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A. Cell just prior to division, showing nucleus (/j) with its chromatic reticulum and
the attraction-sphere and centrosome (c).

B. First phase; the attraction-sphere has divided into two, which have moved
180° apart; the reticulum has been resolved into five chromosomes (black), each
of which has split lengthwise.

C. Second phase; fully developed karyokinetic figure (amphiastcr), with spindle
and asters; the chromosome-halves are moving apart.

D. Final phase; the cell-body is dividing, the spindle disappearing, tlie dauglitor-
nuclei about to be formed.

In its resting state the nucleus contains a network or reticulum of
chromatin (Fig. 37, A), As the cell prepares for division a small body (c)



84 THE BIOLOGY OF AN ANIMAL.

makes its appearance near the nucleus, known as i\iQ attraction- sj:)here or
archoplasm-mass, and in its interior there is often a smaller body, the
centrosome. The first step in cell-division is the fission of the archoplasm-
mass into two, each containing a centrosome (derived by fission of the
original centrosome); after this the two masses move apart to opposite
poles of the nucleus (Fig. 37, B). The reticulum now becomes, in most
cases, resolved into a thread coiled into a skein (not shown in the figure),
which finally breaks up into a number of bodies known as cJiromosomes.
Their form (granular, rodlike, loop-shaped) and number (two, eight, twelve,
sixteen, etc., or often much higher numbers) appear to be constant for
each species of plant and animal. The second principal step is the longi-
tudinal splitting of each chromosome into halves (Fig. 37, B) and the
disappearance of the nuclear membrane.

In the third place starlike rays {aster) appear in the protoplasm around
the archoplasm-masses, a spindle-shaped structure appears between them
(Fig. 37, C), and the double chromosomes arrange themselves around the
equator of the spindle. The structure thus formed is known as the amjjJii-
aster or karyokinetic figure.

Fourthly, the two halves of each chromosome move apart towards the
respective poles of the spindle and the entire cell-body then divides in a
plane passing through the equator of the spindle. Each group of daughter-
chromosomes now gives rise to a reticulum, which becomes surrounded with
a membrane and forms the nucleus of the daughter-cell. The spindle dis-
appears, and in some cases the archoplasm-mass, with its star-rays (aster),
seems to disappear also. In other cases, however, the archoplasm-mass and
centrosome persist and may be found in the resting cell (e.g., in leucocytes
and connective-tissue cells), lying near the nucleus in the cytoplasm.

It appears from the foregoing description that each daughter-cell re-
ceives exactly half the substance of the mother-nucleus (chromatin), mother-
archoplasm, and mother-centrosome. In many cases the cytoplasm also
divides equally, in other cases unequally.

It has been proved in a considerable number of cases that in the fer-
tilization of the ovum each germ-cell contributes the same number of chro-
mosomes, and the wonderful fact has been established with high probability
that the paternal and maternal chromatic substances are equally distributed
to the two cells found at the first segmentation of the ovum. It is further
probable that this equal distribution continues in all the later divisions ;
and if this is true, every cell in the whole adult body contains material
directly derived from both parents, and hence may inherit from both.

Gastrulation. Germ-layers. Diiferentiation. Origin of the
Body. Almost from the first the cells arrange themselves so as
to surround a central cavity known as the segmentation-cavity.
This cavity increases in size in later stages, so that the embryc*
finally appears as a hollow sphere surrounded by a wall consist-



DEVELOPMENT OF THE ORGANS. 85

ing of a single layer of cells. This stage is known as the hhutula
(or hlastosphere) (J., B^ Fig. 35).

The formation of the germ-layers is one of the most im-
portant and signillcant processes in the whole course of devehjp-
ment. Germ-layers like those of Lmnhrlrus^ and callt-d hy
the same names, are found in the embryos of all higher ani-
mals; and it will hereafter appear that this fact has a })n (found
meaning.

