Ernst Heinrich Philipp August Haeckel.

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also to the blood.

The blood-vessel system of the vertebrate has a very elaborate
construction, but seems to have had a very simple form in the
primitive vertebrate, as we find it to-day permanently in the annelids
(for instance, earth-worms) and the amphioxus. We accordingly
distinguish first of all as essential, original parts of it two large
single blood-canals, which lie in the fibrous wall of the gut, and run
along the alimentary canal in the median plane of the body, one above
and the other underneath the canal. These principal canals give out
numerous branches to all parts of the body, and pass into each other
by arches before and behind; we will call them the primitive artery
and the primitive vein. The first corresponds to the dorsal vessel,
the second to the ventral vessel, of the worms. The primitive or
principal artery, usually called the aorta (Figure 1.98 a), lies above
the gut in the middle line of its dorsal side, and conducts oxidised
or arterial blood from the gills to the body. The primitive or
principal vein (Figure 1.100 v) lies below the gut, in the middle line
of its ventral side, and is therefore also called the vena
subintestinalis; it conducts carbonised or venous blood back from the
body to the gills. At the branchial section of the gut in front the
two canals are connected by a number of branches, which rise in arches
between the gill-clefts. These "branchial vascular arches" (kg) run
along the gill-arches, and have a direct share in the work of
respiration. The anterior continuation of the principal vein which
runs on the ventral wall of the gill-gut, and gives off these vascular
arches upwards, is the branchial artery (ka). At the border of the two
sections of the ventral vessel it enlarges into a contractile
spindle-shaped tube (Figures 1.98 and 1.100 h). This is the first
outline of the heart, which afterwards becomes a four-chambered pump
in the higher vertebrates and man. There is no heart in the amphioxus,
probably owing to degeneration. In prospondylus the ventral gill-heart
probably had the simple form in which we still find it in the ascidia
and the embryos of the craniota (Figures 1.98 and 1.100 h).

The kidneys, which act as organs of excretion or urinary organs in all
vertebrates, have a very different and elaborate construction in the
various sections of this stem; we will consider them further in
Chapter 2.29. Here I need only mention that in our hypothetical
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

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.
Chapter 2.29.) Hence the latter was very small, soft, and empty.
Moreover, one could feel nothing of the testicles in the inguinal
canal. On the other hand, the male organ was very small, but normally
developed. It was clear that this apparent hermaphrodite also was a
real male.

Another case of practical gynecomastism has been described by
Alexander von Humboldt. In a South American forest he found a solitary
settler whose wife had died in child-birth. The man had laid the
new-born child on his own breast in despair; and the continuous
stimulus of the child's sucking movements had revived the activity of
the mammary glands. It is possible that nervous suggestion had some
share in it. Similar cases have been often observed in recent years,
even among other male mammals (such as sheep and goats).

The great scientific interest of these facts is in their bearing on
the question of heredity. The stem-history of the mammarium rests
partly on its embryology (Chapter 2.24.) and partly on the facts of
comparative anatomy and physiology. As in the lower and higher mammals
(the monotremes, and most of the marsupials) the whole lactiferous
apparatus is only found in the female; and as there are traces of it
in the male only in a few younger marsupials, there can be no doubt
that these important organs were originally found only in the female
mammal, and that they were acquired by these through a special
adaptation to habits of life.

Later, these female organs were communicated to both sexes by
heredity; and they have been maintained in all persons of either sex,
although they are not physiologically active in the males. This normal
permanence of the female lactiferous organs in BOTH sexes of the
higher mammals and man is independent of any selection, and is a fine
instance of the much-disputed "inheritance of acquired characters."


The three higher classes of vertebrates which we call the
amniotes - the mammals, birds, and reptiles - are notably distinguished
by a number of peculiarities of their development from the five lower
classes of the stem - the animals without an amnion (the anamnia). All
the amniotes have a distinctive embryonic membrane known as the amnion
(or "water-membrane"), and a special embryonic appendage - the
allantois. They have, further, a large yelk-sac, which is filled with
food-yelk in the reptiles and birds, and with a corresponding clear
fluid in the mammals. In consequence of these later-acquired
structures, the original features of the development of the amniotes
are so much altered that it is very difficult to reduce them to the
palingenetic embryonic processes of the lower amnion-less vertebrates.
The gastraea theory shows us how to do this, by representing the
embryology of the lowest vertebrate, the skull-less amphioxus, as the
original form, and deducing from it, through a series of gradual
modifications, the gastrulation and coelomation of the craniota.

It was somewhat fatal to the true conception of the chief embryonic
processes of the vertebrate that all the older embryologists, from
Malpighi (1687) and Wolff (1750) to Baer (1828) and Remak (1850),
always started from the investigation of the hen's egg, and
transferred to man and the other vertebrates the impressions they
gathered from this. This classical object of embryological research
is, as we have seen, a source of dangerous errors. The large round
food-yelk of the bird's egg causes, in the first place, a flat discoid
expansion of the small gastrula, and then so distinctive a development
of this thin round embryonic disk that the controversy as to its
significance occupies a large part of embryological literature.

