In the Selachii also we find a longitudinal row of segmental canals on
each side, which open outwards into the primitive renal ducts
(nephrotomes, Chapter 1.14). The segmental canals (a pair in each
segment of the middle part of the body) open internally by a ciliated
funnel into the body-cavity. From the posterior group of these organs
a compact primitive kidney is formed, the anterior group taking part
in the construction of the sexual organs.
In the same simple form that remains throughout life in the Myxinoides
and partly in the Selachii we find the primitive kidney first
developing in the embryo of man and the higher Craniotes (Figures
2.386 and 2.387). Of the two parts that compose the comb-shaped
primitive kidney the longitudinal channel, or nephroduct, is always
the first to appear; afterwards the transverse "canals," the excreting
nephridia, are formed in the mesoderm; and after this again the
Malpighian capsules with their arterial coils are associated with
these as coelous outgrowths. The primitive renal duct, which appears
first, is found in all craniote embryos at the early stage in which
the differentiation of the medullary tube takes place in the ectoderm,
the severance of the chorda from the visceral layer in the entoderm,
and the first trace of the coelom-pouches arises between the limiting
layers (Figure 2.385). The nephroduct (ung) is seen on each side,
directly under the horny plate, in the shape of a long, thin,
thread-like string of cells. It presently hollows out and becomes a
canal, running straight from front to back, and clearly showing in the
transverse section of the embryo its original position in the space
between horny plate (h), primitive segments (uw), and lateral plates
(hpl). As the originally very short urinary canals lengthen and
multiply, each of the two primitive kidneys assumes the form of a
half-feathered leaf (Figure 2.387). The lines of the leaf are
represented by the urinary canals (u), and the rib by the outlying
nephroduct (w). At the inner edge of the primitive kidneys the
rudiment of the ventral sexual gland (g) can now be seen as a body of
some size. The hindermost end of the nephroduct opens right behind
into the last section of the rectum, thus making a cloaca of it.
However, this opening of the nephroducts into the intestine must be
regarded as a secondary formation. Originally they open, as the
Cyclostomes clearly show, quite independently of the gut, in the
external skin of the abdomen.
(FIGURE 2.395. Primitive kidneys and germinal glands of a human
embryo, three inches in length (beginning of the sixth week),
magnified fifteen times. k germinal gland, u primitive kidney, z
diaphragmatic ligament of same, w Wolffian duct (opened on the right),
g directing ligament (gubernaculum), a allantoic duct. (From
Kollmann.))
In the Myxinoides the primitive kidneys retain this simple comb-shaped
structure, and a part of it is preserved in the Selachii; but in all
the other Craniotes it is only found for a short time in the embryo,
as an ontogenetic reproduction of the earlier phylogenetic structure.
In these the primitive kidney soon assumes the form (by the rapid
growth, lengthening, increase, and serpentining of the urinary canals)
of a large compact gland, of a long, oval or spindle-shaped character,
which passes through the greater part of the embryonic body-cavity
(Figures 1.183 m, 1.184 m, 2.388 n). It lies near the middle line,
directly under the primitive vertebral column, and reaches from the
cardiac region to the cloaca. The right and left kidneys are parallel
to each other, quite close together, and only separated by the
mesentery--the thin narrow layer that attaches the middle gut to the
under surface of the vertebral column. The passage of each primitive
kidney, the nephroduct, runs towards the back on the lower and outer
side of the gland, and opens in the cloaca, close to the
starting-point of the allantois; it afterwards opens into the
allantois itself.
(FIGURES 2.396 TO 2.398. Urinary and sexual organs of ox-embryos.
Figure 2.396, female embryo one and a half inches long; Figure 2.397,
male embryo, one and a half inches long. Figure 2.398 female embryo
two and a half inches long. w primitive kidney, wg Wolffian duct, m
Mullerian duct, m apostrophe upper end of same (opened at t), i lower
and thicker part of same (rudiment of uterus), g genital cord, h
testicle, (h apostrophe, lower and h double apostrophe, upper
testicular ligament), o ovary, o apostrophe lower ovarian ligament, i
inguinal ligament of primitive kidney, d diaphragmatic ligament of
primitive kidney, nn accessory kidneys, n permanent kidneys, under
them the S-shaped ureters, between these the rectum, v bladder, a
umbilical artery. (From Kolliker.))
