REPRODUCTION AND DEVELOPMENT.
We have now to turn to a fresh aspect of animal life, that of
reproduction; and it will be well to connect this process as closely as
possible with the process of life in general, of which it is a direct
outcome.
It will be remembered that, in the last chapter, it was shown that the
essential feature in the process of life is the absorption by living
protoplasm of oxygen on the one hand and nutritive matter on the other
hand, and the kneading of these together, in subtle metabolism, into
unstable compounds, which we likened to explosives. This is the first,
or constructive, stage of the life-process. Thereupon follows the
second, or disruptive, stage. The unstable compounds break down into
more stable products,--they explode, according to our analogy; and
accompanying the explosions are manifestations of motor activity--of
heat, sometimes of light and electrical phenomena. But in the economy of
nature the products of explosion are often utilized, and in the division
of labour among cells the explosions of some of them are directed
specially to the production of substances which shall be of permanent or
temporary use--for digestion, as in the products of the salivary,
gastric, and intestinal glands; for support, as in bone, cartilage, and
skeletal tissue generally; or as a store of nutriment, in fat or yolk.
The constructive products of protoplasmic activity seem for the most
part to be lodged in the spaces between the network of formative
protoplasm. The disruptive products--those of them, that is to say,
which are of temporary or permanent value to the organism--accumulate
either within the cell, sometimes at one pole, sometimes at the centre,
as in the case of the yolk of eggs, or around the cell, as in the case
of cartilage or bone.
Apart from and either preceding or accompanying these phenomena, is the
growth or increase of the formative protoplasm itself; concerning which
the point to be here observed is that it is not indefinite, but limited.
This was first clearly enunciated by Herbert Spencer, and may be called
Spencer's law. In simplest expression it may thus be stated: _Volume
tends to outrun surface._ Take a cube measuring one inch in the side;
its volume is one cubic inch, its surface six square inches. Eight such
cubes will have a surface of (6 × 8) forty-eight square inches. But let
these eight be built into a larger cube, two inches in the side, and it
will be found that the surface exposed is now only twenty-four square
inches. While the volume has been increased eight times, the surface has
been increased only four times. With increase of size, volume tends to
outrun surface. But in the organic cell the nutritive material and
oxygen are absorbed at the surface, while the explosive changes occur
throughout its mass. Increase of size, therefore, cannot be carried
beyond certain limits, for the relatively diminished surface is unable
to supply the relatively augmented mass with material for elaboration
into unstable compounds. Hence the cell divides to afford the same mass
increased surface. This process of cell-division is called fission, and
in some cases cleavage.
We will now proceed to pass in review the phenomena of reproduction and
development in animals.
[Illustration: Fig. 7.--Protozoa.
A, vorticella extended. B, the same contracted. C, D, monads. E,
am[oe]ba. F, _Param[oe]cium_. G, _Gregarina_. c.f., contractile fibre;
c.v., contractile vesicle; d., disc; end., endoplast; f.v.,
food-vacuole; fl., flagellum; gu., gubernaculum; n., nucleus; p.a.,
potential anus; ps., (in A) peristome, (in E) pseudopodium; vs.,
vestibule.]
Attention has already been drawn to the difference between those lowly
organisms, each of which is composed of a single cell--the protozoa, as
they are termed--and those higher organisms, called metazoa, in which
there are many cells with varied functions. Confining our attention at
first to the former group of unicellular animals, we find considerable
diversities of form and habit, from the relatively large, sluggish,
parasitic _Gregarina_, to the active slipper-animalcule, or
_Param[oe]cium_, or the beautiful, stalked bell-animalcule, or
_Vorticella_; and from the small, slow-moving am[oe]ba to the minute,
intensely active monad. In many cases reproduction is by simple fission,
as in the am[oe]ba, where the nucleus first undergoes division; and then
the whole organism splits into two parts, each with its own nucleus. In
other cases, also numerous, the organism passes into a quiescent state,
and becomes surrounded with a more or less toughened cyst. The nucleus
then disappears, and the contents of the cyst break up into a number of
small bodies or spores. Eventually the cyst bursts, and the spores swarm
forth. In the case of some active protozoa the minute creatures that
swarm forth are more or less like the parent; but in the more sluggish
kinds the minute forms are more active than the parent. Thus in the case
of the gregarina, the minute spore-products are like small am[oe]bæ;
while in other instances the embryos, if so we may call them, have a
whip-like cilium like the monads.
Very frequently, however, there is, in the protozoa, a further process,
which would seem to be intimately associated with fission or the
formation of spores, as the case may be. This is known as conjugation.
Among monads, for example, two individuals may meet together, conjugate,
and completely fuse the one into the other. A triangular cyst results.
