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(FIGURE 1.91. A, B, C. Vertical section of the dorsal part of three
triton-embryos. (From Hertwig.) In Figure A the medullary swellings
(the parallel borders of the medullary plate) begin to rise; in Figure
B they grow towards each other; in Figure C they join and form the
medullary tube. mp medullary plate, mf medullary folds, n nerve-tube,
ch chorda, lh body-cavity, mk1 and mk2 parietal and visceral
mesoblasts, uv primitive-segment cavities, ak ectoderm, ik entoderm,
dz yelk-cells, dh gut-cavity.)

From the evolutionary point of view the coelom-pouches are, in any
case, older than the chorda; since they also develop in the same way
as in the chordonia in a number of invertebrates which have no chorda
(for instance, Sagitta, Figures 1.76 to 1.78). Moreover, in the
amphioxus the first outline of the chorda appears later than that of
the coelom-sacs. Hence we must, according to the biogenetic law,
postulate a special intermediate form between the gastrula and the
chordula, which we will call coelomula, an unarticulated, worm-like
body with primitive gut, primitive mouth, and a double body-cavity,
but no chorda. This embryonic form, the bilateral coelomula (Figure
1.81), may in turn be regarded as the ontogenetic reproduction
(maintained by heredity) of an ancient ancestral form of the
coelomaria, the Coelomaea (cf. Chapter 2.20).

In Sagitta and other worm-like animals the two coelom-pouches
(presumably gonads or sex-glands) are separated by a complete median
partition, the dorsal and ventral mesentery (Figure 1.78 dm and vm);
but in the vertebrates only the upper part of this vertical partition
is maintained, and forms the dorsal mesentery. This mesentery
afterwards takes the form of a thin membrane, which fastens the
visceral tube to the chorda (or the vertebral column). At the under
side of the visceral tube the coelom-sacs blend together, their inner
or median walls breaking down and disappearing. The body-cavity then
forms a single simple hollow, in which the gut is quite free, or only
attached to the dorsal wall by means of the mesentery.

The development of the body-cavity and the formation of the chordula
in the higher vertebrates is, like that of the gastrula, chiefly
modified by the pressure of the food-yelk on the embryonic structures,
which forces its hinder part into a discoid expansion. These
cenogenetic modifications seem to be so great that until twenty years
ago these important processes were totally misunderstood. It was
generally believed that the body-cavity in man and the higher
vertebrates was due to the division of a simple middle layer, and that
the latter arose by cleavage from one or both of the primary germinal
layers. The truth was brought to light at last by the comparative
embryological research of the Hertwigs. They showed in their Coelom
Theory (1881) that all vertebrates are true enterocoela, and that in
every case a pair of coelom-pouches are developed from the primitive
gut by folding. The cenogenetic chordula-forms of the craniotes must
therefore be derived from the palingenetic embryology of the amphioxus
in the same way as I had previously proved for their gastrula-forms.

The chief difference between the coelomation of the acrania
(amphioxus) and the other vertebrates (with skulls - craniotes) is that
the two coelom-folds of the primitive gut in the former are from the
first hollow vesicles, filled with fluid, but in the latter are empty
pouches, the layers of which (inner and outer) close with each other.
In common parlance we still call a pouch or pocket by that name,
whether it is full or empty. It is different in ontogeny; in some of
our embryological literature ordinary logic does not count for very
much. In many of the manuals and large treatises on this science it is
proved that vesicles, pouches, or sacs deserve that name only when
they are inflated and filled with a clear fluid. When they are not so
filled (for instance, when the primitive gut of the gastrula is filled
with yelk, or when the walls of the empty coelom-pouches are pressed
together), these vesicles must not be cavities any longer, but "solid
structures."