The primitive or primordial kidneys of the amniote embryo were
formerly called the "Wolffian bodies," and sometimes "Oken's bodies."
They act for a time as kidneys, absorbing unusable juices from the
embryonic body and conducting them to the cloaca--afterwards to the
allantois. There the primitive urine accumulates, and thus the
allantois acts as bladder or urinary sac in the embryos of man and the
other Amniotes. It has, however, no genetic connection with the
primitive kidneys, but is a pouch-like growth from the anterior wall
of the rectum (Figure 1.147 u). Thus it is a product of the visceral
layer, whereas the primitive kidneys are a product of the middle
layer. Phylogenetically we must suppose that the allantois originated
as a pouch-like growth from the cloaca-wall in consequence of the
expansion caused by the urine accumulated in it and excreted by the
kidneys. It is originally a blind sac of the rectum. The real bladder
of the vertebrate certainly made its first appearance among the
Dipneusts (in Lepidosiren), and has been transmitted from them to the
Amphibia, and from these to the Amniotes. In the embryo of the latter
it protrudes far out of the not yet closed ventral wall. It is true
that many of the fishes also have a "bladder." But this is merely a
local enlargement of the lower section of the nephroducts, and so
totally different in origin and composition from the real bladder. The
two structures can be compared from the physiological point of view,
and so are ANALOGOUS, as they have the same function; but not from the
morphological point of view, and are therefore not HOMOLOGOUS. The
false bladder of the fishes is a mesodermic product of the
nephroducts; the true bladder of the Dipneusts, Amphibia, and Amniotes
is an entodermic blind sac of the rectum.
In all the Anamnia (the lower amnionless Craniotes, Cyclostomes,
Fishes, Dipneusts, and Amphibia) the urinary organs remain at a lower
stage of development to this extent, that the primitive kidneys
(protonephri) act permanently as urinary glands. This is only so as a
passing phase of the early embryonic life in the three higher classes
of Vertebrates, the Amniotes. In these the permanent or after or
secondary (really tertiary) kidneys (renes or metanephri) that are
distinctive of these three classes soon make their appearance. They
represent the third and last generation of the vertebrate kidneys. The
permanent kidneys do not arise (as was long supposed) as independent
glands from the alimentary tube, but from the last section of the
primitive kidneys and the nephroduct. Here a simple tube, the
secondary renal duct, develops, near the point of its entry into the
cloaca; and this tube grows considerably forward. With its blind upper
or anterior end is connected a glandular renal growth, that owes its
origin to a differentiation of the last part of the primitive kidneys.
This rudiment of the permanent kidneys consists of coiled urinary
canals with Malpighian capsules and vascular coils (without ciliated
funnels), of the same structure as the segmental mesonephridia of the
primitive kidneys. The further growth of these metanephridia gives
rise to the compact permanent kidneys, which have the familiar
bean-shape in man and most of the higher mammals, but consist of a
number of separate folds in the lower mammals, birds, and reptiles. As
the permanent kidneys grow rapidly and advance forward, their passage,
the ureter, detaches altogether from its birth-place, the posterior
end of the nephroduct; it passes to the posterior surface of the
allantois. At first in the oldest Amniotes this ureter opens into the
cloaca together with the last section of the nephroduct, but
afterwards separately from this, and finally into the permanent
bladder apart from the rectum altogether. The bladder originates from
the hindmost and lowest part of the allantoic pedicle (urachus), which
enlarges in spindle shape before the entry into the cloaca. The
anterior or upper part of the pedicle, which runs to the navel in the
ventral wall of the embryo, atrophies subsequently, and only a useless
string-like relic of it is left as a rudimentary organ; that is the
single vesico-umbilical ligament. To the right and left of it in the
adult male are a couple of other rudimentary organs, the lateral
vesico-umbilical ligaments. These are the degenerate string-like
relics of the earlier umbilical arteries.