After a while, the cyst bursts, and an apparently homogeneous fluid
escapes. The highest powers of the microscope fail to disclose in it any
germ of life; and there, at first sight, would seem to be an end of the
matter. But wait and watch; and there will appear in the field of the
microscope, suddenly and as if by magic, countless minute points, which
prolonged watching shows to be growing. And when they have further
grown, each distinct point is seen to be a monad.
In the slipper-animalcule, conjugation is temporary. But during the
temporary fusion of the two individuals important changes are said to
occur. In these infusorians there is, beside the nucleus, a smaller
body, the paranucleus. This, in the case of conjugating param[oe]cia,
appears to divide into two portions, of which one is mutually exchanged.
Thus when two slipper-animalcules are in conjugation, the paranucleus of
each breaks into two parts, _a_ and _b_, of which _a_ is retained and
_b_ handed over in exchange. The old _a_ and the new _b_ then unite, and
each param[oe]cium goes on its separate way. M. Maupas, who has lately
reinvestigated this matter, considers, as the result of his observations
on another infusorian (_Stylonichia_), that without conjugation these
organisms become exhausted, and multiplication by fission comes to a
standstill. If this be so, conjugation is, in these organisms, necessary
for the continuance of the race. But Richard Hertwig has recently shown
that this is, at any rate, not universally true.
In the bell-animalcule, fission takes place in such a manner as to
divide the bell into two equal portions. Thus there are two bells to one
stalk. But the fate of the two is not the same. One remains attached to
the stalk, and expands into a complete vorticella. The other remains
pear-shaped, and develops round the posterior region of the body a
girdle of powerful vibratile cilia, by the lashing of which the
animalcule tears itself away from the parent stem, and swims off through
the water. After a short active existence, it settles down in a
convenient spot, adhering by its posterior extremity. The hinder girdle
of cilia is lost or absorbed, a stalk is rapidly developed, and the
organism expands into a perfect vorticella.
In some cases, however, the fission is of a different character, with
different results. It may be very unequal, so that a minute,
free-swimming animalcule is disengaged; or minute animalcules may result
by repetition of division. In either case the minute form conjugates
with an ordinary vorticella, its smaller mass being completely merged in
the larger volume of its mate.
There are, of course, many variations in detail in the modes of
protozoan reproduction; but we may say that, omitting such details,
reproduction is either by simple fission or by spore-formation; and that
these processes are in some cases associated with, and perhaps dependent
on, the temporary or permanent union of two individuals in conjugation.
It is essential to notice that the results of fission or of
spore-formation separate, each going on its own way. Hence such
development as we find in the protozoa results from differentiations
within the limits of the single cell. Thus the bell-animalcule has a
well-defined and constant form; a definite arrangement of cilia round
the rim and in the vestibule by which food finds entrance to the body.
The outer layer of the body forms a transparent cuticle, beneath which
is a so-called "myophan" layer, continuous with a contractile thread in
the stalk. Within the substance of the body is a pulsating cavity, or
contractile vesicle, and a nucleus. Such is the nature of the
differentiation which may go on within the protozoan cell.
When we pass to the metazoa, we find that the method of differentiation
is different. These organisms are composed of many cells; and instead of
the parts of the cell differentiating in several directions, the several
cells differentiate each in its own special direction. This is known as
the physiological division of labour. The cells merge their
individuality in the general good of the organism. Each, so to speak,
cultivates some special protoplasmic activity, and neglects everything
else in the attainment of this end. The adult metazoan, therefore,
consists of a number of cells which have diverged in several, sometimes
many, directions.
In some of the lower metazoans, reproduction may be effected by fission.
Thus the fresh-water hydra is said to divide into two parts, each of
which grows up into a perfect hydra. It is very doubtful, however,
whether this takes place normally in natural life. But there is no doubt
that if a hydra be artificially divided into a number of special pieces,
each will grow up into a perfect organism, so long as each piece has
fair samples of the different cells which constitute the body-wall.
Sponges and sea-anemones may also be divided and subdivided, each part
having the power of reproducing the parts that are thus cut away. When a
worm is cut in half by the gardener's spade, the head end grows a new
tail; and it is even stated that a worm not only survived the removal of
the first five rings, including the brain, mouth, and pharynx, but
within fifty-eight days had completely regenerated these parts.
Higher up in the scale of metazoan life, animals have the power of
regenerating lost limbs. The lobster that has lost a claw reproduces a
new one in its stead. A snail will reproduce an amputated "horn," or
tentacle, many times in succession, reproducing in each case the eye,
with its lens and retina. Even a lizard will regenerate a lost tail or a
portion of a leg. In higher forms, regeneration is restricted to the
healing of wounds and the mending of broken bones.
