation
of these parts.

The genital ridge becomes diiferentiated into two promi-
nent folds or pouches placed one on either side of the root of
the penis. In the fourth month these unite to form the
scrotum, the line of union being indicated by the raphe.
Failure of union of the two halves of the scrotum is one of
the features of certain forms of so-called hermaphroditism.

The glands of Cowper, which correspond to the glands of
Bartholin of the female, are developed, like the latter, as
evaginations of the terminal part of the urogenital sinus.

The accompanying tabulation exhibits a comparison of the
organs of the two sexes on the basis of their common origin.
Male and female parts that develop from the same fetal
structure are said to be homologous with each other.



Homologies of the Sexual System.

Fetal Structure.

Indiflerent sexual gland.
Wolffian body —

Its middle series of tu-
bules and

Corresponding part of
Wolffian duct.

Remainder of Wolffian

IJpper series of short

tubules (pronephros).

Loiver series of tubules.

Duct of Miiller—
Its upper extremity.
Succeeding portion.

Female Organs.

Short tubules of par-
ovarium and

Horizontal or long tube
of pai'ovarium.

Usually altogether dis-
appears ; if persistent,
Gartner's duct.

Stalked hydatid of Mor-


Fimbria of oviduct.

Remaining portion, by
fusion with its fellow.

Uterus and vagina.

Male Organs.


Vasa efferentia, rete testis

and coni vasculosi.
Tube of epididymis.

Vas deferens, seminal

Online LibraryJohn Clement HeislerA text-book of embryology for students of medicine → online text (page 17 of 33)