Though in man and all the other Amniotes the primitive kidneys are
thus early replaced by the permanent kidneys, and these alone then act
as urinary organs, all the parts of the former are by no means lost.
The nephroducts become very important physiologically by being
converted into the passages of the sexual glands. In all the
Gnathostomes--or all the Vertebrates from the fishes up to man--a
second similar canal develops beside the nephroduct at an early stage
of embryonic evolution. The latter is usually called the Mullerian
duct, after its discoverer, Johannes Muller, while the former is
called the Wolffian duct. The origin of the Mullerian duct is still
obscure; comparative anatomy and ontogeny seem to indicate that it
originates by differentiation from the Wolffian duct. Perhaps it would
be best to say: "The original primary nephroduct divides by
differentiation (or longitudinal cleavage) into two secondary
nephroducts, the Wolffian and the Mullerian ducts." The latter (Figure
2.387 m) lies just on the inner side of the former (Figure 2.387 w).
Both open behind into the cloaca.
However uncertain the origin of the nephroduct and its two products,
the Mullerian and the Wolffian ducts, may be, its later development is
clear enough. In all the Gnathostomes the Wolffian duct is converted
into the spermaduct, and the Mullerian duct into the oviduct. Only one
of them is retained in each sex; the other either disappears
altogether, or only leaves relics in the shape of rudimentary organs.
In the male sex, in which the two Wolffian ducts become the
spermaducts, we often find traces of the Mullerian ducts, which I have
called "Rathke's canals" (Figure 2.394 c). In the female sex, in which
the two Mullerian ducts form the oviducts, there are relics of the
Wolffian ducts, which are called "the ducts of Gaertner."
(FIGURE 2.399. Female sexual organs of a Monotreme (Ornithorhynchus,
Figure 2.269). o ovaries, t oviducts, u womb, sug urogenital sinus; at
u apostrophe is the outlet of the two wombs, and between them the
bladder (vu). cl cloaca. (From Gegenbaur.)
FIGURES 2.400 AND 2.401. Original position of the sexual glands in the
ventral cavity of the human embryo (three months old).
FIGURE 2.400 male (natural size). h testicles, gh conducting ligament
of the testicles, wg spermaduct, h bladder, uh inferior vena cava, nn
accessory kidneys, n kidneys.
FIGURE 2.401 female, slightly magnified. r round maternal ligament
(underneath it the bladder, over it the ovaries). r apostrophe
kidneys, s accessory kidneys, c caecum, o small reticle, om large
reticle (stomach between the two), l spleen. (From Kolliker.))
We obtain the most interesting information with regard to this
remarkable evolution of the nephroducts and their association with the
sexual glands from the Amphibia (Figures 2.390 to 2.395). The first
structure of the nephroduct and its differentiation into Mullerian and
Wolffian ducts are just the same in both sexes in the Amphibia, as in
the mammal embryos (Figures 2.392 and 2.396). In the female Amphibia
the Mullerian duct develops on either side into a large oviduct
(Figure 2.393 od), while the Wolffian duct acts permanently as ureter
(u). In the male Amphibia the Mullerian duct only remains as a
rudimentary organ without any functional significance, as Rathke's
canal (Figure 2.394 c); the Wolffian duct serves also as ureter, but
at the same time as spermaduct, the sperm-canals (ve) that proceed
from the testicles (t) entering the fore part of the primitive kidneys
and combining there with the urinary canals.
In the mammals these permanent amphibian features are only seen as
brief phases of the earlier period of embryonic development (Figure
2.392). Here the primitive kidneys, which act as excretory organs of
urine throughout life in the amnion-less Vertebrates, are replaced in
the mammals by the permanent kidneys. The real primitive kidneys
disappear for the most part at an early stage of development, and only
small relics of them remain. In the male mammal the epididymis
develops from the uppermost part of the primitive kidney; in the
female a useless rudimentary organ, the epovarium, is formed from the
same part. The atrophied relic of the former is known as the
paradidymis, that of the latter as the parovarium.