Closely connected with this process of regeneration of lost parts is the
widely prevalent process of reproduction by budding. The cut stump of
the amputated tentacle of the hydra or the snail buds forth a new organ.
But in the hydra, during the summer months, under normal circumstances,
a bud may make its appearance and give rise to a new individual, which
will become detached from the parent, to lead a separate existence. In
other organisms allied to the hydra the buds may remain in attachment,
and a colony will result. This, too, is the result of budding in many of
the sponges. In some worms, too, budding may occur. In the fresh-water
worm (_Chætogaster limnæi_) the animal, as we ordinarily see it, is a
train of individuals, one budded off behind the other--the first fully
developed, those behind it in various stages of development. The
individuals finally separate by transverse division. Another more lowly
worm (_Microstomum lineare_, a Turbellarian) may bud off in similar
fashion a chain of ten or fifteen individuals. In these cases budding is
not far removed from fission.
Now, in the case of reproduction by budding, as in the hydra, a new
individual is produced from some group of cells in the parent organism.
From this it is but a step--a step, however, of the utmost
importance--to the production of a new individual from a single cell
from the tissues of the parental organism. Such a reproductive cell is
called an egg-cell, or ovum. In the great majority of cases, to enable
the ovum to develop into a new individual, it is necessary that the
egg-cell should conjugate or fuse with a minute, active sperm-cell,
generally derived from a different parent. This process of fusion of
germinal cells is called fertilization (see Fig. 5, p. 13).
In sponges, the cells which become ova or sperms lie scattered in the
mid-layer between the ciliated layers which line the cavities and spaces
of the organism. Sometimes the individual sponge produces only ova;
sometimes only sperms; sometimes both, but at different periods. The
cells which become ova increase in size, are passive, and rich in
reserve material elaborated by their protoplasm. The cells which become
sperms divide again and again, and thus produce minute active bodies,
adance with restless motion. These opposite tendencies are repeated and
emphasized throughout the animal kingdom--ova relatively large, passive,
and accumulative of reserve material; sperms minute, active, and the
result of repeated fission. The active sperm, when it unites with the
ovum, imports into it a tendency to fission, or cleavage; but the
resulting cells do not part and scatter--they remain associated
together, and in mutual union give rise to a new sponge.
[Illustration: Fig. 8.--Hydra viridis.
A, hydra half retracted, with a bud and an ovum attached to the shrunken
ovary; B, a small hydra firmly retracted; C, a hydra fully extended. b.,
bud; f., foot; h.s., hypostome; ovm., ovum; ovy., ovary; t., tentacles;
ts., testis.]
In the hydra, generally near the foot or base of attachment, a rounded
swelling often makes its appearance in autumn. Within this swelling one
central cell increases enormously at the expense of the others. It
becomes an ovum. Eventually it bursts through the swelling, but remains
attached for a time. Rarely in the same hydra, more frequently in
another, one or two swellings may be seen higher up, beneath the circle
of tentacles. Within these, instead of the single ovum may be seen a
swarm of sperms, minute and highly active. When these are discharged,
one may fuse with and fertilize an ovum, occasionally in the same, but
more frequently in another individual, with the result that it develops
into a new hydra. Here there are definite organs--an ovary and a
testis--producing the ova or the sperms. But they are indefinite and not
permanent in position.
In higher forms of life the organs which are set apart for the
production of ova or sperms become definite in position and definite in
structure. Occasionally, as in the snail, the same organ produces both
sperms and ova, but then generally in separate parts of its structure.
The two products also ripen at different times. Not infrequently, as in
the earthworm, each individual has both testes and ovaries, and thus
produces both ova and sperms, but from different organs. The ova of one
animal are, however, fertilized by sperms from another. But in the
higher invertebrates and vertebrates there is a sex-differentiation
among the individuals, the adult males being possessed of testes only
and producing sperms, the adult females possessed of ovaries only and
producing ova. There are also, in many cases, accessory structures for
ensuring that the ova shall be fertilized by sperms, while sexual
appetences are developed to further the same end. But however the matter
may thus be complicated, the essential feature is the same--the union of
a sluggish, passive cell, more or less laden with nutritive matter, with
a minute active cell with an hereditary tendency to fission.[F]
It is not, however, necessary in all cases that fertilization of the
ovum should take place. The plant-lice, or _Aphides_ of our rose trees,
may produce generation after generation, and their offspring in turn
reproduce in like manner, without any union or fusion of ovum or sperm.