The accumulation of food-yelk in the ventral wall of the primitive gut
(Figures 1.85 and 1.86) is the simple cause that converts the
sac-shaped coelom-pouches of the acrania into the leaf-shaped
coelom-streaks of the craniotes. To convince ourselves of this we need
only compare, with Hertwig, the palingenetic coelomula of the
amphioxus (Figures 1.80 and 1.81) with the corresponding cenogenetic
form of the amphibia (Figures 1.89 to 1.90), and construct the simple
diagram that connects the two (Figures 1.87 and 1.88). If we imagine
the ventral half of the primitive gut-wall in the amphioxus embryo
(Figures 1.79 to 1.84) distended with food-yelk, the vesicular
coelom-pouches (lh) must be pressed together by this, and forced to
extend in the shape of a thin double plate between the gut-wall and
body-wall (Figures 1.86 and 1.87). This expansion follows a downward
and forward direction. They are not directly connected with these two
walls. The real unbroken connection between the two middle layers and
the primary germ-layers is found right at the back, in the region of
the primitive mouth (Figure 1.87 u). At this important spot we have
the source of embryonic development (blastocrene), or "zone of
growth," from which the coelomation (and also the gastrulation)
originally proceeds.

(FIGURE 1.92. Transverse section of the chordula-embryo of a bird
(from a hen's egg at the close of the first day of incubation). (From
Kolliker,) h horn-plate (ectoderm), m medullary plate, Rf dorsal folds
of same, Pv medullary furrow, ch chorda, uwp median (inner) part of
the middle layer (median wall of the coelom-pouches), sp lateral
(outer) part of same, or lateral plates, uwh structure of the
body-cavity, dd gut-gland-layer.)

Hertwig even succeeded in showing, in the coelomula-embryo of the
water salamander (Triton), between the first structures of the two
middle layers, the relic of the body-cavity, which is represented in
the diagrammatic transitional form (Figures 1.87 and 1.88). In
sections both through the primitive mouth itself (Figure 1.89) and in
front of it (Figure 1.90) the two middle layers (pb and vb) diverge
from each other, and disclose the two body-cavities as narrow clefts.
At the primitive-mouth itself (Figure 1.90 u) we can penetrate into
them from without. It is only here at the border of the primitive
mouth that we can show the direct transition of the two middle layers
into the two limiting layers or primary germinal layers.

The structure of the chorda also shows the same features in these
coelomula-embryos of the amphibia (Figure 1.91) as in the amphioxus
(Figures 1.79 to 1.82). It arises from the entodermic cell-streak,
which forms the middle dorsal-line of the primitive gut, and occupies
the space between the flat coelom-pouches (Figure 1.91 A). While the
nervous centre is formed here in the middle line of the back and
separated from the ectoderm as "medullary tube," there takes place at
the same time, directly underneath, the severance of the chorda from
the entoderm (Figure 1.91 A, B, C). Under the chorda is formed (out of
the ventral entodermic half of the gastrula) the permanent gut or
visceral cavity (enteron) (Figure 1.91 B, dh). This is done by the
coalescence, under the chorda in the median line, of the two dorsal
side-borders of the gut-gland-layer (ik), which were previously
separated by the chorda-plate (Figure 1.91 A, ch); these now alone
form the clothing of the visceral cavity (dh) (enteroderm, Figure 1.91
C). All these important modifications take place at first in the fore
or head-part of the embryo, and spread backwards from there; here at
the hinder end, the region of the primitive mouth, the important
border of the mouth (or properistoma) remains for a long time the
source of development or the zone of fresh construction, in the
further building-up of the organism. One has only to compare carefully
the illustrations given (Figures 1.85 to 1.91) to see that, as a fact,
the cenogenetic coelomation of the amphibia can be deduced directly
from the palingenetic form of the acrania (Figures 1.79 to 1.84).

(FIGURE 1.93. Transverse section of the vertebrate-embryo of a bird
(from a hen's egg on the second day of incubation). (From Kolliker.) h
horn-plate, mr medullary tube, ch chorda, uw primitive segments, uwh
primitive-segment cavity (median relic of the coelom), sp lateral
coelom-cleft, hpl skin-fibre-layer, df gut-fibre-layer, ung
primitive-kidney passage, ao primitive aorta, dd gut-gland-layer.)

