An introduction to general biology online

regularly recurring cycles, for every individual life has its limit.
In vouth the constructive processes preponderate over the de-
structive and the organism grows. Tlie normal adult attains a
state of apparent physiological balance in which the processes of
waste and repair are approximately equal. Sooner or later,
however, this balance is disturbed. Even though the organism
escapes every injury or special disease the constructive process
falls behind the destructive, old age ensues, and the individual
dies from sheer inability to live. Why the vital machine should
thus w^ear out is a mystery, but that it has a definite cause and
meaning is indicated by the familiar fact that the sjDan of natural
life varies with the species ; man lives longer than the dog, the
elephant longer than man.

It is a w^onderful fact that living things have the power to
detach from themselves portions or fragments of their own
bodies endowed with fresh powers of growth and develoj^ment
and capable of running througli tlie same cycle as the parent.
There is therefore an unbroken material (protoplasmic) continuity
from one generation to another, that forms tlie physical basis of
inheritance, and upon which the integrity of the s]3ecies depends.
As far as known, li\dng things never arise save through this
process; in other words every mass of existing protoplasm is
the last link in an unbroken chain that extends backward in the
past to the first origin of life.

The detached portions of the parent that are to give rise to
offspring are sometimes masses of cells, as in the separation of
branches or buds among plants, but more commonly they are single

72



REPRODUCTION. 73

cells, known as germ-cells, like the eggs of animals and tlie
spores of ferns and mosses. Only the germ-cells (which may
conveniently be distinguished from those forming the rest of the
body, or the somatic cells), escape death, and that only under
certain conditions.

All forms of reproduction fall under one or the other of two
heads, viz., Agamogenesis {asexual reproduction) or Gamogenesis
{sexual reproduction). In the former case the detached portion
(which may be either a single cell or a group of cells) has the
power to develop into a new individual without the influence of
other living matter. In the latter, the detached portion, in tliis
case always a single cell (ovum, ousphere, etc.), is acted u})(>n
by a second portion of living matter, likewise a single cell, wliich
in most cases has been detached from the body of another in-
dividual. The germ is called the y<2m«Z^ germ- cell ; the cell act-
ing upon it the "male gerin-cell / and in the sexual process the
two fuse together {fertilization, imjjr eg nation) to form a single
new cell endowed with the power of developing into a new in-
dividual. In some organisms (e.g., the yeast-plant and bacteria)
only agamogenesis has been observed ; in others (e. g. , vertebrates)
only gamogenesis ; in others still both processes take place as in
many higher plants.

The earthworm is not known to multiply by any natural
process of agamogenesis. It possesses in a high degree, however,
the closely related power of regeneration / for if a worm be cut
transversely into two pieces, the anterior piece will usually make
good or rege'iierate the missing portion, while the j^osterior })i(.'ce
may regenerate the anterior region. Thus the worm can to a
certain limited extent be artificially propagated, like a plant, by
cuttings, a process closely related to true agamc jgenesis. -^ Its
usual and normal mode of reproduction is by gamogenesis, that
is, by the formation of male germ-cells {sj)er)?iatozoa) and female
germ-cells (ova). In higher animals the two kinds uf germ-
cells are produced by different individuals of opi)osite sex. The
earth w^orm on the contrary is hermaphrodite or lisexual; e\ery

* Many worms nearly related to Lumhricns—e.g:., the genus Duo, and other
Naids — spontaneously divide themselves into two parts each of which becomes
a perfect animal. This process is true agamogenesis, though obviously closely-
related to regeneration.



74



THE BIOLOGY OF AN ANIMAL.



individual is hoth male and female^ producing both eggs and
sjDermatozoa. The ova arise in special organs, the ovaries^ the
spermatozoa in spermaries or testes.

The ripe ovum (Fig. 33, B) is a relatively large spherical
cell, agreeing closely with the e^g of the star-lish (Fig. 12), but
having a thinner and more delicate membrane. It is still cus-
tomary to apply to ova the old terminology, calling the cell-
substance ly'dellus^ the membrane vitelline membrane, the nucleus
germinal vesicle, and the nucleolus germinal spot.

The ripe spermatozoon (Fig. 33, 6^) is an extremely minute
elongated cell or filament thickening towards one end to form
the head (71), which contains the nucleus of the cell enveloped by a
thin layer of protoplasm. This is followed by a short ' ' middle
piece ' ' (m) to which is attached a long vibratory fiagellum or tail
(t). The tail is virtually a long cilium (p. 31), which by vigorous
lashing di'ives the whole cell along head-foremost, very much as
a tadpole is driven by its tail.