The same is true of the little water-fleas, or _Daphnids_; while in some
kinds of rotifers fertilization is said never to occur. It is a curious
and interesting fact, which seems now to be established beyond question,
that drone bees are developed from unfertilized ova, the fertilized ova
producing either queens or workers, according to the nature of the food
with which the grubs are supplied. Where, as in the case of aphids and
daphnids, fertilization occasionally takes place, it would seem that
lowered temperature and diminished food-supply are the determining
conditions. Fertilization, therefore, generally takes place in the
autumn; the fertilized ovum living on in a quiescent state during the
winter, and developing with the warmth of the succeeding spring. In the
artificial summer of a greenhouse, reproduction may continue for three
or four years without the occurrence of any fertilization.
[Illustration: Fig. 9.--Aurelia: Life-cycle.
a, embryo; b, _Hydra tuba_; c, _Hydra tuba_, with medusoid segments; d,
medusa separated to lead free existence.]
Mention may here be made of some peculiarly modified modes of
reproduction among the metazoa. The aurelia is a well-known and
tolerably common jelly-fish. These produce ova, which are duly
fertilized by sperms from a different individual. A minute,
free-swimming embryo develops from the ovum, which settles down and
becomes a little polyp-like organism, the _Hydra tuba_. As growth
proceeds, this divides or segments into a number of separable, but at
first connected, parts. As these attain their full development, first
one and then another is detached from the free end, floats off, and
becomes a medusoid aurelia. Thus the fertilized ovum of aurelia
develops, not into one, but into a number of medusæ,[G] passing through
the _Hydra tuba_ condition as an intermediate stage.
Many of the hydroid zoophytes, forming colonies of hydra-like organisms,
give rise in the warm months to medusoid jelly-fish, capable of
producing ova and sperms. Fertilization takes place; and the fertilized
ova develop into little hydras, which produce, by budding, new colonies.
In these new colonies, again, the parts which are to become ovaries or
testes float off, and ripen their products in free-swimming, medusoid
organisms. Such a rhythm between development from ova and development by
budding is spoken of as an alternation of generations.
The fresh-water sponge (_Spongilla_) exhibits an analogous rhythm. The
ova are fertilized by sperms from a different short-lived individual.
They develop into sponges which have no power of producing ova or
sperms. But on the approach of winter in Europe, and of the dry season
in India, a number of cells collect and group themselves into a
so-called gemmule. Round this is formed a sort of crust beset with
spicules, which, in some cases, have the form of two toothed discs
united by an axial shaft. When these gemmules have thus been formed, the
sponge dies; but the gemmules live on in a quiescent state during the
winter or the dry season, and with the advent of spring develop into
sponges, male or female. These have the power of producing sperms or
ova, but no power of producing gemmules. The power of producing ova, and
that of producing gemmules, thus alternates in rhythmic fashion.
[Illustration: Fig. 10.--Liver-fluke: Embryonic stages. (After A. P.
Thomas.)
A. ovum: em., embryo; op., operculum. B. _Limnæus truncatulus_ (natural
size). C. Free embryo: e.s., eye-spot; ex., excretory vessel; g.c.,
germinal cells; h.p., head-papilla. D. Embryo preparing to become a
sporocyst: g.c., germinal cells. E. Sporocyst: g., gastrula; m., morula;
re., redia. F. Redia: b.o., birth-opening; ce., cercaria; col., collar;
di., digestive sac; ph., pharynx; p.pr., posterior processes; re.,
daughter redia. G. Cercaria: cys., cystogenous organ; di., digestive
sac; o.s., oral sucker; p.s., posterior sucker; ph., pharynx.]
But one more example of these modified forms of reproduction can here be
cited (from the author's text-book on "Animal Biology"). The liver-fluke
is a parasitic organism, found in the liver of sheep. Here it reaches
sexual maturity, each individual producing many thousands of eggs, which
pass with the bile into the alimentary canal of the _host_, and are
distributed over the fields with the excreta. Here, in damp places,
pools, and ditches, free and active embryos are hatched out of the eggs.