Development of the Organs. (Organogeny.) The embry(j gradu-
ally increases in size and at the same time elongates. As it
lengthens, the blastopore (in this case the mouth) remains at one
end, wliicli is therefore to be regarded as anterior, and the
elongation is backwards. The cells of all three germ-layers
continually increase in number by division, new matter and
energy being supplied from the food, which is swalhjwed by the
embryo in such quantities as to swell up the body like a bladdrr.
The archenteron enlarges until it comes into contact with the
ectoblast and the segmentation-cavity is obliterated.

The two primary mesoblastic cells are carried backwards,
and always remain at the extreme posterior end (???, Fig. .ST)).
The mesoblast is in the form of tw^o bands Ivino: on either side
of the archenteron, and extending forwards from the primary
mesoblastic cells.

This is clearly seen in a cross-section of the embryo, as in
Fig. 36, B^ C. The mesoblastic bands are at lirst solid, but
after a time a series of paired cavities appears in them, con-
tinually increasing in number by the formation of new cavities
near the hinder end of the bands as they increase in length. \
cross-section passing through one pair of these cavities is shown
at B^ Fig. 35. As the bands lengthen they also extend up-
wards and downwards (6^, Fig. 35), until finally they meet al)ove
and below the archenteron. The cavities at the sanio time
continue to increase in size, and finally meet above and below
the archenteron, which thus becomes surrounded by the ImxIv-
cavity or coelom {D). The cavities are separated by the double
23artition-walls of mesoblast. These partitions are the dissepi-
ments, and the cavities themselves constitute the c(elom. The
outer mesoblastic Avail of each cavitv is known as tha so?ii at ic
layer (.§.??!.); it unites with the ectoblast to constitute the body-



86



THE BIOLOGY OF AN ANIMAL.



wall (somatojdeure). The inner wall, or splanchnic layer
{spl.?n), unites with the entoblast to constitute the wall of the
alimentary canal (splanchnopleure). An ingrowth of ectoblast
{stomod(BU7n) takes place into the blastopore to form the pharynx,
and a similar ingrowth at the opposite extremity {proctodceum)
unites with the blind end of the archenteron to form the anus
and terminal part of the intestine.

As to its origin, therefore, the alimentary canal consists of
three portions, viz. : (1) the archenteron, consisting of the




B.VJU

Fig. 38.— Diagram of a cross-section of Lumhricus, showing tlie relation of the
various organs, etc., to the germ-layers. Ectoblastic structures shaded with fine
parallel lines, entoblastic with coarser parallel lines, mesoblastic with cross-lines;
a?.c, alimentary canals ; ch, chloragogue layer ; co?, coelom ; c.w, circular muscles
of body- wall; c.ma, circular muscles of alimentary wall; ep, lining epithelium of
alimentary canal; d.r, dorsal vessel; /ly, hypodermis or skin; l.m, longitudinal
muscles of body-wall; l.m.a, longitudinal muscles of alimentary wall; ?i, central
part of nerve-cord ; np, nephridium ; ns, sheath of nerve-cord ; p.e, peritoneal
epithelium ; r, reproductive organs ; s.i.v, sub-intestinal vessel.

original entoblast; (2) the stomodgeum or pharyngeal region,
lined by ectoblast ; and (3) the proctodseum or hindmost part,
also lined by ectoblast. These three parts are called the fore-
gut (stomodaeum), mid-gut or meUsenteron (archenteron), and
hind-gut (proctodseum), and it is a remarkable fact that these
same parts can be distinguished in all higher animals, not ex-
cepting man.

