primitive vertebrate they probably had the same form as in the actual
amphioxus--the primitive kidneys (protonephra). These are originally
made up of a double row of little canals, which directly convey the
used-up juices or the urine out of the body-cavity (Figure 1.102 n).
The inner aperture of these pronephridial canals opens with a ciliated
funnel into the body-cavity; the external aperture opens in lateral
grooves of the epidermis, a couple of longitudinal grooves in the
lateral surface of the outer skin (Figure 1.102 b). The pronephridial
duct is formed by the closing of this groove to the right and left at
the sides. In all the craniota it develops at an early stage in the
horny plate; in the amphioxus it seems to be converted into a wide
cavity, the atrium, or peribranchial space.
Next to the kidneys we have the sexual organs of the vertebrate. In
most of the members of this stem the two are united in a single
urogenital system; it is only in a few groups that the urinary and
sexual organs are separated (in the amphioxus, the cyclostoma, and
some sections of the fish-class). In man and all the higher
vertebrates the sexual apparatus is made up of various parts, which we
will consider in Chapter 2.29. But in the two lowest classes of our
stem, the acrania and cyclostoma, they consist merely of simple sexual
glands or gonads, the ovaries of the female sex and the testicles
(spermaria) of the male; the former provide the ova, the latter the
sperm. In the craniota we always find only one pair of gonads; in the
amphioxus several pairs, arranged in succession. They must have had
the same form in our hypothetical prospondylus (Figures 1.98 and 1.100
s). These segmental pairs of gonads are the original ventral halves of
the coelom-pouches.
The organs which we have now enumerated in this general survey, and of
which we have noted the characteristic disposition, are those parts of
the organism that are found in all vertebrates without exception in
the same relation to each other, however much they may be modified. We
have chiefly had in view the transverse section of the body (Figures
1.101 and 1.102), because in this we see most clearly the distinctive
arrangement of them. But to complete our picture we must also consider
the segmentation or metamera-formation of them, which has yet been
hardly noticed, and which is seen best in the longitudinal section. In
man and all the more advanced vertebrates the body is made up of a
series or chain of similar members, which succeed each other in the
long axis of the body--the segments or metamera of the organism. In
man these homogeneous parts number thirty-three in the trunk, but they
run to several hundred in many of the vertebrates (such as serpents or
eels). As this internal articulation or metamerism is mainly found in
the vertebral column and the surrounding muscles, the sections or
metamera were formerly called pro-vertebrae. As a fact, the
articulation is by no means chiefly determined and caused by the
skeleton, but by the muscular system and the segmental arrangement of
the kidneys and gonads. However, the composition from these
pro-vertebrae or internal metamera is usually, and rightly, put
forward as a prominent character of the vertebrate, and the manifold
division or differentiation of them is of great importance in the
various groups of the vertebrates. But as far as our present task--the
derivation of the simple body of the primitive vertebrate from the
chordula--is concerned, the articulate parts or metamera are of
secondary interest, and we need not go into them just now.
(FIGURE 1.103 A, B, C, D. Instances of redundant mammary glands and
nipples (hypermastism). A a
pair of small redundant breasts (with two nipples on the left) above
the large normal ones; from a 45-year-old Berlin woman, who had had
children 17 times (twins twice). (From Hansemann.) B the highest
number: ten nipples (all giving milk), three pairs above, one pair
below, the large normal breasts; from a 22-year-old servant at
Warschau. (From Neugebaur.) C three pairs of nipples: two pairs on the
normal glands and one pair above; from a 19-year-old Japanese girl. D
four pairs of nipples: one pair above the normal and two pairs of
small accessory nipples underneath; from a 22-year-old Bavarian
soldier. (From Wiedersheim.))
The characteristic composition of the vertebrate body develops from
the embryonic structure in the same way in man as in all the other
vertebrates. As all competent experts now admit the monophyletic
origin of the vertebrates on the strength of this significant
agreement, and this "common descent of all the vertebrates from one
original stem-form" is admitted as an historical fact, we have found
the answer to "the question of questions." We may, moreover, point out
that this answer is just as certain and precise in the case of the
origin of man from the mammals. This advanced vertebrate class is also
monophyletic, or has evolved from one common stem-group of lower
vertebrates (reptiles, and, earlier still, amphibia). This follows
from the fact that the mammals are clearly distinguished from the
other classes of the stem, not merely in one striking particular, but
in a whole group of distinctive characters.
It is only in the mammals that we find the skin covered with hair, the
breast-cavity separated from the abdominal cavity by a complete
diaphragm, and the larynx provided with an epiglottis. The mammals
alone have three small auscultory bones in the tympanic cavity--a
feature that is connected with the characteristic modification of
their maxillary joint. Their red blood-cells have no nucleus, whereas
this is retained in all other vertebrates. Finally, it is only in the
mammals that we find the remarkable function of the breast structure
which has given its name to the whole class--the feeding of the young
by the mother's milk. The mammary glands which serve this purpose are
interesting in so many ways that we may devote a few lines to them
here.