(FIGURE 2.402. Urogenital system of a human embryo of three inches in
length, double natural size. h testicles, wg spermaducts, gh
conducting ligament, p processus vaginalis, b bladder, au umbilical
arteries, m mesorchium, d intestine, u ureter, n kidney, nn accessory
kidney. (From Kollman.))
The Mullerian ducts undergo very important changes in the female
mammal. The oviducts proper are developed only from their upper part;
the lower part dilates into a spindle-shaped tube with thick muscular
wall, in which the impregnated ovum develops into the embryo. This is
the womb (uterus). At first the two wombs (Figure 2.399 u) are
completely separate, and open into the cloaca on either side of the
bladder (vu), as is still the case in the lowest living mammals, the
Monotremes. But in the Marsupials a communication is opened between
the two Mullerian ducts, and in the Placentals they combine below with
the rudimentary Wolffian ducts to form a single "genital cord." The
original independence of the two wombs and the vaginal canals formed
from their lower ends are retained in many of the lower Placentals,
but in the higher they gradually blend and form a single organ. The
conjunction proceeds from below (or behind) upwards (or forwards). In
many of the Rodents (such as the rabbit and squirrel) two separate
wombs still open into the simple and single vaginal canal; but in
others, and in the Carnivora, Cetacea, and Ungulates, the lower halves
of the wombs have already fused into a single piece, though the upper
halves (or "horns") are still separate ("two-horned" womb, uteris
bicornis). In the bats and lemurs the "horns" are very short, and the
lower common part is longer. Finally, in the apes and in man the
blending of the two halves is complete, and there is only the one
simple, pear-shaped uterine pouch, into which the oviducts open on
each side. This simple uterus is a late evolutionary product, and is
found ONLY in the ape and man.
(FIGURES 2.403 TO 2.406. Origin of human ova in the female ovary.
FIGURE 2.403. Vertical section of the ovary of a new-born female
infant, a ovarian epithelium, b rudimentary string of ova, c young ova
in the epithelium, d long string of ova with follicle-formation
(Pfluger's tube), e group of young follicles, f isolated young
follicle, g blood-vessels in connective tissue (stroma) of the ovary.
In the strings the young ova are distinguished by their considerable
size from the surrounding follicle-cells. (From Waldeyer.)
FIGURE 2.404. Two young Graafian follicles, isolated. In 1 the
follicle-cells still form a simple, and in 2 a double, stratum round
the young ovum; in 2 they are beginning to form the ovolemma or the
zona pellucida (a).
FIGURES 2.405 AND 2.406. Two older Graafian follicles, in which fluid
is beginning to accumulate inside the eccentrically thickened
epithelial mass of the follicle-cells (Figure 2.405 with little, 2.406
with much, follicle-water). ei the young ovum, with embryonic vesicle
and spot, zp ovolemma or zona pellucida, dp discus proligerus, formed
of an accumulation of follicle-cells, which surround the ovum, ff
follicle-liquid (liquor folliculi), gathered inside the stratified
follicle-epithelium (fe), fk connective-tissue fibrous capsule of the
Graafian follicle (theca folliculi).)
In the male mammals there is the same fusion of the Mullerian and
Wolffian ducts at their lower ends. Here again they form a single
genital cord (Figure 2.397 g), and this opens similarly into the
original urogenital sinus, which develops from the lowest section of
the bladder (v). But while in the male mammal the Wolffian ducts
develop into the permanent spermaducts, there are only rudimentary
relics left of the Mullerian ducts. The most notable of these is the
"male womb" (uterus masculinus), which originates from the lowest
fused part of the ducts, and corresponds to the female uterus. It is a
small, flask-shaped vesicle without any physiological significance,
which opens into the ureter between the two spermaducts and the
prostate folds (vesicula prostatica).
(FIGURE 2.407. A ripe human Graafian follicle. a the mature ovum, b
the surrounding follicle-cells, c the epithelial cells of the
follicle, d the fibrous membrane of the follicle, e its outer
surface.)