Each embryo (Fig. 10, C., much enlarged) is covered with cilia, except
at the anterior end, which is provided with a head-papilla (h.p.). When
the embryo comes in contact with any object, it, as a rule, pauses for a
moment, and then darts off again. But if that object be the minute
water-snail, _Limnæus truncatulus_ (Fig. 10, B., natural size), instead
of darting off, the embryo bores its way into the tissues until it
reaches the pulmonary chamber, or more rarely the body-cavity. Here its
activity ceases. It passes into a quiescent state, and is now known as a
_sporocyst_ (Fig. 10, E.). The active embryo has degenerated into a mere
brood-sac, in which the next generation is to be produced. For within
the sporocyst special cells undergo division, and become converted into
embryos of a new type, which are known as _rediæ_ (F.), and which, so
soon as they are sufficiently developed, break through the wall of the
sporocyst. They then increase rapidly in size, and browse on the
digestive gland of the water-snail (known as the _intermediate host_),
to which congenial spot they have in the mean time migrated. The series
of developmental changes is even yet not complete. For within the rediæ
(besides, at times, daughter rediæ) embryos of yet another type are
produced by a process of cell-division. These are known as _cercariæ_
(Fig. 10, G.). Each has a long tail, by means of which it can swim
freely in water. It leaves the intermediate host, and, after leading a
short, active life, becomes encysted on blades of grass. The cyst is
formed by a special larval organ, and is glistening snowy white. Within
the cyst lies the transparent embryonic liver-fluke, which has lost its
tail in the process of encystment.
The last chapter in this life-history is that in which the sheep crops
the blade of grass on which the parasite lies encysted; whereupon the
cyst is dissolved in the stomach of the host, the little liver-fluke
becomes active, passes through the bile-duct into the liver of the
sheep, and there, growing rapidly, reaches sexual maturity, and lays its
thousands of eggs, from each of which a fresh cycle may take its origin.
The sequence of phenomena is characterized by discontinuity of
development. Instead of the embryo growing up continuously into the
adult, with only the atrophy of provisional organs (e.g. the gills and
tail of the tadpole, or embryo frog), it produces germs from which the
adult is developed. Not merely provisional organs, but provisional
organisms, undergo atrophy. In the case of the liver-fluke there are two
such provisional organisms, the embryo sporocyst and the redia.
We may summarize the life-cycle thus--
1. _Ovum_ laid in liver of sheep, passes with bile into intestine,
and thence out with the excreta.
2. _Free ciliated embryo_, in water or on damp earth, passes into
pulmonary cavity of _Limnæus truncatulus_, and develops into
3. _Sporocyst_, in which secondary embryos are developed, known as
4. _Rediæ_, which pass into the digestive glands of _Limnæus_, and
within which, besides daughter rediæ, there are developed tertiary
embryos, or
5. _Cercariæ_, which pass out of the intermediate host and become
6. _Encysted_ on blades of grass, which are eaten by sheep. The cyst
dissolves, and the young flukes pass into the liver of their host,
each developing into
7. A _liver-fluke_, sexual, but hermaphrodite.
Here, again, we notice that one fertilized ovum gives rise to not one,
but a number of liver-flukes.
We must now pass on to consider the growth and development of organisms.
Simple growth results from the multiplication of similar cells. As the
child, for example, grows, the framework of the body and the several
organs increase in size by continuous cell-multiplication. Development
is differential growth; and this may be seen either in the organs or
parts of an organism or in the cells themselves. As the child grows up
into a man, there is a progressive change in his relative proportions.
The head becomes relatively smaller, the hind limbs relatively longer,
and there are changes in the proportional size of other organs.
In the development of the embryo from the ovum, the differentiation is
of a deeper and more fundamental character. Cells at first similar
become progressively dissimilar, and out of a primitively homogeneous
mass of cells is developed a heterogeneous system of different but
mutually related tissues.
This view of development is, however, the outcome of comparatively
modern investigation and perfected microscopical appliances. The older
view was that development in all cases is nothing more than differential
growth, that there is no differentiation of primitively similar into
ultimately different parts. Within the fertilized ovum of the horse or
bird lay, it was supposed, in all perfection of structure, a miniature
racer or chick, the parts all there, but too minute to be visible. All
that was required was that each part should grow in due proportion.
Those who held this view, however, divided into two schools. The one
believed that the miniature organism was contained within the ovum, the
function of the sperm being merely to stimulate its subsequent
developmental growth. The other held that the sperm was the miniature
organism, the ovum merely affording the food-material necessary for its
developmental growth. In either case, this unfolding of the invisible
organic bud was the _evolution_ of the older writers on organic life.
More than this. As Messrs. Geddes and Thomson remind us,[H] "the germ
was more than a marvellous bud-like miniature of the adult. It
necessarily included, in its turn, the next generation, and this the
next--in short, all future generations. Germ within germ, in ever
smaller miniature, after the fashion of an infinite juggler's box, was
the corollary logically appended to this theory of preformation and
unfolding."
Modern embryology has completely negatived any such view as that of
preformation, and as completely established that the evolution is not
the unfolding of a miniature germ, but the growth and differentiation of
primitively similar cell-elements. In different animals, as might be
expected, the manner and course of development are different. We may
here illustrate it by a very generalized and so to speak diagrammatic
description of the development of a primitive vertebrate.
[Illustration: Fig. 11.--Diagram of development. See text. The fine line
across G. indicates the plane of section shown in H.]
