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sponges, etc.) was a later evolution from the gastrula. It was also
shown that what is called delamination - the rise of the two primary
germinal layers by the folding of the surface of the blastoderm (for
instance, in the Geryonidae and other medusae) - was a secondary
formation, due to cenogenetic variations from the original
invagination of the blastula. The same may be said of what is called
"immigration," in which certain cells or groups of cells are detached
from the simple layer of the blastoderm, and travel into the interior
of the blastula; they attach themselves to the inner wall of the
blastula, and form a second internal epithelial layer - that is to say,
the entoderm. In these and many other controversies of modern
embryology the first requisite for clear and natural explanation is a
careful and discriminative distinction between palingenetic
(hereditary) and cenogenetic (adaptive) processes. If this is properly
attended to, we find evidence everywhere of the biogenetic law.


CHAPTER 1.10. THE COELOM THEORY.

The two "primary germinal layers" which the gastraea theory has shown
to be the first foundation in the construction of the body are found
in this simplest form throughout life only in animals of the lowest
grade - in the gastraeads, olynthus (the stem-form of the sponges),
hydra, and similar very simple animals. In all the other animals new
strata of cells are formed subsequently between these two primary
body-layers, and these are generally comprehended under the title of
the middle layer, or mesoderm. As a rule, the various products of this
middle layer afterwards constitute the great bulk of the animal frame,
while the original entoderm, or internal germinal layer, is restricted
to the clothing of the alimentary canal and its glandular appendages;
and, on the other hand, the ectoderm, or external germinal layer,
furnishes the outer clothing of the body, the skin and nervous system.

In some large groups of the lower animals, such as the sponges,
corals, and flat-worms, the middle germinal layer remains a single
connected mass, and most of the body is developed from it; these have
been called the three-layered metazoa, in opposition to the
two-layered animals described. Like the two-layered animals, they have
no body-cavity - that is to say, no cavity distinct from the alimentary
system. On the other hand, all the higher animals have this real
body-cavity (coeloma), and so are called coelomaria. In all these we
can distinguish four secondary germinal layers, which develop from the
two primary layers. To the same class belong all true vermalia
(excepting the platodes), and also the higher typical animal stems
that have been evolved from them - molluscs, echinoderms, articulates,
tunicates, and vertebrates.

(FIGURES 1.74 AND 1.75. Diagram of the four secondary germinal layers,
transverse section through the metazoic embryo: Figure 1.74 of an
annelid, Figure 1.75 of a vermalian. a primitive gut, dd ventral
glandular layer, df ventral fibre-layer, hm skin-fibre-layer, hs
skin-sense-layer, u beginning of the rudimentary kidneys, n beginning
of the nerve-plates.)

The body-cavity (coeloma) is therefore a new acquisition of the animal
body, much younger than the alimentary system, and of great
importance. I first pointed out this fundamental significance of the
coelom in my Monograph on the Sponges (1872), in the section which
draws a distinction between the body-cavity and the gut-cavity, and
which follows immediately on the germ-layer theory and the ancestral
tree of the animal kingdom (the first sketch of the gastraea theory).
Up to that time these two principal cavities of the animal body had
been confused, or very imperfectly distinguished; chiefly because
Leuckart, the founder of the coelenterata group (1848), has attributed
a body-cavity, but not a gut-cavity, to these lowest metazoa. In
reality, the truth is just the other way about.

The ventral cavity, the original organ of nutrition in the
multicellular animal-body, is the oldest and most important organ of
all the metazoa, and, together with the primitive mouth, is formed in
every case in the gastrula as the primitive gut; it is only at a much
later stage that the body-cavity, which is entirely wanting in the
coelenterata, is developed in some of the metazoa between the ventral
and the body wall. The two cavities are entirely different in content
and purport. The alimentary cavity (enteron) serves the purpose of
digestion; it contains water and food taken from without, as well as
the pulp (chymus) formed from this by digestion. On the other hand,
the body-cavity, quite distinct from the gut and closed externally,
has nothing to do with digestion; it encloses the gut itself and its
glandular appendages, and also contains the sexual products and a
certain amount of blood or lymph, a fluid that is transuded through
the ventral wall.