The internal sexual organs of the mammals undergo very distinctive
changes of position. At first the germinal glands of both sexes lie
deep inside the ventral cavity, at the inner edge of the primitive
kidneys (Figures 2.386 g and 2.392 k), attached to the vertebral
column by a short mesentery (mesorchium in the male, mesovarium in the
female). But this primary arrangement is retained permanently only in
the Monotremes (and the lower Vertebrates). In all other mammals (both
Marsupials and Placentals) they leave their original cradle and travel
more or less far down (or behind), following the direction of a
ligament that goes from the primitive kidneys to the inguinal region
of the ventral wall. This is the inguinal ligament of the primitive
kidneys, known in the male as the Hunterian ligament (Figure 2.400
gh), and in the female as the "round maternal ligament" (Figure 2.401
r). In woman the ovaries travel more or less towards the small pelvis,
or enter into it altogether. In the male the testicles pass out of the
ventral cavity, and penetrate by the inguinal canal into a sac-shaped
fold of the outer skin. When the right and left folds ("sexual
swellings") join together they form the scrotum. The various mammals
bring before us the successive stages of this displacement. In the
elephant and the whale the testicles descend very little, and remain
underneath the kidneys. In many of the rodents and carnassia they
enter the inguinal canal. In most of the higher mammals they pass
through this into the scrotum. As a rule, the inguinal canal closes
up. When it remains open the testicles may periodically pass into the
scrotum, and withdraw into the ventral cavity again in time of rut (as
in many of the marsupials, rodents, bats, etc.).
The structure of the external sexual organs, the copulative organs
that convey the fecundating sperm from the male to the female organism
in the act of copulation, is also peculiar to the mammals. There are
no organs of this character in most of the other Vertebrates. In those
that live in water (such as the Acrania and Cyclostomes, and most of
the fishes) the ova and sperm-cells are simply ejected into the water,
where their conjunction and fertilisation are left to chance. But in
many of the fishes and amphibia, which are viviparous, there is a
direct conveyance of the male sperm into the female body; and this is
the case with all the Amniotes (reptiles, birds, and mammals). In
these the urinary and sexual organs always open originally into the
last section of the rectum, which thus forms a cloaca (Chapter 2.22).
Among the mammals this arrangement is permanent only in the
Monotremes, which take their name from it (Figure 2.399 cl). In all
the other mammals a frontal partition is developed in the cloaca (in
the human embryo about the beginning of the third month), and this
divides it into two cavities. The anterior cavity receives the
urogenital canal, and is the sole outlet of the urine and the sexual
products; the hind or anus-cavity passes the excrements only.
Even before this partition has been formed in the Marsupials and
Placentals, we see the first trace of the external sexual organs.
First a conical protuberance rises at the anterior border of the
cloaca-outlet--the sexual prominence (phallus, Figure 2.402 A, e, B,
e). At the tip it is swollen in the shape of a club ("acorn" glans).
On its under side there is a furrow, the sexual groove (sulcus
genitalis, f), and on each side of this a fold of skin, the "sexual
pad" (torus genitalis, h l). The sexual protuberance or phallus is the
chief organ of the sexual sense (Chapter 2.25); the sexual nerves
spread on it, and these are the principal organs of the specific
sexual sensation. As erectile bodies (corpora cavernosa) are developed
in the male phallus by peculiar modifications of the blood-vessels, it
becomes capable of erecting periodically on a strong accession of
blood, becoming stiff, so as to penetrate into the female vagina and
thus effect copulation. In the male the phallus becomes the penis; in
the female it becomes the much smaller clitoris; this is only found to
be very large in certain apes (Ateles). A prepuce ("foreskin") is
developed in both sexes as a protecting fold on the anterior surface
of the phallus.
(FIGURE 408. The human ovum after issuing from the Graafian follicle,
surrounded by the clinging cells of the discus proligerus (in two
radiating crowns). z ovolemma (zona pellucida, with radial porous
canals), p cytosoma (protoplasm of the cell-body, darker within,
lighter without), k nucleus of the ovum (embryonic vesicle). (From
Nagel, magnified 250 times.) (Cf. Figures 1.1 and 1.14.)