The ovum before fertilization is a simple spherical cell, without any
large amount of nutritive material in the form of food-yolk (A.). It
contains a nucleus. Previous to fertilization, however, in many forms of
life, portions of the nucleus, amounting to three parts of its mass, are
got rid of in little "polar cells" budded off from the ovum. The import
of this process we shall have to consider in connection with the subject
of heredity. The sperm is also a nucleated cell; and on its entrance
into the ovum there are for a short time two nuclei--the female nucleus
proper to the ovum, and the male nucleus introduced by the sperm. These
two unite and fuse to form a joint nucleus. Thus the fertilized ovum
starts with a perfect blending of the nuclear elements from two cells
produced by different parents.
Then sets in what is known as the segmentation or cleavage of the ovum.
First the nucleus and then the cell itself divides into two equal halves
(B.), each of these shortly afterwards again dividing into two. We may
call the points of intersection of these two planes of division the
"poles," and the planes "vertical planes." We thus have four cells
produced by two vertical planes (C.). The next plane of division is
equatorial, midway between the poles. By this plane the four cells are
subdivided into eight (D.). Then follow two more vertical planes
intermediate between the first two. By them the eight cells are divided
into sixteen. These are succeeded by two more horizontal planes midway
between the equator and the poles. Thus we get thirty-two cells. So the
process continues until, by fresh vertical and horizontal planes of
division, the ovum is divided into a great number of cells.
But meanwhile a cavity has formed in the midst of the ovum. This makes
its appearance at about the eight-cell stage, the eight cells not quite
meeting in the centre of the ovum. The central cavity so formed is thus
surrounded by a single layer of cells, and it remains as a single layer
throughout the process of segmentation, so that there results a hollow
vesicle composed of a membrane constituted by a single layer of cells
(E.).
The cells on one side of the vesicle are rather larger than the others,
and the next step in the process is the apparent pushing in of this part
of the hollow sphere; just as one might take a hollow squash indiarubber
ball, and push in one side so as to form a hollow, two-layered cup (F.).
The vesicle, then, is converted into a cup, the mouth of which gradually
closes in and becomes smaller, while the cup itself elongates (G.).[I]
Thus a hollow, two-layered, stumpy, worm-like embryo is produced, the
outer layer of which may be ciliated, so that by the lashing of these
cilia it is enabled to swim freely in the water. The inner cavity is the
primitive digestive cavity.
A cross-section through the middle of the embryo at this stage will show
this central cavity surrounded by a two-layered body-wall (H.). A little
later the following changes take place (J. K.): Along a definite line
on the surface of the embryo, marking the region of the back, the outer
layer becomes thickened; the edges of the thickened band so produced
rise up on either side, so as to give rise to a median groove between
them; and then, overarching and closing over the groove, convert it into
a tube. This tube is called the neural tube, because it gives rise to
the central nervous system. In the region of the head it expands; and
from its walls, by the growth and differentiation of the cells, there is
formed--in the region of the head, the brain, and along the back, the
spinal cord. Immediately beneath it there is formed a rod of cells,
derived from the inner layer. This rod, which is called the notochord,
is the primitive axial support of the body. Around it eventually is
formed the vertebral column, the arches of the vertebræ embracing and
protecting the spinal cord.
Meanwhile there has appeared between the two primitive body-layers a
third or middle layer.[J] The cells of which it is composed arise from
the inner layer, or from the lips of the primitive cup when the outer
and inner layer pass the one into the other. This middle layer at first
forms a more or less continuous sheet of cells between the inner and the
outer layers. But ere long it splits into two sheets, of which one
remains adherent to the inner layer and one to the outer layer. The
former becomes the muscular part of the intestinal or digestive tube,
the latter the lining of the body-wall. The space between the two is
known as the body-cavity. Beneath the throat the heart is fashioned out
of this middle layer.
Very frequently--that is to say, in many animals--the opening by which
the primitive digestive tube communicated with the exterior has during
these changes closed up, so that the digestive cavity does not any
longer communicate in any way with the exterior. This is remedied by the
formation of a special depression or pit at the front end for the mouth,
and a similar pit at the hinder end.[K] These pits then open into the
canal, and communications with the exterior are thus established. The
lungs and liver are formed as special outgrowths from the digestive
tube. The ovaries or testes make their appearance _at a very early
period_ as ridges of the middle layer projecting into the body-cavity.
For some time it is impossible to say whether they will produce sperms
or ova; and it is said that in many cases they pass through a stage in
which one portion has the special sperm-producing, and another the
special ovum-producing, structure. But eventually one or other prevails,
and the organs become either ovaries or testes.