Since the ovaries and spermaries give rise to the germ-cells,

they are called the essential organs of
reproduction. Besides these, ZtimhricuSj
like most animals, has accessory organs of
reproduction which act as reservoirs or
carriers of the germs, assist in securing
cross-fertilization, and minister to the
wants of the young worms.

Essential Reproductive Organs. The
ovaries are two in number and lie one on
eitlier side in the 13th somite attached to
the hinder face of the anterior dissepiment
{ov, Fig. 29). They are about 2"^"^ in
length, distinctly pear-shaped, and at-
tached by the broader end (Fig. 32). The
narrow^ extremity contains a single row of
ova and is called the eqq-strinq ies). In

Fig. 32.— The ovary, much , . , . ^^ ^ \ J

enlarged, /j, the basal part; this the ova are ripe or nearly so; behind

a body of the ovary con- ^|^ g|^^^g ^^ -^^^^ ^l^^g^ ^^^^ ^^^
taming immature ova; es, ^ "^

egg-string; oi\ ripe ovum immature, till these are lost in a mass of

rea > to a off. nearly undifferentiated cells {primitive

ova), constituting the great bulk of the ovary. Each of these,



ov



ea








'V V; ■' y*' •■'*:•• ^'>i1








'^0:iy0Mm



REPRODUCTIVE ORGANS. 75

however, is surrounded witli still smaller cells constitutiii*'- its
nutrient envelope or follicle. As the ova mature the follicles
still persist, and they may be detected even in the ci^j^sti-iiiir.
When fully ripe the ovum bursts the follicle and is shed from
the end of the egg-string into the body-cavity. It is ultimately
taken into the oviduct and carried to the exterior.

The development of the ovary shows it to be morphologically
a thickening of the peritoneal epithelium. The eggs therefore
are originally epithelial cells.

The sjjennaries or testes {t,t, Fig. 29) are four in mnnl)eraiid
in outward appearance are somewhat similar to the ovaries.
They are small ilattened bodies with somewhat irregular or lohed
borders, lying one on either side the nerve-cord in a position
corresponding with that of the ovaries, but in somites 10 and 11.
Like the ovary the testis is a solid mass of cells, which are shed
into the body-cavity and are finally carried to the exterior.
The sperm-cells leave the testis, however, at a very early period
and undergo the later stages of maturation within the cavities of
the seminal vesicles described below.

Accessory Reproductive Organs. The most important of the
accessory organs are the genital ducts, by which the germ-cells
are passed out to the exterior. Both the female ducts {oviducts)
and the male {sperm-ducts) are tubular organs opening at one
end to the outside, through tlie body-wall, and at the other end
into the coelom by means of a ciliated funnel somewhat similar
to a nephridial funnel, but nnicli larger. By means of these
ciliated funnels the germ-cells after their discharge t'runi the
ovary or testis are taken up and passed to the exterior.

The oviducts {od, Fig. 29, Fig. 23) are two short trumpet-
shaped tubes lying innnediately posterior to the ovaries and p;uss-
ing through the dissepiment between the 13th and 14th somites.
Tlie inner end opens freely into the cavity of the 13th somite,
by means of a wide and much-folded ciliated funnel, fi-om the
centre of which a slender tube passes backward througli the
dissepiment, turns rather sharply towards the outer side and,
passing through the body-wall, opens to the outside on the 14th
somite (see p. 43). Innnediately behind the dissepiment the
oviduct gives off at its dorsal and outer side a small ])oueh,
richly supplied with blood-vessels. In this, the receptacul um



76 THE BIOLOGY OF AN ANIMAL.

ovorum^ the ova taken up by the funnel are temporarily stored
before passing out to the exterior.

It is probable tliat the eggs never float freely in the coelom,
but drop out of the ovary at maturity directly into the mouth of
the funnel, Tliey pass thence into the receptaculum^ where they
may remain for a considerable period.

The sperm-ducts {vasa deferentia) {sd, Fig. 29) are very
long slender tubes, open like the oviducts at both ends. The
outer opening is a conspicuous slit surrounded by fleshy lips
(Fig. 21), on the ventral side of the loth somite. From this
point the duct runs straight forwards to the 12tli somite, where
it branches like a Y, the two branches passing forwards to ter-
minate, one in the 11th somite, the other in the 10th. I^s^ear its
end each branch is twisted into a peculiar knot and Anally ter-
minates in an innnense ciliated funnel (the so-called "ciliated
rosette"), the borders of wliicli are folded in so complicated a
manner tliat they form a labyrinthine body, the true nature of
wliieh can only be made out in microscopic sections.