The body now becomes jointed by the appearance of trans-
verse folds opposite the dissepiments, and tlie metamerism of the
body becomes evident on the exterior. The young worm has
thus reached a stage (^, Fig. 36) where its resemblance to the



FATE OF THE GERM- LAYERS. 87

adult is obvious. It has an elongated, jointed l)ody, traversed
by the alimentary canal, which opens in front by the mouth and
behind by the anus. The metamerism is expressed exterimlly
by tlie jointed appearance, internally l)y the presence of paired
cavities (coelom) separated by dissepiments. Both the body-wall
and the alimentary wall consist of two layers: the former of
ectoblast without and somatic mesoblast within; the latter of
splanchnic mesoblast without (i.e., towards the bodv-cavitv)
and either entoblast or ectoblast within, according as we con-
sider the mid-gut on the one hand, or the fore- and hind-gut on
tlie other. This is shown in Fig. 38, which represents a cross-
section of the embryo through the mid-gut. If this be clearly
borne in mind the development of all the other organs is easy to
understand, since they are formed as thickenings, outgrowths,
etc., of the parts already existing. For instance, the blood-
vessels make their appearance everywhere throughout the meso-
blast, and the reproductive organs are at first mere thickenings
on the somatic layer of the mesoblast, afterwards separating
more or less from it so as to lie in the cavity of the coelom.
The nervous system is produced by thickenings and ingrowths
from the ectoblast. The origin of the different parts is shown
in the following scheme : —

THE GERM-LAYERS AND THEIR DERIVATIVES.



Ectoblast.



Mesoblast.



Entoblast.



Outer skin (Hypodermis and Cuticle).

Nerves and Ganglia.

Lining membrane of pharynx (fore-gut).

Lining membrane of anus and hinder part of intestine (hind-gut).



Muscles.

Blood-vessels.

Reproductive organs, ,

Outer layers of alimentary canal.



Lining membrane of greater part of the alimentary canal (mid-gut).



The above statements* as to the origin of the various organs
acquire great interest in view of the fact that they are essen-

* Tlie nepbridia have been omitted since their precise origin is in dispute.
It is certain that tbe outer portion of the tube (inu.scular part) is an ingrowth
from the ectoblast. The latest researches seem to show that the entire ne-
phridium has the same origin, though some autbors describe the inner portion
as arising from mesoblast.



88 THE BIOLOGY OF AN ANIMAL.

tiallj true of all animals above the earthworm, as well as of
many below it — of all, in a word, in which the three germ-
layers are developed, i.e., all those above the Cmleiiterata^ or
polyps, jelly-fishes, hydroids, sponges, etc. In man, as in the
earthworm and all intermediate forms, the ectoblast gives rise
to the outer skin (epidermis), the brain and nerves, fore- and
hind-gut ; the entoblast gives rise to the lining membrane of the
stomach, intestines, and other parts pertaining to the mid-gut;
while the somatic and splanchnic layers of the mesoblast give
rise to the muscles, kidneys, reproductive organs, heart, blood-
vessels, etc. It is now generally held that the germ-layers
throughout the animal kingdom (with the partial exception of
the Coelenterata already mentioned) are essentially identical in
origin and fate. This view is known as the Germ-layer Theory.
It is one of the most significant and important generalizations
which the study of Embryology has brought to light, since it
recognizes a structural identity of the most fundamental kind
among all the higher animals.

Sooner or later the young earthworm bursts through the
walls of the capsule and makes its entry into the world. When
first hatched it is about an inch long and has no clitellum.

It is a curious fact that in certain species of Lumbricus the young
worms are almost always hatched as twins, two individuals being derived
from a single egg by a process which is described by Kleinenberg in the
Quarterly Journal of Microscopical Science, Vol, XIX., 1879. It often
happens that the twins are permanently united by a band of tissue, as in
the case of the well-known Siamese twins.

We have now traced roughly the evolution of a complex
many-celled animal from a suuple one-celled germ. It is im-
portant to notice at this point a few general principles which are
true of higher animals in general.

1. The embryological history is a true process of develop-
ment, — not a mere growth or unfolding of a pre-existing rudi-
ment as the leaf is unfolded from the bud. ISTeither the ovum
nor any of the earlier stages of development bears the slightest
resemblance to an earthworm. The embryo undergoes a trans-
formation of structure as well as an increase of size.