Thus from the outer layer of the primitive embryo is produced the outer
skin, together with the hairs, scales, or feathers which it carries;
from it also is produced the nervous system, and the end-organs of the
special senses. From the inner layer is formed the digestive lining of
the alimentary tube and the glands connected therewith; from it also the
primitive axial support of the body. But this primitive support gives
place to the vertebral column formed round the notochord; and this is of
mid-layer origin. Out of the middle layer are fashioned the muscles and
framework of the body; out of it, too, the heart and reproductive
organs. The tissues of many of the organs are cunningly woven out of
cells from all three layers. The lens of the eye, for example, is a
little piece of the outer layer pinched off and rendered transparent.
The retina of that organ is an outgrowth from the brain, which, as we
have seen, was itself developed from the outer layer. But round the
retina and the lens there is woven from the middle layer the tough
capsule of the eye and the circular curtain or iris. The lining cells of
the digestive tube are cells of the inner layer, but the muscular and
elastic coats are of middle-layer origin. The lining cells of the
salivary glands arise from the outer layer where it is pushed in to form
the mouth-pit; but the supporting framework of the glands is derived
from the cells of the middle layer.
Enough has now been said to give some idea of the manner in which the
different tissues and organs of the organism are elaborated by the
gradual differentiation of the initially homogeneous ovum. The cells
into which the fertilized egg segments are at first all alike; then
comes the divergence between those which are pushed in to line the
hollow of the cup, and those which form its outer layer. Thereafter
follows the differentiation of a special band of outer cells to form the
nervous system, and a special rod, derived from the inner cells, to form
the primitive axial support. And when the middle layer has come into
existence, its cells group themselves and differentiate along special
lines to form gristle or bone, blood or muscle.
The description above given is a very generalized and diagrammatic
description. There are various ways in which complexity is introduced
into the developmental process. The store of nutritive material present
in the egg, for example, profoundly modifies the segmentation so that
where, as in the case of birds' eggs, there is a large amount of
food-yolk, not all the ovum, but only a little patch on its surface,
undergoes segmentation. In this little patch the embryo is formed. Break
open an egg upon which a hen has been sitting for five or six days, and
you will see the little embryo chick lying on the surface of the yolk.
The large mass of yolk to which it is attached is simply a store of
food-material from which the growing chick may draw its supplies.
For it is clear that the growing and developing embryo must obtain, in
some way and from some source, the food-stuff for its nutrition. And
this is effected, among different animals, in one of three ways. Either
the embryo becomes at a very early stage a little, active, voracious,
free-swimming larva, obtaining for itself in these early days of life
its own living; as is the case, for example, with the oyster or the
star-fish. Or the egg from which it is developed contains a large store
of food-yolk, on which it can draw without stint; as is the case with
birds. Or else the embryo becomes attached to the maternal organism in
such a way that it can draw on her for all the nutriment which it may
require; as is the case with the higher mammals.
In both these latter cases the food-material is drawn from the maternal
organism, and is the result of parental sacrifice; but in different
ways. In the case of the bird, the protoplasm of the ovum has acquired
the power of storing up the by-products of its vital activity. The ovum
of such an animal seems at first sight a standing contradiction to the
statement, made some pages back, that the cell cannot grow to any great
extent without undergoing division or fission; and this because volume
tends to outrun surface. For the yolk of a bird's egg is a single cell,
and is often of large size. But when we come to examine carefully these
exceptional cases of very large cells--for what we call the yolk of an
egg is, I repeat, composed of a single cell--we find that the formative
protoplasm is arranged as a thin patch on one side of the yolk in the
case of the bird's egg, or as a thin pellicle surrounding the yolk in
the case of that of the lobster or the insect. All the rest is a product
of protoplasmic life stowed away beneath the patch or within the
pellicle. And this stored material is relatively stable and inert, not
undergoing those vital disruptive changes which are characteristic of
living formative protoplasm. The mass of formative protoplasm, even in
the large eggs of birds, is not very great, and is so arranged as to
offer a relatively extensive surface. All the rest, the main mass of the
visible egg-yolk, is the stored product of a specialized activity of the
formative protoplasm. But all this material is of parental origin--is
elaborated from the nutriment absorbed and digested by the mother.
Thus we see, in the higher types of life, parental sacrifice, fosterage,
and protection. For in the case of mammals and many birds, especially
those which are born in a callow, half-fledged condition, even when the
connection of mother and offspring is severed, or the supplies of
food-yolk are exhausted, and the young are born or hatched, there is
still a more or less prolonged period during which the weakly offspring
are nourished by milk, by a secretion from the crop ("pigeon's milk"),
or by food-stuff brought with assiduous care by the parents. There is a
longer or shorter period of fosterage and protection--longer in the case
of man than in that of any of the lower animals--ere the offspring are
fitted to fend for themselves in life's struggle.