As soon as the body-cavity appears, the ventral wall is found to be
separated from the enclosing body-wall, but the two continue to be
directly connected at various points. We can also then always
distinguish a number of different layers of tissue in both walls - at
least two in each. These tissue-layers are formed originally from four
different simple cell-layers, which are the much-discussed four
secondary germinal layers. The outermost of these, the
skin-sense-layer (Figures 1.74 and 1.75 hs), and the innermost, the
gut-gland-layer (dd), remain at first simple epithelia or
covering-layers. The one covers the outer surface of the body, the
other the inner surface of the ventral wall; hence they are called
confining or limiting layers. Between them are the two middle-layers,
or mesoblasts, which enclose the body-cavity.

(FIGURE 1.76. Coelomula of sagitta (gastrula with a couple of
coelom-pouches. (From Kowalevsky.) bl.p primitive mouth, al primitive
gut, pv coelom-folds, m permanent mouth.)

The four secondary germinal layers are so distributed in the structure
of the body in all the coelomaria (or all metazoa that have a
body-cavity) that the outer two, joined fast together, constitute the
body-wall, and the inner two the ventral wall; the two walls are
separated by the cavity of the coelom. Each of the walls is made up of
a limiting layer and a middle layer. The two limiting layers chiefly
give rise to epithelia, or covering-tissues, and glands and nerves,
while the middle layers form the great bulk of the fibrous tissue,
muscles, and connective matter. Hence the latter have also been called
fibrous or muscular layers. The outer middle layer, which lies on the
inner side of the skin-sense-layer, is the skin fibre-layer; the inner
middle layer, which attaches from without to the ventral glandular
layer, is the ventral fibre layer. The former is usually called
briefly the parietal, and the latter the visceral layer or mesoderm.
Of the many different names that have been given to the four secondary
germinal layers, the following are those most in use to-day: -

1. Skin-sense-layer (outer limiting layer) and 2. Skin-fibre-layer
(outer middle layer).

I. Neural layer (neuroblast) and II. Parietal layer (myoblast). The
two secondary germinal layers of the body-wall: 1. Epithelial. 2.
Fibrous.

3. Gut-fibre-layer (inner middle layer) and 4. Gut-gland-layer (inner
limiting layer).

III. Visceral layer (gonoblast) and IV. Enteral layer (enteroblast).
The two secondary germinal layers of the gut-wall: 3. Fibrous. 4.
Epithelial.

The first scientist to recognise and clearly distinguish the four
secondary germinal layers was Baer. It is true that he was not quite
clear as to their origin and further significance, and made several
mistakes in detail in explaining them. But, on the whole, their great
importance did not escape him. However, in later years his view had to
be given up in consequence of more accurate observations. Remak then
propounded a three-layer theory, which was generally accepted. These
theories of cleavage, however, began to give way thirty years ago,
when Kowalevsky (1871) showed that in the case of Sagitta (a very
clear and typical subject of gastrulation) the two middle germinal
layers and the two limiting layers arise not by cleavage, but by
folding - by a secondary invagination of the primary inner germ-layer.
This invagination or folding proceeds from the primitive mouth, at the
two sides of which (right and left) a couple of pouches are formed. As
these coelom-pouches or coelom-sacs detach themselves from the
primitive gut, a double body-cavity is formed (Figures 1.74 to 1.76).

(FIGURE 1.77. Coelomula of sagitta, in section. (From Hertwig.) D
dorsal side, V ventral side, ik inner germinal layer, mv visceral
mesoblast, lh body-cavity, mp parietal mesoblast, ak outer germinal
layer.)