2. It is a progress from a one-celled to a many-celled con-
dition.



SUMMARY OF DEVELOPMENT. 89

3. It is a progress from relative simplicity to relative eom-
plexity. The ovmn is certainly vastly more coinplcx than it
appears to the eye, but no one can doubt that the full-grown
worm is more complex still.

4. It is a progress from a slightly differentiated to a liii^dily
differentiated condition. The life of the ovum is that of a
single cell. The blastula is composed of a number of nearly
similar cells, which in the gastrula become differentiated into
two distinct tissues. In later stages the cells become differenti-
ated into many different tissues, which in turn build up different
organs performing unlike functions.

5. Lastly, the development forms a cycle, beginning with
the germ-cell, and after many complicated changes resulting in
the production of new germ- cells, which repeat the process and
give rise to a new generation. All other cells in the body must
sooner or later die. The germ-cells alone persist as the starting-
point to which the cycle of life continually returns (cf. p. 78).
Their protoplasm, the ' ' gerin-])lasin^ ' ' is the bond of continuity
that links together the successive generations.



CHAPTEK YI.

THE BIOLOGY OF AN ANIMAL {Conti7\ued).

The Earthworm.
Microscopic Stkuctuke or Histology.

"We have followed the develoj)nient of the one-celled germ
through a stage, the hlastula^ in which it consists of a mass of
nearly similar cells out of which the various tissues of the adult
eventually arise. The first step in this direction is the differen-
tiation of the germ-layers or three primitive tissues (p. 84).
As the embryo develops, the cells of these three tissues become
differentiated in structure to fit them for difl'erent duties in the
physiological division of labor. And when this process of dif-
ferentiation is accomplished and the adult state is reached we
find six well-marked varieties of tissue, as follows : —

Principal Tissues of Liimhricus,

I. EpitheliaL Layer of cells covering free surfaces.

(a) Pavement Epithelium. Cells thin and flat, arranged like the

stones of a pavement.
(6) Columnar Epithelium. Cells elongated, standing side by side,

palisade-like,
(c) Ciliated Epithelium. Columnar or cuboid, and bearing cilia.

II. Muscular. Cells contractile and elongated to form fibres. Often
arranged in parallel masses or bundles.

III. Nervous. Cells pear-shaped or irregular, with large nuclei ; hav-
ing processes prolonged into slender cords or fibres, bundles of which con-
stitute the nerves.

IV. Germinal. Including the germ-cells. At first in the form of epi-
thelial cells covering the ccelomic surface, but afterwards differentiated
into ova and spermatozoa.

V. Blood. Isolated cells Of corpuscles floating in a fluid intercellular
substance, \h^ plasma.

VI. Connective Tissue. Cells of different shapes, often branched but
sometimes rounded, separated from one another by more or less lifeless
(intercellular) substance in the form of threads or homogeneous material.

90



ARRANGEMENT OF TISSUES.



91



These six kinds of tissue constitute tlie main Imlk of the
earthworm, as of liigher animals generally ; hut there are \n ad-
dition other tissues which will be treated of hereafter.

Arrangement of the Tissues. The sini})lest and most diivct
mode of discovering the arrangement of the tissues is hv the nji-
croscopical study of thin transverse or longitudinal sections. A



am



pe




Ftg. 39.— Transverse section of the body behind the clitellum. a.(\ caN-ity of the ali-
mentary canal ; c, cuticle ; em, ccelom ; cm, circular muscles ; c.r, cinular vessel ;
ci.u, dorsal vessel; luj, hypodermis; l.m, longitudinal muscles; ».r, ventral nerve-
chain; p.e, peritoneal epithelium; s, seta; s.y, setigerous gland; nA.v, sub-intes-
tinal vessel ; s.w, muscle connecting the two groups of setae on the same aide ; f j/,
typhlosole.

transverse section taken throuo^h the rei^ion of the stomach-
intestine is represented in Fig. 39. Its composition is as
follows : —
A. Body -WALL.