The external sexual member (phallus) is found at various stages of
development within the mammal class, both in regard to size and shape,
and the differentiation and structure of its various parts; this
applies especially to the terminal part of the phallus, the glans,
both the larger glans penis of the male and the smaller glans
clitoridis of the female. The part of the cloaca from the upper wall
of which it forms belongs to the proctodaeum, the ectodermic
invagination of the rectum (Chapter 2.27); hence its epithelial
covering can develop the same horny growths as the corneous layer of
the epidermis. Thus the glans, which is quite smooth in man and the
higher apes, is covered with spines in many of the lower apes and in
the cat, and in many of the rodents with hairs (marmot) or scales
(guinea-pig) or solid horny warts (beaver). Many of the Ungulates have
a free conical projection on the glans, and in many of the Ruminants
this "phallus-tentacle" grows into a long cone, bent hook-wise at the
base (as in the goat, antelope, gazelle, etc.). The different forms of
the phallus are connected with variations in the structure and
distribution of the sensory corpuscles--i.e. the real organs of the
sexual sense, which develop in certain papillae of the corium of the
phallus, and have been evolved from ordinary tactile corpuscles of the
corium by erotic adaptation (Chapter 2.25).
The formation of the corpora cavernosa, which cause the stiffness of
the phallus and its capability of penetrating the vagina, by certain
special structures of their spongy vascular spaces, also shows a good
deal of variety within the vertebrate stem. This stiffness is
increased in many orders of mammals (especially the carnassia and
rodents) by the ossification of a part of the fibrous body (corpus
fibrosum). This penis-bone (os priapi) is very large in the badger and
dog, and bent like a hook in the marten; it is also very large in some
of the lower apes, and protrudes far out into the glans. It is wanting
in most of the anthropoid apes; it seems to have been lost in their
case (and in man) by atrophy.
The sexual groove on the under side of the phallus receives in the
male the mouth of the urogenital canal, and is changed into a
continuation of this, becoming a closed canal by the juncture of its
parallel edges, the male urethra. In the female this only takes place
in a few cases (some of the lemurs, rodents, and moles); as a rule,
the groove remains open, and the borders of this "vestibule of the
vagina" develop into the smaller labia (nymphae). The large labia of
the female develop from the sexual pads (tori genitales), the two
parallel folds of the skin that are found on each side of the genital
groove. They join together in the male, and form the closed scrotum.
These striking differences between the two sexes cannot yet be
detected in the human embryo of the ninth week. We begin to trace them
in the tenth week of development, and they are accentuated in
proportion as the difference of the sexes develops.
Sometimes the normal juncture of the two sexual pads in the male fails
to take place, and the sexual groove may also remain open
(hypospadia). In these cases the external male genitals resemble the
female, and they are often wrongly regarded as cases of hermaphrodism.
Other malformations of various kinds are not infrequently found in the
human external sexual organs, and some of them have a great
morphological interest. The reverse of hypospadia, in which the penis
is split open below, is seen in epispadia, in which the urethra is
open above. In this case the urogenital canal opens above at the
dorsal root of the penis; in the former case down below. These and
similar obstructions interfere with a man's generative power, and thus
prejudicially affect his whole development. They clearly prove that
our history is not guided by a "kind Providence," but left to the play
of blind chance.
We must carefully distinguish the rarer cases of real hermaphrodism
from the preceding. This is only found when the essential organs of
reproduction, the genital glands of both kinds, are united in one
individual. In these cases either an ovary is developed on the right
and a testicle on the left (or vice versa); or else there are
testicles and ovaries on both sides, some more and others less
developed. As hermaphrodism was probably the original arrangement in
all the Vertebrates, and the division of the sexes only followed by
later differentiation of this, these curious cases offer no
theoretical difficulty. But they are rarely found in man and the
higher mammals. On the other hand, we constantly find the original
hermaphrodism in some of the lower Vertebrates, such as the
Myxinoides, many fishes of the perch-type (serranus), and some of the
Amphibia (ringed snake, toad). In these cases the male often has a
rudimentary ovary at the fore end of the testicle; and the female
sometimes has a rudimentary, inactive testicle. In the carp also and
some other fishes this is found occasionally. We have already seen how
traces of the earlier hemaphrodism can be traced in the passages of
the Amphibia.