The same kind of coelom-formation as in sagitta was afterwards found
by Kowalevsky in brachiopods and other invertebrates, and in the
lowest vertebrate - the amphioxus. Further instances were discovered by
two English embryologists, to whom we owe very considerable advance in
ontogeny - E. Ray-Lankester and F. Balfour. On the strength of these
and other studies, as well as most extensive research of their own,
the brothers Oscar and Richard Hertwig constructed in 1881 the Coelom
Theory. In order to appreciate fully the great merit of this
illuminating and helpful theory, one must remember what a chaos of
contradictory views was then represented by the "problem of the
mesoderm," or the much-disputed "question of the origin of the middle
germinal layer." The coelom theory brought some light and order into
this infinite confusion by establishing the following points: 1. The
body-cavity originates in the great majority of animals (especially in
all the vertebrates) in the same way as in sagitta: a couple of
pouches or sacs are formed by folding inwards at the primitive mouth,
between the two primary germinal layers; as these pouches detach from
the primitive gut, a pair of coelom-sacs (right and left) are formed;
the coalescence of these produces a simple body-cavity. 2. When these
coelom-embryos develop, not as a pair of hollow pouches, but as solid
layers of cells (in the shape of a pair of mesodermal streaks) - as
happens in the higher vertebrates - we have a secondary (cenogenetic)
modification of the primary (palingenetic) structure; the two walls of
the pouches, inner and outer, have been pressed together by the
expansion of the large food-yelk. 3. Hence the mesoderm consists from
the first of TWO genetically distinct layers, which do not originate
by the cleavage of a primary simple middle layer (as Remak supposed).
4. These two middle layers have, in all vertebrates, and the great
majority of the invertebrates, the same radical significance for the
construction of the animal body; the inner middle layer, or the
visceral mesoderm, (gut-fibre layer), attaches itself to the original
entoderm, and forms the fibrous, muscular, and connective part of the
visceral wall; the outer middle layer, or the parietal mesoderm
(skin-fibre-layer), attaches itself to the original ectoderm and forms
the fibrous, muscular, and connective part of the body-wall. 5. It is
only at the point of origination, the primitive mouth and its
vicinity, that the four secondary germinal layers are directly
connected; from this point the two middle layers advance forward
separately between the two primary germinal layers, to which they
severally attach themselves. 6. The further separation or
differentiation of the four secondary germinal layers and their
division into the various tissues and organs take place especially in
the later fore-part or head of the embryo, and extend backwards from
there towards the primitive mouth.

(FIGURE 1.78. Section of a young sagitta. (From Hertwig.) dh visceral
cavity, ik and ak inner and outer limiting layers, mv and mp inner and
outer middle layers, lk body-cavity, dm and vm dorsal and visceral
mesentery.)

All animals in which the body-cavity demonstrably arises in this way
from the primitive gut (vertebrates, tunicates, echinoderms,
articulates, and a part of the vermalia) were comprised by the
Hertwigs under the title of enterocoela, and were contrasted with the
other groups of the pseudocoela (with false body-cavity) and the
coelenterata (with no body-cavity). However, this radical distinction
and the views as to classification which it occasioned have been shown
to be untenable. Further, the absolute differences in tissue-formation
which the Hertwigs set up between the enterocoela and pseudocoela
cannot be sustained in this connection. For these and other reasons
their coelom-theory has been much criticised and partly abandoned.
Nevertheless, it has rendered a great and lasting service in the
solution of the difficult problem of the mesoderm, and a material part
of it will certainly be retained. I consider it an especial merit of
the theory that it has established the identity of the development of
the two middle layers in all the vertebrates, and has traced them as
cenogenetic modifications back to the original palingenetic form of
development that we still find in the amphioxus. Carl Rabl comes to
the same conclusion in his able Theory of the Mesoderm, and so do
Ray-Lankester, Rauber, Kupffer, Ruckert, Selenka, Hatschek, and
others. There is a general agreement in these and many other recent
writers that all the different forms of coelom-construction, like
those of gastrulation, follow one and the same strict hereditary law
in the vast vertebrate stem; in spite of their apparent differences,
they are all only cenogenetic modifications of one palingenetic type,
and this original type has been preserved for us down to the present
day by the invaluable amphioxus.

(FIGURES 1.79 AND 1.80. Transverse section of amphioxus-larvae. (From
Hatschek.) Figure 1.79 at the commencement of coelom formation (still
without segments), Figure 1.80 at the stage with four primitive
segments. ak, ik, mk outer, inner, and middle germinal layer, hp horn
plate, mp medullary plate, ch chorda, asterisk and asterisk,
disposition of the coelom-pouches, lh body-cavity.)

But before we go into the regular coelomation of the amphioxus, we
will glance at that of the arrow-worm (Sagitta), a remarkable deep-sea
worm that is interesting in many ways for comparative anatomy and
ontogeny. On the one hand, the transparency of the body and the
embryo, and, on the other hand, the typical simplicity of its
embryonic development, make the sagitta a most instructive object in
connection with various problems. The class of the chaetogatha, which
is only represented by the cognate genera of Sagitta and Spadella, is
in another respect also a most remarkable branch of the extensive
vermalia stem. It was therefore very gratifying that Oscar Hertwig
(1880) fully explained the anatomy, classification, and evolution of
the chaetognatha in his careful monograph.