This consists of five layers, viz. (beginning with the out-
side), — •*

1. Cuticle ((?). A very thin transparent nieml)rane. not
composed of cells and perforated by fine pctres. It is a product
or secretion of the —



92 THE BIOLOGY OF AN ANIMAL.

2. Hypodermis {]nj) (epidermis or sMn). A layer of colum-
nar epithelium, composed of several kinds of elongated cells, set
vertically to tlie surface of the body. Some of these, known as
gland-cells.^ have the power of producing within their substance
a glairy fluid (mucus), which exudes to the exterior through the
pores in the cuticle. Others (sensory cells) give off from their
inner ends nerve-iibres which may be traced inwards to the
ganglia (Fig. 43).

The Clitellum is produced by an enormous thickening of the hypoder
mis, caused especially by a great development of the gland-cells. Three
forms of these may be distinguished, which probably produce different
secretions. The tissue is permeated by numerous minute blood-vessels
which ramify between the cells.

3. CirGidar Ihiscles {cm). A layer of parallel muscle-
libres running around the body. On the upper side they are
intermingled with connective-tissue cells containing a granular
brownish substance (pigment) which gives to the dorsal aspect
its darker tint.

4. Longitudinal Muscles (l-.m). A layer of muscle-fibres
running lengthwise of the body. They are arranged in compli-
cated bundles, which in cross-sections have a feathery appear-
ance. In longitudinal sections they appear as a simple layer, and
resemble the circular fibres as seen in the cross-section.

The circular muscles are arranged in somewhat similar bun-
dles, as may be seen in longitudinal sections.

5. Coelomic or Peritoneal Epitlieliuin {p.e.). A very thin
layer of flattened cells next the coelomic cavdty.

The hypodermis, and therefore also the cuticle to which it
gives rise, is derived from the ectoblast. The othei layers (3,
4, 5) arise from the somatic layer of the mesoblast.
B. Alimentary Canal.

The wall of this tube appears in cross-section as a ring sur-
rounded by the coelom. The typhlosole {ty) is seen to be a deep
infolding of its upper portion. In the middle region the wall is
composed of five layers as follows, starting from the alimentary
cavity (Fig. 40) : —

1. Lining Epithelium {ep). A layer of closely packed, nar-
row ciliated columnar cells with oval nuclei.

2. Vascular Layer {v. I). Numerous minute blood-vessels.



HISTOLOGY OF THE ALIMENTARY CANAL.



93



3. Cii'cular Muscles (c.m). A thin layer uf iimscle-fibres
running around the gut.

4. Longitudinal Muscles {J.m). A thin layer of muscle-
fibres running along the gut.

5. Chloragogue Layer (eh). Composed of large polyhedral
or rounded cells containing yellowish-green granules. The cells
fill the hollow of the typhlosole, and cover the surface of the
dorsal and lateral blood-vessels. This layer represents the
splanchnic part of the peritoneal epithelium.

The same general arrangemeut exists in all parts of the alimentary
canal, but is sometimes greatly modified. For instance, the gizzard and
pharynx are lined by a tough, thick cuticle, and the muscular layers are
enormously developed. In a part of the gizzard the chloragogue-layer is
nearly or quite absent and the typlilosole disappears. A fuller description
of these modifications will be found in Brooks's Handbook of Invertebrate
Zoology., and a complete account in Claparede, Zeitschrift far wiasen-
schaftliche Zoologie, Vol. XIX., 1869.

The lining epithelium is derived from the entoblast. The
remaining layers arise by differentiation of the splanchnic layer
of inesoblast.




■■■■ r.'. r..- '.•■.•,■.■■ .*•:-: 'Ji -



■^•iii'->i^^'-^iy'*iij - \^=iaiif ij.



r. :â– â– â–  : â– .