The same principle holds good for the amniotes, the reptiles, birds,
and mammals, although in this case the processes of coelomation are
more modified and more difficult to identify on account of the
colossal accumulation of food-yelk and the corresponding notable
flattening of the germinal disk. However, as the whole group of the
amniotes has been developed at a comparatively late date from the
class of the amphibia, their coelomation must also be directly
traceable to that of the latter. This is really possible as a matter
of fact; even the older illustrations showed an essential identity of
features. Thus forty years ago Kolliker gave, in the first edition of
his Human Embryology (1861), some sections of the chicken-embryo, the
features of which could at once be reduced to those already described
and explained in the sense of Hertwig's coelom-theory. A section
through the embryo in the hatched hen's egg towards the close of the
first day of incubation shows in the middle of the dorsal surface a
broad ectodermic medullary groove (Figure 1.92 Rf), and underneath the
middle of the chorda (ch) and at each side of it a couple of broad
mesodermic layers (sp). These enclose a narrow space or cleft (uwh),
which is nothing else than the structure of the body-cavity. The two
layers that enclose it - the upper parietal layer (hpl) and the lower
visceral layer (df) - are pressed together from without, but clearly
distinguishable. This is even clearer a little later, when the
medullary furrow is closed into the nerve-tube (Figure 1.93 mr).

Special importance attaches to the fact that here again the four
secondary germinal layers are already sharply distinct, and easily
separated from each other. There is only one very restricted area in
which they are connected, and actually pass into each other; this is
the region of the primitive mouth, which is contracted in the amniotes
into a dorsal longitudinal cleft, the primitive groove. Its two
lateral lip-borders form the primitive streak, which has long been
recognised as the most important embryonic source and starting-point
of further processes. Sections through this primitive streak (Figures
1.94 and 1.95) show that the two primary germinal layers grow at an
early stage (in the discoid gastrula of the chick, a few hours after
incubation) into the primitive streak (x), and that the two middle
layers extend outward from this thickened axial plate (y) to the right
and left between the former. The plates of the coelom-layers, the
parietal skin-fibre-layer (m) and the visceral gut-fibre-layer (f),
are seen to be still pressed close together, and only diverge later to
form the body-cavity. Between the inner borders of the two flat
coelom-pouches lies the chorda (Figure 1.95 x), which here again
develops from the middle line of the dorsal wall of the primitive gut.

(FIGURES 1.94 AND 1.95. Transverse section of the primitive-streak
(primitive mouth) of the chick. Figure 1.94 a few hours after the
commencement of incubation, Figure 1.95 a little later. (From
Waldeyer.) h horn-plate, n nerve-plate, m skin-fibre-layer, f
gut-fibre-layer, d gut-gland-layer, y primitive streak or axial plate,
in which all four germinal layers meet, x structure of the chorda, u
region of the later primitive kidneys.)

Coelomation takes place in the vertebrates in just the same way as in
the birds and reptiles. This was to be expected, as the characteristic
gastrulation of the mammal has descended from that of the reptiles. In
both cases a discoid gastrula with primitive streak arises from the
segmented ovum, a two-layered germinal disk with long and small hinder
primitive mouth. Here again the two primary germinal layers are only
directly connected (Figure 1.96 pr) along the primitive streak (at the
folding-point of the blastula), and from this spot (the border of the
primitive mouth) the middle germinal layers (mk) grow out to right and
left between the preceding. In the fine illustration of the coelomula
of the rabbit which Van Beneden has given us (Figure 1.96) one can
clearly see that each of the four secondary germinal layers consists
of a single stratum of cells.

Finally, we must point out, as a fact of the utmost importance for our
anthropogeny and of great general interest, that the four-layered
coelomula of man has just the same construction as that of the rabbit
(Figure 1.96). A vertical section that Count Spee made through the
primitive mouth or streak of a very young human germinal disk (Figure
1.97) clearly shows that here again the four secondary germ-layers are
inseparably connected only at the primitive streak, and that here also
the two flattened coelom-pouches (mk) extend outwards to right and
left from the primitive mouth between the outer and inner germinal
layers. In this case, too, the middle germinal layer consists from the
first of two separate strata of cells, the parietal (mp) and visceral
(mv) mesoblasts.

(FIGURE 1.96. Transverse section of the primitive groove (or primitive
mouth) of a rabbit. (From Van Beneden.) pr primitive mouth, ul lips of
same (primitive lips), ak and ik outer and inner germinal layers, mk
middle germinal layer, mp parietal layer, mv visceral layer of the
mesoderm.

FIGURE 1.97. Transverse section of the primitive mouth (or groove) of
a human embryo (at the
coelomula stage). (From Count Spee.) pr primitive mouth, ul lips of
same (primitive folds), ak and ik outer and inner germinal layers, mk
middle layer, mp parietal layer, mv visceral layer of the mesoblasts.)