The spherical blastula that arises from the impregnated ovum of the
sagitta is converted by a folding at one pole into a typical
archigastrula, entirely similar to that of the Monoxenia which I
described (Chapter 1.8, Figure 1.29). This oval, uni-axial cup-larva
(circular in section) becomes bilateral (or tri-axial) by the growth
of a couple of coelom-pouches from the primitive gut (Figures 1.76 and
1.77). To the right and left a sac-shaped fold appears towards the top
pole (where the permanent mouth, m, afterwards arises). The two sacs
are at first separated by a couple of folds of the entoderm (Figure
1.76 pv), and are still connected with the primitive gut by wide
apertures; they also communicate for a short time with the dorsal side
(Figure 1.77 d). Soon, however, the coelom-pouches completely separate
from each other and from the primitive gut; at the same time they
enlarge so much that they close round the primitive gut (Figure 1.78).
But in the middle line of the dorsal and ventral sides the pouches
remain separated, their approaching walls joining here to form a thin
vertical partition, the mesentery (dm and vm). Thus Sagitta has
throughout life a double body-cavity (Figure 1.78 lk), and the gut is
fastened to the body-wall both above and below by a mesentery - below
by the ventral mesentery (vm), and above by the dorsal mesentery (dm).
The inner layer of the two coelom-pouches (mv) attaches itself to the
entoderm (ik), and forms with it the visceral wall. The outer layer
(mp) attaches itself to the ectoderm (ak), and forms with it the outer
body-wall. Thus we have in Sagitta a perfectly clear and simple
illustration of the original coelomation of the enterocoela. This
palingenetic fact is the more important, as the greater part of the
two body-cavities in Sagitta changes afterwards into sexual
glands - the fore or female part into a pair of ovaries, and the hind
or male part into a pair of testicles.

Coelomation takes place with equal clearness and transparency in the
case of the amphioxus, the lowest vertebrate, and its nearest
relatives, the invertebrate tunicates, the sea-squirts. However, in
these two stems, which we class together as Chordonia, this important
process is more complex, as two other processes are associated with
it - the development of the chorda from the entoderm and the separation
of the medullary plate or nervous centre from the ectoderm. Here again
the skulless amphioxus has preserved to our own time by tenacious
heredity the chief phenomena in their original form, while it has been
more or less modified by embryonic adaptation in all the other
vertebrates (with skulls). Hence we must once more thoroughly
understand the palingenetic embryonic features of the lancelet before
we go on to consider the cenogenetic forms of the craniota.

(FIGURES 1.81 AND 1.82. Transverse section of amphioxus embryo. Figure
1.81 at the stage with five somites, Figure 1.82 at the stage with
eleven somites. (From Hatschek.) ak outer germinal layer, mp medullary
plate, n nerve-tube, ik inner germinal layer, dh visceral cavity, lh
body-cavity, mk middle germinal layer (mk1 parietal, mk2 visceral), us
primitive segment, ch chorda.)

The coelomation of the amphioxus, which was first observed by
Kowalevsky in 1867, has been very carefully studied since by Hatschek
(1881). According to him, there are first formed on the bilateral
gastrula we have already considered (Figures 1.36 and 1.37) three
parallel longitudinal folds - one single ectodermal fold in the central
line of the dorsal surface, and a pair of entodermic folds at the two
sides of the former. The broad ectodermal fold that first appears in
the middle line of the flattened dorsal surface, and forms a shallow
longitudinal groove, is the beginning of the central nervous system,
the medullary tube. Thus the primary outer germinal layer divides into
two parts, the middle medullary plate (Figure 1.81 mp) and the
horny-plate (ak), the beginning of the outer skin or epidermis. As the
parallel borders of the concave medullary plate fold towards each
other and grow underneath the horny-plate, a cylindrical tube is
formed, the medullary tube (Figure 1.82 n); this quickly detaches
itself altogether from the horny-plate. At each side of the medullary
tube, between it and the alimentary tube (Figures 1.79 to 1.82 dh),
the two parallel longitudinal folds grow out of the dorsal wall of the
alimentary tube, and these form the two coelom-pouches (Figures 1.80
and 1.81 lh). This part of the entoderm, which thus represents the
first structure of the middle germinal layer, is shown darker than the
rest of the inner germinal layer in Figures 1.79 to 1.82. The edges of
the folds meet, and thus form closed tubes (Figure 1.81 in section).