(FIGURE 1.105. Severance of the discoid mammal embryo from the
yelk-sac, in transverse section (diagrammatic). A The germinal disk
(h, hf) lies flat on one side of the branchial-gut vesicle (kb). B In
the middle of the germinal disk we find the medullary groove (mr), and
underneath it the chorda (ch). C The gut-fibre-layer (df) has been
enclosed by the gut-gland-layer (dd). D The skin-fibre-layer (hf) and
gut-fibre-layer (df) divide at the periphery; the gut (d) begins to
separate from the yelk-sac or umbilical vesicle (nb). E The medullary
tube (mr) is closed; the body-cavity (c) begins to form. F The
provertebrae (w) begin to grow round the medullary tube (mr) and the
chorda (ch): the gut (d) is cut off from the umbilical vesicle (nb). H
The vertebrae (w) have grown round the medullary tube (mr) and chorda;
the body-cavity is closed, and the umbilical vesicle has disappeared.
The amnion and serous membrane are omitted. The letters have the same
meaning throughout: h horn-plate, mr medullary tube, hf
skin-fibre-layer, w provertebrae, ch chorda, c body-cavity or coeloma,
df gut-fibre-layer, dd gut-gland-layer, d gut-cavity, nb umbilical

One of the most unfortunate errors that this led to was the idea of an
original antithesis of germ and yelk. The latter was regarded as a
foreign body, extrinsic to the real germ, whereas it is properly a
part of it, an embryonic organ of nutrition. Many authors said there
was no trace of the embryo until a later stage, and outside the yelk;
sometimes the two-layered embryonic disk itself, at other times only
the central portion of it (as distinguished from the germinative area,
which we will describe presently), was taken to be the first outline
of the embryo. In the light of the gastraea theory it is hardly
necessary to dwell on the defects of this earlier view and the
erroneous conclusions drawn from it. In reality, the first
segmentation-cell, and even the stem-cell itself and all that issues
therefrom, belong to the embryo. As the large original yelk-mass in
the undivided egg of the bird only represents an inclosure in the
greatly enlarged ovum, so the later contents of its embryonic yelk-sac
(whether yet segmented or not) are only a part of the entoderm which
forms the primitive gut. This is clearly shown by the ova of the
amphibia and cyclostoma, which explain the transition from the
yelk-less ova of the amphioxus to the large yelk-filled ova of the
reptiles and birds.

It is precisely in the study of these difficult features that we see
the incalculable value of phylogenetic considerations in explaining
complex ontogenetic facts, and the need of separating cenogenetic
phenomena from palingenetic. This is particularly clear as regards the
comparative embryology of the vertebrates, because here the
phylogenetic unity of the stem has been already established by the
well-known facts of paleontology and comparative anatomy. If this
unity of the stem, on the basis of the amphioxus, were always borne in
mind, we should not have these errors constantly recurring.

In many cases the cenogenetic relation of the embryo to the food-yelk
has until now given rise to a quite wrong idea of the first and most
important embryonic processes in the higher vertebrates, and has
occasioned a number of false theories in connection with them. Until
thirty years ago the embryology of the higher vertebrates always
started from the position that the first structure of the embryo is a
flat, leaf-shaped disk; it was for this reason that the cell-layers
that compose this germinal disk (also called germinative area) are
called "germinal layers." This flat germinal disk, which is round at
first and then oval, and which is often described as the tread or
cicatricula in the laid hen's egg, is found at a certain part of the
surface of the large globular food-yelk. I am convinced that it is
nothing else than the discoid, flattened gastrula of the birds. At the
beginning of germination the flat embryonic disk curves outwards, and
separates on the inner side from the underlying large yelk-ball. In
this way the flat layers are converted into tubes, their edges folding
and joining together (Figure 1.105). As the embryo grows at the
expense of the food-yelk, the latter becomes smaller and smaller; it
is completely surrounded by the germinal layers. Later still, the
remainder of the food-yelk only forms a small round sac, the yelk-sac
or umbilical vesicle (Figure 1.105 nb). This is enclosed by the
visceral layer, is connected by a thin stalk, the yelk-duct, with the
central part of the gut-tube, and is finally, in most of the
vertebrates, entirely absorbed by this (H). The point at which this
takes place, and where the gut finally closes, is the visceral navel.
In the mammals, in which the remainder of the yelk-sac remains without
and atrophies, the yelk-duct at length penetrates the outer ventral
wall. At birth the umbilical cord proceeds from here, and the point of
closure remains throughout life in the skin as the navel.

As the older embryology of the higher vertebrates was mainly based on
the chick, and regarded the antithesis of embryo (or formative-yelk)
and food-yelk (or yelk-sac) as original, it had also to look upon the
flat leaf-shaped structure of the germinal disk as the primitive
embryonic form, and emphasise the fact that hollow grooves were formed
of these flat layers by folding, and closed tubes by the joining
together of their edges.

This idea, which dominated the whole treatment of the embryology of
the higher vertebrates until thirty years ago, was totally false. The
gastraea theory, which has its chief application here, teaches us that
it is the very reverse of the truth. The cup-shaped gastrula, in the
body-wall of which the two primary germinal layers appear from the
first as closed tubes, is the original embryonic form of all the
vertebrates, and all the multicellular invertebrates; and the flat
germinal disk with its superficially expanded germinal layers is a
later, secondary form, due to the cenogenetic formation of the large
food-yelk and the gradual spread of the germ-layers over its surface.
Hence the actual folding of the germinal layers and their conversion
into tubes is not an original and primary, but a much later and
tertiary, evolutionary process. In the phylogeny of the vertebrate
embryonic process we may distinguish the following three stages: -

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