As is well known, the lower mammals, especially those which beget a
number of young at a time, have several mammary glands at the breast.
Hedgehogs and sows have five pairs, mice four or five pairs, dogs and
squirrels four pairs, cats and bears three pairs, most of the
ruminants and many of the rodents two pairs, each provided with a teat
or nipple (mastos). In the various genera of the half-apes (lemurs)
the number varies a good deal. On the other hand, the bats and apes,
which only beget one young at a time as a rule, have only one pair of
mammary glands, and these are found at the breast, as in man.
These variations in the number or structure of the mammary apparatus
(mammarium) have become doubly interesting in the light of recent
research in comparative anatomy. It has been shown that in man and the
apes we often find redundant mammary glands (hyper-mastism) and
corresponding teats (hyper-thelism) in both sexes. Figure 1.103 shows
four cases of this kind--A, B, and C of three women, and D of a man.
They prove that all the above-mentioned numbers may be found
occasionally in man. Figure 1.103 A shows the breast of a Berlin woman
who had had children seventeen times, and who has a pair of small
accessory breasts (with two nipples on the left one) above the two
normal breasts; this is a common occurrence, and the small soft pad
above the breast is not infrequently represented in ancient statues of
Venus. In Figure 1.103 C we have the same phenomenon in a Japanese
girl of nineteen, who has two nipples on each breast besides (three
pairs altogether). Figure 1.103 D is a man of twenty-two with four
pairs of nipples (as in the dog), a small pair above and two small
pairs beneath the large normal teats. The maximum number of five pairs
(as in the sow and hedgehog) was found in a Polish servant of
twenty-two who had had several children; milk was given by each
nipple; there were three pairs of redundant nipples above and one pair
underneath the normal and very large breasts (Figure 1.103 B).
A number of recent investigations (especially among recruits) have
shown that these things are not uncommon in the male as well as the
female sex. They can only be explained by evolution, which attributes
them to atavism and latent heredity. The earlier ancestors of all the
primates (including man) were lower placentals, which had, like the
hedgehog (one of the oldest forms of the living placentals), several
mammary glands (five or more pairs) in the abdominal skin. In the apes
and man only a couple of them are normally developed, but from time to
time we get a development of the atrophied structures. Special notice
should be taken of the arrangement of these accessory mammae; they
form, as is clearly seen in Figure 1.103 B and D, two long rows, which
diverge forward (towards the arm-pit), and converge behind in the
middle line (towards the loins). The milk-glands of the polymastic
lower placentals are arranged in similar lines.
The phylogenetic explanation of polymastism, as given in comparative
anatomy, has lately found considerable support in ontogeny. Hans
Strahl, E. Schmitt, and others, have found that there are always in
the human embryo at the sixth week (when it is three-fifths of an inch
long) the microscopic traces of five pairs of mammary glands, and that
they are arranged at regular distances in two lateral and divergent
lines, which correspond to the mammary lines. Only one pair of
them--the central pair--are normally developed, the others atrophying.
Hence there is for a time in the human embryo a normal hyperthelism,
and this can only be explained by the descent of man from lower
primates (lemurs) with several pairs.
But the milk-gland of the mammal has a great morphological interest
from another point of view. This organ for feeding the young in man
and the higher mammals is, as is known, found in both sexes. However,
it is usually active only in the female sex, and yields the valuable
"mother's milk"; in the male sex it is small and inactive, a real
rudimentary organ of no physiological interest. Nevertheless, in
certain cases we find the breast as fully developed in man as in
woman, and it may give milk for feeding the young.
(FIGURE 1.104. A Greek gynecomast.)
We have a striking instance of this gynecomastism (large milk-giving
breasts in a male) in Figure 1.104. I owe the photograph (taken from
life) to the kindness of Dr. Ornstein, of Athens, a German physician,
who has rendered service by a number of anthropological observations,
(for instance, in several cases of tailed men). The gynecomast in
question is a Greek recruit in his twentieth year, who has both
normally developed male organs and very pronounced female breasts. It
is noteworthy that the other features of his structure are in accord
with the softer forms of the female sex. It reminds us of the marble
statues of hermaphrodites which the ancient Greek and Roman sculptors
often produced. But the man would only be a real hermaphrodite if he
had ovaries internally besides the (externally visible) testicles.
I observed a very similar case during my stay in Ceylon (at
Belligemma) in 1881. A young Cinghalese in his twenty-fifth year was
brought to me as a curious hermaphrodite, half-man and half-woman. His
large breasts gave plenty of milk; he was employed as "male nurse" to
suckle a new-born infant whose mother had died at birth. The outline
of his body was softer and more feminine than in the Greek shown in
Figure 1.104. As the Cinghalese are small of stature and of graceful
build, and as the men often resemble the women in clothing (upper part
of the body naked, female dress on the lower part) and the dressing of
the hair (with a comb), I first took the beardless youth to be a
woman. The illusion was greater, as in this remarkable case
gynecomastism was associated with cryptorchism--that is to say, the
testicles had kept to their original place in the visceral cavity, and
had not travelled in the normal way down into the scrotum. (Cf.