The two pairs of sperm-funnels (Fig. 29) lie in the 10th
and 11th somites, innnediately posterior to the respective testes,
i.e., they have essentially the same relation to the testes as that
of the oviduct-funnels to the ovaries.

The testes and sperm-funnels can be readily made out only in young
specimens. In mature worms they are completely enveloped by the semi-
nal vesicles described below.

Seminal vesicles. Tliese, the most conspicuous part of the
reproductive apparatus, are A^oluminous pouches in which the
spenn-cells undergo their later development, after leaving the
testis. They are large white bodies lying in somites 9 to 12 and
usually overlapping the oesophagus in that region. In all cases
there are three pairs of lateral seminal vesicles, viz., an anterior
pair in somite 9, a middle pair in somite 11, and a posterior pair
in somite 12. In unmature specimens tliese six are entirely
separate, and allow tlie testes to be easily seen. In mature
worms (as shown in Fig. 29) the posterior pair of lateral
vesicles grow together in the middle line, thus forming a 2)os-
terior median vesicle lying below the alimentary canal in the
11th somite. In like manner an anterior' median vesicle is
formed in the 10th somite by the union of the two anterior pairs



EGG-LA YING. 77

of lateral vesicles. The two median vesicles thus formed eiivehji)
the testes and sperm-funnels uf their respective somites and hide
them from view.

The sperm-cells leave the testis at a very early period and float freely
in the cavities of the seminal vesicles, where many stages of their develop-
ment may easily be observed. They are developed in balls known as
spermatospheves, each of which consists of a central solid mass of proto-
plasm surrounded by a single layer of sperm-cells. When mature the
spermatozoa separate from the central mass and are drawn into the fun-
nels of the sperm-ducts. The manner in which this action is controlled is
not understood.

The seminal rece;ptacles are accessory organs of reproduction
in the shape of small rounded sacs or potiches, open to the out-
side only, at about the level of the upper row of set*. They
lie between the 9th and 10th, and lOtli and lltli somites (*./•,
Figs. 24 and 29), where their openings may be sought for (Fig.
21). Their function is explained under the head of co])ulati()n.

Accessory glands. Besides all the structures so far described
there are many glands which play a part in tlie reproductive
functions. The setigerous glands from about the 7th to about
the 19th somite (sometimes fewer, sometimes none at all) are
often greatly enlarged, and form the glandular j^rominences men-
tioned at p. 46. They seem to be used as organs of adhesion
during copulation. The clitellum is tilled with gland-cells which
probably serve in j)art to secrete a nourishing tluid for the young
worms, and in part to provide a tough protecting membrane to
cover them.

Copulation. Egg-laying^. Inasmuch as each individual earth-
worm produces both ova and spermatozoa, it might be supposed
that copulation, or the sexual union of two different individuals,
would not be necessary. This, however, is not the case. The
ova of one individual are invariably fertilized by the s])ermatozoa
of another individual after a process of copulation and exchange
of spermatozoa, as follows : During the night-time, and usually
in the spring, the worms leave their burrows and ])air. placing
themselves so that their heads point in opposite directions and
holding firmly together by the enlarged setigerous glands and the
thickened lower lateral margins of the clitellum. During this
act the seminal receptacles of each w<^rm are filled witli s]>erma-
tozoa from the sperm-ducts of the other, after which the wnrms



\7I%



78 THE BIOLOGY OF AN ANIMAL.

separate. [The spermatozoa thus received are simply stored up
and do not perform their function until the time of egg-laying.]
When the worm is ready to lay its eggs the glands of the
clitelJmn become very active, pouring out a thick glairy fluid
which soon hardens into a tough membrane and forms a girdle
around the body. Besides this a large quantity of a thick jelly -
like nutrient fluid is poured out and retained in the s]3ace be-
tween the gu-dle and the body of the worm. The girdle is
thereupon gradually worked forward toward the head of the
worm by contractions of the body. As it passes the 1-ith somite
a number of ova are received from the oviducts, and between
the 9th and 11th somites a quantity of spermatozoa are added
from the seminal receptacles where they have been stored since
the time of copulation, when they were obtained from another
worm. The girdle is next stripped forwards over the anterior

end and is finally thrown
completely off. As it
passes off its oj^en ends
immediately contract
tightly together, and the
girdle becomes a closed
capsule (Fig. 33) contain-
ing both ova and sj^erma-
tozoa floatinoj in a nutri-

Fio. 33.—^, egg-capsule enlarged 5 diameters . n • j 'n m

(a few eggs, 01% enlarged to the same scale are tive liuiQ Or milk. Ihe

shown near by on the right) ;B, an ovum very niembraiie SOOU aSSUmCS a

much enlarged ; C, a spematozoon, enormously

magnified ; 71, head ; m, middle piece ; f, tail. light yellowisll Or Lrown

color, becomes hard and tough, and serves to protect the de-
veloping embryos. The capsules may be found in Iviay or June
in earth under logs or stones, or especially in heaps of manure.
Within the capsules the fertilization and development of the ova
take place.