During this interesting process the outline of a third very important
organ, the chorda or axial rod, is being formed between the two
coelom-pouches. This first foundation of the skeleton, a solid
cylindrical cartilaginous rod, is formed in the middle line of the
dorsal primitive gut-wall, from the entodermal cell-streak that
remains here between the two coelom-pouches (Figures 1.79 to 1.82 ch).
The chorda appears at first in the shape of a flat longitudinal fold
or a shallow groove (Figures 1.80 and 1.81); it does not become a
solid cylindrical cord until after separation from the primitive gut
(Figure 1.82). Hence we might say that the dorsal wall of the
primitive gut forms three parallel longitudinal folds at this
important period - one single fold and a pair of folds. The single
middle fold becomes the chorda, and lies immediately below the groove
of the ectoderm, which becomes the medullary tube; the pair of folds
to the right and left lie at the sides between the former and the
latter, and form the coelom-pouches. The part of the primitive gut
that remains after the cutting off of these three dorsal primitive
organs is the permanent gut; its entoderm is the gut-gland-layer or
enteric layer.

(FIGURES 1.83 AND 1.84. Chordula of the amphioxus. Figure 1.83 median
longitudinal section (seen from the left). Figure 1.84 transverse
section. (From Hatschek.) In Figure 1.83 the coelom-pouches are
omitted, in order to show the chordula more clearly. Figure 1.84 is
rather diagrammatic. h horny-plate, m medullary tube, n wall of same
(n apostrophe, dorsal, n double apostrophe, ventral), ch chorda, np
neuroporus, ne canalis neurentericus, d gut-cavity, r gut dorsal wall,
b gut ventral wall, z yelk-cells in the latter, u primitive mouth, o
mouth-pit, p promesoblasts (primitive or polar cells of the mesoderm),
w parietal layer, v visceral layer of the mesoderm, c coelom, f rest
of the segmentation-cavity.

FIGURES 1.85 AND 1.86. Chordula of the amphibia (the ringed adder).
(From Goette.) Figure 85 median longitudinal section (seen from the
left), Figure 1.86 transverse section (slightly diagrammatic).
Lettering as in Figures 1.83 and 1.84.

FIGURES 1.87 AND 1.88. Diagrammatic vertical section of
coelomula-embryos of vertebrates. (From Hertwig.) Figure 1.87,
vertical section THROUGH the primitive mouth, Figure 1.88, vertical
section BEFORE the primitive mouth. u primitive mouth, ud primitive
gut. d yelk, dk yelk-nuclei, dh gut-cavity, lh body-cavity, mp
medullary plate, ch chorda plate, ak and ik outer and inner germinal
layers, pb parietal and vb visceral mesoblast.

FIGURES 1.89 AND 1.90. Transverse section of coelomula embryos of
triton. (From Hertwig.) Figure 1.89, section THROUGH the primitive
mouth. Figure 1.90, section in front of the primitive mouth, u
primitive mouth. dh gut-cavity, dz yelk-cells, dp yelk-stopper, ak
outer and ik inner germinal layer, pb parietal and vb visceral middle
layer, m medullary plate, ch chorda.)

I give the name of chordula or chorda-larva to the embryonic stage of
the vertebrate organism which is represented by the amphioxus larva at
this period (Figures 1.83 and 1.84, in the third period of development
according to Hatschek). (Strabo and Plinius give the name of cordula
or cordyla to young fish larvae.) I ascribe the utmost phylogenetic
significance to it, as it is found in all the chorda-animals
(tunicates as well as vertebrates) in essentially the same form.
Although the accumulation of food-yelk greatly modifies the form of
the chordula in the higher vertebrates, it remains the same in its
main features throughout. In all cases the nerve-tube (m) lies on the
dorsal side of the bilateral, worm-like body, the gut-tube (d) on the
ventral side, the chorda (ch) between the two, on the long axis, and
the coelom pouches (c) at each side. In every case these primitive
organs develop in the same way from the germinal layers, and the same
organs always arise from them in the mature chorda-animal. Hence we
may conclude, according to the laws of the theory of descent, that all
these chordonia or chordata (tunicates and vertebrates) descend from
an ancient common ancestral form, which we may call Chordaea. We
should regard this long-extinct Chordaea, if it were still in
existence, as a special class of unarticulated worm (chordaria). It is
especially noteworthy that neither the dorsal nerve-tube nor the
ventral gut-tube, nor even the chorda that lies between them, shows
any trace of articulation or segmentation; even the two coelom-sacs
are not segmented at first (though in the amphioxus they quickly
divide into a series of parts by transverse folding). These
ontogenetic facts are of the greatest importance for the purpose of
learning those ancestral forms of the vertebrates which we have to
seek in the group of the unarticulated vermalia. The coelom-pouches
were originally sexual glands in these ancient chordonia.



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