These concordant results of the best recent investigations (which have
been confirmed by the observations of a number of scientists I have
not enumerated) prove the unity of the vertebrate-stem in point of
coelomation, no less than of gastrulation. In both respects the
invaluable amphioxus - the sole survivor of the acrania - is found to be
the original model that has preserved for us in palingenetic form by a
tenacious heredity these most important embryonic processes. From this
primary model of construction we can cenogenetically deduce all the
embryonic forms of the other vertebrates, the craniota, by secondary
modifications. My thesis of the universal formation of the gastrula by
folding of the blastula has now been clearly proved for all the
vertebrates; so also has been Hertwig's thesis of the origin of the
middle germinal layers by the folding of a couple of coelom-pouches
which appear at the border of the primitive mouth. Just as the
gastraea-theory explains the origin and identity of the two primary
layers, so the coelom-theory explains those of the four secondary
layers. The point of origin is always the properistoma, the border of
the original primitive mouth of the gastrula, at which the two primary
layers pass directly into each other.

Moreover, the coelomula is important as the immediate source of the
chordula, the embryonic reproduction of the ancient, typical,
unarticulated, worm-like form, which has an axial chorda between the
dorsal nerve-tube and the ventral gut-tube. This instructive chordula
(Figures 1.83 to 1.86) provides a valuable support of our phylogeny;
it indicates the important moment in our stem-history at which the
stem of the chordonia (tunicates and vertebrates) parted for ever from
the divergent stems of the other metazoa (articulates, echinoderms,
and molluscs).

I may express here my opinion, in the form of a chordaea-theory, that
the characteristic chordula-larva of the chordonia has in reality this
great significance - it is the typical reproduction (preserved by
heredity) of the ancient common stem-form of all the vertebrates and
tunicates, the long-extinct Chordaea. We will return in Chapter 2.20
to these worm-like ancestors, which stand out as luminous points in
the obscure stem-history of the invertebrate ancestors of our race.


CHAPTER 1.11. THE VERTEBRATE CHARACTER OF MAN.

We have now secured a number of firm standing-places in the
labyrinthian course of our individual development by our study of the
important embryonic forms which we have called the cytula, morula,
blastula, gastrula, coelomula, and chordula. But we have still in
front of us the difficult task of deriving the complicated frame of
the human body, with all its different parts, organs, members, etc.,
from the simple form of the chordula. We have previously considered
the origin of this four-layered embryonic form from the two-layered
gastrula. The two primary germinal layers, which form the entire body
of the gastrula, and the two middle layers of the coelomula that
develop between them, are the four simple cell-strata, or epithelia,
which alone go to the formation of the complex body of man and the
higher animals. It is so difficult to understand this construction
that we will first seek a companion who may help us out of many
difficulties.

This helpful associate is the science of comparative anatomy. Its task
is, by comparing the fully-developed bodily forms in the various
groups of animals, to learn the general laws of organisation according
to which the body is constructed; at the same time, it has to
determine the affinities of the various groups by critical
appreciation of the degrees of difference between them. Formerly, this
work was conceived in a teleological sense, and it was sought to find
traces of the plan of the Creator in the actual purposive organisation
of animals. But comparative anatomy has gone much deeper since the
establishment of the theory of descent; its philosophic aim now is to
explain the variety of organic forms by adaptation, and their
similarity by heredity. At the same time, it has to recognise in the
shades of difference in form the degree of blood-relationship, and
make an effort to construct the ancestral tree of the animal world. In
this way, comparative anatomy enters into the closest relations with
comparative embryology on the one hand, and with the science of
classification on the other.