Fertilization and Embryological Development. The s]3erma-
tozoa swim actively about in the nutrient fluid of the capsule^
approach an ovum, and attach themselves to its surface by their
heads. Several of the spermatozoa then enter the vitellus (cf .
p. 80), but it has been proved that only one of these is con-
cerned in fertilization, the others dying and becoming absorbed
by the ovum.




B



FERTILIZATION OF THE EGO.



79



It is probable that the tail plays no part in the actual fertili-
zation, but is merely a locomotor apparatus fur tlie head (nucleus)
and middle-piece.

Within the ovum the head of the spermatozoun })ersi8ts as
the s^erin-nucleus (or male pro-nucleus)^ while the protoplaMn in
its neighborhood assumes a peculiar and characteristic ra<liate
arrangement like a star, probably through the influence of the
middle-piece.

After the entrance of the spermatozoon the ^g'g segments ulf







Fig. 34.— Fertilization of the ovum. A., entrance of the spermatozoon (in the sea-
urchin, after Fol). B, the sea-urchin egg after entrance of the sperniatozoim ;
within and to the left is the egg-nucleus; above is the sperm-nucleus, with a cen-
trosomenear it (modified from Hertwig). r, diagram of the ovum after extrusion
of the polar cells (p.c), and union of the two pro-nuclei to form the segmenta-
tion-nucleus. The smaller and darker portion of the latter is derived from the
sperm-nucleus. Two asters or archoplasm-spheres are shown near the nuiU-us.
These arise by the division of a single aster derived from the middle-piece of the
spermatozoon. D, two-celled stage of the earthworm, after the first fission of
the ovum. (After Vejdovsky.)

at one side two small cells, one after the other, known as the
^olar cells oy jyolar hodles. These take no part in tlie formation
of the embryo, and their formation prol)ab]y serves, in some way
not yet wholly clear, to prej)are tlie egg for the lju>t act of
fertilization. After the formation of the polar cells the egg-
nucleus (now often called t\\.Q female pi'o-nucle us) and tlie sperm-
nucleus approach one another and iinally become intimately



80 THE BIOLOOT OF AN ANIMAL.

associated to form tlie segmentation- or cleavage-nucleus ; by this

act fertilization is completed.

The process of fertilization appears to be essentially the same among
all higher animals, and in a broader sense to be identical with the sexual
process among all higher and many lower plants (compare the fern, p. 139),
but its precise nature is still in dispute. It is certain that one essential
part of it is the union of two nuclei derived from the two respective parents.
This has led to the view, now held by many investigators, that inheritance
has its seat in the nucleus, and that chromatin (p. 23), is its physical
basis. Later researches have shown that another element known as the
archoplasm- or attraction-sphere is concerned in fertilization, and this is
apparently always derived from the middle-piece. It is not yet certain
whether the archoplasm is to be regarded as a nuclear or a cytoplasmic
structure, and it is equally doubtful whether it plays an essential or merely
a subsidiary role in fertilization and inheritance (cf. p. 84).

Cleavage of the Fertilized Ovum. Soon after fertilization the
ovum begins the remarkable process of segmentation which
has already been briefly sketched on j). 25. The segmen-
tation-nucleus divides into two parts, and this is followed by
a division of the vitellus, each half of the original nucleus becom-
ing the nucleus of one of the halves of the vitellus ; that is, the
original cell divides into two smaller but similar cells (see Fig.
3-1:). These divide in turn into four, and these into eight, and
so on, but yet remain closely connected in one mass. In the
case of the earthworm, the cells do not multiply in regular
geometrical progression, but show many irregularities ; and more-
over they become unequal in size at an early period.

The blastula (pp. 25, 85,) shows scarcely any differentiation
of parts, though the cells of one hemisphere are somewhat smaller
than the others. From this time forwards the whole course of
development is a process of differentiation, both of the cells and of
the organs into wliicli they soon arrange themselves. One of
the first steps in this process is a flattening of the embryo at the
lower pole — i.e., the half consisting of larger cells (Fig. 35, D),
The large cells are then folded into the segmentation-cavity so
as to form a pouch opening to the exterior ; at the same time
the embryo becomes somewhat elongated (Fig. 35, ^, F).