Man has faithfully preserved the main features of his stem-history in
the ontogeny of his urinary and sexual organs. We can follow their
development step by step in the human embryo in the same advancing
gradation that is presented to us by the comparison of the urogenital
organs in the Acrania, Cyclostomes; Fishes, Amphibia, Reptiles, and
then (within the mammal series) in the Monotremes, Marsupials, and the
various Placentals. All the peculiarities of urogenital structure that
distinguish the mammals from the rest of the Vertebrates are found in
man; and in all special structural features he resembles the apes,
particularly the anthropoid apes. In proof of the fact that the
special features of the mammals have been inherited by man, I will, in
conclusion, point out the identical way in which the ova are formed in
the ovary. In all the mammals the mature ova are contained in special
capsules, which are known as the Graafian
follicles, after their discoverer, Roger de Graaf (1677). They were
formerly supposed to be the ova themselves; but Baer discovered the
ova within the follicles (Chapter 1.3). Each follicle (Figure 2.407)
consists of a round fibrous capsule (d), which contains fluid and is
lined with several strata of cells (c). The layer is thickened like a
knob at one point (b); this ovum-capsule encloses the ovum proper (a).
The mammal ovary is originally a very simple oval body (Figure 2.387
g), formed only of connective tissue and blood-vessels, covered with a
layer of cells, the ovarian epithelium or the female germ epithelium.
From this germ epithelium strings of cells grow out into the
connective tissue or "stroma" of the ovary (Figure 2.403 b). Some of
the cells of these strings (or Pfluger's tubes) grow larger and become
ova (primitive ova, c); but the great majority remain small, and form
a protective and nutritive stratum of cells round each ovum--the
"follicle-epithelium" (e).
The follicle-epithelium of the mammal has at first one stratum (Figure
2.404 1), but afterwards several (2). It is true that in all the other
Vertebrates the ova are enclosed in a membrane, or "follicle," that
consists of smaller cells. But it is only in the mammals that fluid
accumulates between the growing follicle-cells, and distends the
follicle into a large round capsule, on the inside wall of which the
ovum lies, at one side (Figures 2.405 and 2.406). There again, as in
the whole of his morphology, man proves indubitably his descent from
the mammals.
In the lower Vertebrates the formation of ova in the germ-epithelium
of the ovary continues throughout life; but in the higher it is
restricted to the earlier stages, or even to the period of embryonic
development. In man it seems to cease in the first year; in the second
year we find no new-formed ova or chains of ova (Pfluger's tubes).
However, the number of ova in the two ovaries is very large in the
young girl; there are calculated to be 72,000 in the sexually-mature
maiden. In the production of the ova men resemble most of the
anthropoid apes.
Generally speaking, the natural history of the human sexual organs is
one of those parts of anthropology that furnish the most convincing
proofs of the animal origin of the human race. Any man who is
acquainted with the facts and impartially weighs them will conclude
from them alone that we have been evolved from the lower Vertebrates.
The larger and the detailed structure, the action, and the
embryological development of the sexual organs are just the same in
man as in the apes. This applies equally to the male and the female,
the internal and the external organs. The differences we find in this
respect between man and the anthropoid apes are much slighter than the
differences between the various species of apes. But all the apes have
certainly a common origin, and have been evolved from a long-extinct
early-Tertiary stem-form, which we must trace to a branch of the
lemurs. If we had this unknown pithecoid stem-form before us, we
should certainly put it in the order of the true apes in the primate
system; but within this order we cannot, for the anatomic and
ontogenetic reasons we have seen, separate man from the group of the
anthropoid apes. Here again, therefore, on the ground of the
pithecometra-principle, comparative anatomy and ontogeny teach with
full confidence the descent of man from the ape.