Now, when we ask what position man occupies among the other organisms
according to the latest teaching of comparative anatomy and
classification, and how man's place in the zoological system is
determined by comparison of the mature bodily forms, we get a very
definite and significant reply; and this reply gives us extremely
important conclusions that enable us to understand the embryonic
development and its evolutionary purport. Since Cuvier and Baer, since
the immense progress that was effected in the early decades of the
nineteenth century by these two great zoologists, the opinion has
generally prevailed that the whole animal kingdom may be distributed
in a small number of great divisions or types. They are called types
because a certain typical or characteristic structure is constantly
preserved within each of these large sections. Since we applied the
theory of descent to this doctrine of types, we have learned that this
common type is an outcome of heredity; all the animals of one type are
blood-relatives, or members of one stem, and can be traced to a common
ancestral form. Cuvier and Baer set up four of these types: the
vertebrates, articulates, molluscs, and radiates. The first three of
these are still retained, and may be conceived as natural phylogenetic
unities, as stems or phyla in the sense of the theory of descent. It
is quite otherwise with the fourth type - the radiata. These animals,
little known as yet at the beginning of the nineteenth century, were
made to form a sort of lumber-room, into which were cast all the lower
animals that did not belong to the other three types. As we obtained a
closer acquaintance with them in the course of the last sixty years,
it was found that we must distinguish among them from four to eight
different types. In this way the total number of animal stems or phyla
has been raised to eight or twelve (cf. Chapter 2.20).

These twelve stems of the animal kingdom are, however, by no means
co-ordinate and independent types, but have definite relations, partly
of subordination, to each other, and a very different phylogenetic
meaning. Hence they must not be arranged simply in a row one after the
other, as was generally done until thirty years ago, and is still done
in some manuals. We must distribute them in three subordinate
principal groups of very different value, and arrange the various
stems phylogenetically on the principles which I laid down in my
Monograph on the Sponges, and developed in the Study of the Gastraea
Theory. We have first to distinguish the unicellular animals
(protozoa) from the multicellular tissue-forming (metazoa). Only the
latter exhibit the important processes of segmentation and
gastrulation; and they alone have a primitive gut, and form germinal
layers and tissues.

The metazoa, the tissue-animals or gut-animals, then sub-divide into
two main sections, according as a body-cavity is or is not developed
between the primary germinal layers. We may call these the coelenteria
and coelomaria, the former are often also called zoophytes or
coelenterata, and the latter bilaterals. This division is the more
important as the coelenteria (without coelom) have no blood and
blood-vessels, nor an anus. The coelomaria (with body-cavity) have
generally an anus, and blood and blood-vessels. There are four stems
belonging to the coelenteria: the gastraeads ("primitive-gut
animals"), sponges, cnidaria, and platodes. Of the coelomaria we can
distinguish six stems: the vermalia at the bottom represent the common
stem-group (derived from the platodes) of these, the other five
typical stems of the coelomaria - the molluscs, echinoderms,
articulates, tunicates, and vertebrates - being evolved from them.

Man is, in his whole structure, a true vertebrate, and develops from
an impregnated ovum in just the same characteristic way as the other
vertebrates. There can no longer be the slightest doubt about this
fundamental fact, nor of the fact that all the vertebrates form a
natural phylogenetic unity, a single stem. The whole of the members of
this stem, from the amphioxus and the cyclostoma to the apes and man,
have the same characteristic disposition, connection, and development
of the central organs, and arise in the same way from the common
embryonic form of the chordula. Without going into the difficult
question of the origin of this stem, we must emphasise the fact that
the vertebrate stem has no direct affinity whatever to five of the
other ten stems; these five isolated phyla are the sponges, cnidaria,
molluscs, articulates, and echinoderms. On the other hand, there are
important and, to an extent, close phylogenetic relations to the other
five stems - the protozoa (through the amoebae), the gastraeads
(through the blastula and gastrula), the platodes and vermalia
(through the coelomula), and the tunicates (through the chordula).

How we are to explain these phylogenetic relations in the present
state of our knowledge, and what place is assigned to the vertebrates
in the animal ancestral tree, will be considered later (Chapter 2.20).
For the present our task is to make plainer the vertebrate character
of man, and especially to point out the chief peculiarities of
organisation by which the vertebrate stem is profoundly separated from
the other eleven stems of the animal kingdom. Only after these
comparative-anatomical considerations shall we be in a position to
attack the difficult question of our embryology. The development of
even the simplest and lowest vertebrate from the simple chordula
(Figures 1.83 to 1.86) is so complicated and difficult to follow that
it is necessary to understand the organic features of the fully-formed
vertebrate in order to grasp the course of its embryonic evolution.
But it is equally necessary to confine our attention, in this general
anatomic description of the vertebrate-body, to the essential facts,
and pass by all the unessential. Hence, in giving now an ideal
anatomic description of the chief features of the vertebrate and its


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