This process is known as gastrulation^ and at its completion
the embryo is called the gastrula. The infolded pouch (called
the archenteron) is the future alimentary canal ; its opening (now
known as the hlastopoi^e) will become the mouth ; and tlie layei



THE GERM-LAYERS.



81



of small cells over the outside will form the skin or (\\\X{'y laver
of the body-wall.

The embryo very soon begins to swallow, through the bla-sto-
pore, the milkhke tiuid in which it tloats, and to di<»"est it with-
in the cavity of the archenteron.

It is obvious that the embryo already shows a distinct (hlfcr-






B





Fig. 35.— Diagrams of the early stages of development in the earthworm. .1, accu-
rate drawing of the blastula, surrounded by the vitelline membrane i after Vej-
dovsky) ; B, blastula in optical section showing the large segmentation-t-avity
{s.c.\ and the parent-cell of the mesoblast (m.); C, later blastula, showing forma-
tion of mesoblast-cells ; D, flattening of the blastula preparatory to invagination ;

E, the gastrula in side view; as the infolding takes place the two niesoblast-
bands are left at the sides of the body, in the position shown by the dotted lines;

F, section of E along the line s-s, showing the mesoblast-bands and pole-cells.

entiation of parts which perform unlike functions. In fact we
may regard the gastrula as composed of two tissues still nearly
similar in structure though unlike in function. One of these
consists of the layer of cells which forms the outer covering;
this tissue is known as the ectohlast {ec\ Fig. 35). The second
tissue is the layer of cells forming the wall of the archenteron ;
it is called the entoblast {en). The ectoblast and eiitol>last to-
gether are known as \h(i primary germ-layers.

Meanwhile changes are taking place which result in the for-
mation of a third germ-layer lying in the segmentation-cavity
between the ectoblast and entoblast and therefore called the
mesoblast {m, Figs. 35, 30). In some animals the mesobltu^t
does not arise until after the completion of gastrulation. In



S2



THE BIOLOGY OF AN ANIMAL.



Lumhricus, however, it goes on during gastrulation and begins
€ven before gastrulation. Even in the blastula stage two large
cells may be distinguished which afterwards give rise ^ to the
niesoblast and are hence called the primary mesoblastic cells.
They soon bud forth smaller cells hito the segmentation- cavity,
and as tlie blastula flattens they themselves sink below the sur-
face. At this period, therefore, the niesoblast forms two bands
of cells {mesoUast-hands) each terminating behind in the large
mother-cell or pole-cell. Throughout the later stages the pole-
cells continue to bud forth smaller cells which are added to the
hinder ends of the mesoblast-bands (Figs. 35, 36).




Ftg. 36.— Diagrams of later embryonic stages. A^ late stage in longitudinal section,
showing the appearance of the cavities of the somites ; JB, the same in cross-sec-
tion ; t;, diagram of a young worm in longitudinal section after the formation of
the stomodaeum, proctodaeum, and anus ; C, the same in cross-section, showing
the beginning of the nervous system; D, cross-section of later stage with the
nervous system completely established, a/, alimentary canal ; 07% archenteron ;
an, anus; c<£^ coelom; cc., ectoblast; en, entoblast; ?ni, primary mesoblastic cells;
m", mesoblast; mh., mouth ; 7J, nervous system; s, cavity of somite; s.w, somatic
layer of the mesoblast, which with the ectoblast forms the somatopleure ; syA.m,
splanchnic layer of the mesoblast, which with the entoblast forms the splanch-
nopleure.

After each division the pole-cells increase in size, so that up
to a late stage in development they may be distinguished from



CELL-DIVISION. EAR 70KINESIS.



83



the cells to which thej give rise. The twu masses of iiiesohlastic
cells gradually increase in size and finally fill the segmentation-
cavity.

The internal phenomena of cell-division are of great complexity and
can here be given only in outline. The ordinary type of cell-division, as
shown in the segmentation of the ovum and in the multiplication of most
tissue-cells, involves a complicated series of changes in the nucleus known
as karyokinesis or mitosis. These changes, which appear to be of essen-
tially the same character in nearly all kinds of cells, and both in jjlants and
in animals, are illustrated by the following diagrams :







C D

Fig. 37.— Diagrams of indirect cell-division or karyokinesis.