C. //i.




Fig. 40.— Highly magnified cross-section through tlio wall of the alimentary cnnnl.
c7i, chloragogue layer; cm, circular muscles; c.p, lining epithelium; Lm, longi-
tudinal muscles ; v.l, vascular layer.

Blood-vessels appear in the section as rounded or irregular
cavities bounded by thin walls. They consist of a delicate lining
epithelium covered by a thin layer of muscle-fibres. In the
walls of the stomach-intestine the vessels are often completely
invested by chloragogue-cells, which radiate from them with



94



THE BIOLOGY OF AN ANIMAL.



great regularity (Fig. 39). The liner brandies have no muscu-
lar layer, consisting of the epithelium alone.

Lissepimeiits. These often appear in cross or longitudinal
sections. They consist chiefly of muscle-flbres irregularly dis-
posed, intermingled with connective-tissue cells and fibres, and
covered on both sides with the peritoneal epithelium.

Nervous System. A cross-section of a ganglion (Fig. 41)
shows it to be comjDosed of two distinct parts, viz. , (1) the gan-




FiG. 41.— Highly magnified cross-section of a ventral ganglion, g.f, giant-fibres ; Z.n,
lateral nerve ; n.c, nerve-cells ; s, muscular sheath of the ganglion ; s.v, sub-neu-
ral vessel ; s.n.i', supra-neural vessel.

glion proper on the inside, and (2) a sheath which envelops it.
The sheath (^, Fig. 41) consists of two layers, viz. : —

1. Peritoneal Ejnthelium. On the outside.

2. Muscular Layer ^ or sheath, a thick layer of irregularly
arranged muscle-fibres intermingled with connective tissue. Im-
bedded in it are the sub-neural blood-vessel on the lower side
and the supra-neural blood-vessels on each side above. In the
middle line are three rounded spaces (^, y. Fig. 41), which are
the cross- sections of three hollow fibres running along the entire
length of the ventral nerve-chain. They are called ' ' giant-
fibres, ' ' and possibly serve to support the soft parts of the nerve-
cord.

The Ganglion proper is distinctly bilobed, and consists of
two portions, viz. : —

1. Nerve-cells {n.c), I^umerous pear-shaped nerve-cells near
the surface, with their narrow ends turned towards the centre,
into which each sends a single branch or nerve-fibre. They are
confined chiefly to the ventral and lateral parts of the ganglion.



HISTOLOGY OF THE N Eli VOL'S SYSTEM.



95



2. Flhwiis Portion. This occupies the central part. It
consists of a close and complicated network of nerve-libres inter-
mingled with connective tissue. Some of these li])res communi-
cate with branches of the nerve-cells, as stated al«n'e; otiiers
run out into the lateral nerves, while still others run alunf*- the
connnissui'es to connect with libres from other irantrlia




Fig. 42.— Two of the ventral ganglia (I, II) of Lumhrkns with the lateral nerves,
showing some of the motor nerve-cells and fibres (black), a sends fibres for-
wards and backwards within the nerve-cord; /», a fibre into one of tlie double
nerves on its own side ; c and d, fibres that cross to the nerves of the opposite side.
(After Retzius.)

According to the latest researches (of Lenhossek and Retzius) most if
not all of the nerve-cells of the ventral cord are motor in function. Near
the centre of each ganglion (Fig. 42, e) in a sinirle large multipolar cell of
doubtful nature. All the other cells are either bipolar or unipolar, in the
latter case sending out a single branch which soon divides into two. In
every case one of the branches breaks up into fine sub-divisions within the
cord. The other branch in most cases passes out of the cord through one
of the lateral nerves to the muscles or other peripheral organs, either



96



THE BIOLOGY OF AN ANIMAL,



crossing within the cord to the opposite side of the body or making exit
on its own side. Some of the cells, however, are purely " commissural,"
i.e., neither branch leaves the cord.

The sensory fibres entering from the periphery terminate freely (not in
â– nerve-cells), breaking up into numerous fine branches on the same side of
the cord. (Fig. 43.)