And accompanying this parental sacrifice, first in supplying food for
embryonic development, and then in affording fosterage and protection
during the early stages of growth, there is, as might well be supposed,
a reduction in the number of ova produced and of young brought forth or
hatched. Many of the lower organisms lay hundreds of thousands of eggs,
each of which produces a living active embryo. The condor has but two
downy fledglings in a year; the gannet lays annually but a single egg;
while the elephant, in the hundred years of its life, brings forth but
half a dozen young.
We shall have to consider by what means these opposite tendencies (a
tendency to produce enormous numbers of tender, ill-equipped embryos,
and a tendency to produce few well-equipped offspring) have been
emphasized. The point now to be noted is that every organism, even the
slowest breeder that exists, produces more young than are sufficient to
keep up the numbers of the species. If every pair of organisms gave
birth to a similar pair, and if this pair survived to do likewise, the
number of individuals in the species would have no tendency either to
increase or to diminish. But, as a matter of fact, animals actually do
produce from three or four times to hundreds or even thousands of times
as many new individuals as are necessary in this way to keep the numbers
constant. This is the _law of increase_. It may be thus stated: _The
number of individuals in every race or species of animals is tending to
increase._ Practically this is only a tendency. By war, by struggle, by
competition, by the preying of animals upon each other, by the stress of
external circumstances, the numbers are thinned down, so that, though
the births are many, the deaths are many also, and the survivals few. In
the case of those species the numbers of which are remaining constant,
out of the total number born only two survive to procreate their kind.
We may judge, then, of the amount of extermination that goes on among
those animals which produce embryos by the thousand or even the hundred
thousand. The effects of this enormous death-rate on the progress of the
race or species we shall have to consider in the next chapter, when the
question of the differentiation of species is before us.
There is one form of differentiation, however, which we may glance at
before closing this chapter--the differentiation of sex. We are not in a
position to discuss the ultimate causes of sex-differentiation, but we
may here note the proximate causes as they seem to be indicated in
certain cases.
Among honey-bees there are males (drones), fertile females (queens), and
imperfect or infertile females (workers). It has now been shown, beyond
question, that the eggs from which drones develop are not fertilized.
The presence or absence of fertilization in this case determines the
sex. During the nuptial flight, a special reservoir, possessed by the
queen bee, is stored with sperms in sufficient number to last her
egg-laying life. It is in her power either to fertilize the eggs as they
are laid or to withhold fertilization. If the nuptial flight is
prevented, and the reservoir is never stored with sperms, she is
incapable of laying anything but drone eggs. The cells in which drones
are developed are somewhat smaller than those for ordinary workers; but
what may be the nature of the stimulus that prompts the queen to
withhold fertilization we at present do not know. The difference between
the fertile queen and the unfertile worker seems to be entirely a matter
of nutrition. If all the queen-embryos should die, the workers will tear
down the partitions so as to throw three ordinary worker-cells into one;
they will destroy two of the embryos, and will feed the third on highly
nutritious and stimulating diet; with the result that the ovaries and
accessory parts are fully developed, and the grub that would have become
an infertile worker becomes a fertile queen. And one of the most
interesting points about this change, thus wrought by a stimulating
diet, is that not only are the reproductive powers thus stimulated, but
the whole organism is modified. Size, general structure, sense-organs,
habits, instincts, and character are all changed with the development of
the power of laying eggs. The organism is a connected whole, and you
cannot modify one part without deeply influencing all parts. This is the
_law of correlated variation_.
Herr Yung has made some interesting experiments on tadpoles. Under
normal circumstances, the relation of females to males is about 57 to
43. But when the tadpoles were well fed on beef, the proportion of
females to males rose so as to become 78 to 32; and on the highly
nutritious flesh of frogs the proportion became 92 to 8. A highly
nutritious diet and plenty of it caused a very large preponderance of
females.
Mrs. Treat, in America, found that if caterpillars were half-starved
before entering upon the chrysalis state, the proportion of males was
much increased; while, if they were supplied with abundant nutritious
food, the proportion of female insects was thereby largely increased.
The same law is said to hold good for mammals. Favourable vital
conditions are associated with the birth of females; unfavourable, with
that of males. Herr Ploss attempts to show that, among human folk, in
hard times there are more boys born; in good times, more girls.
On the whole, we may say that there is some evidence to show that in
certain cases favourable conditions of temperature, and especially
nutrition, tend to increase the number of females. We have seen that
many animals pass through a stage where the reproductive organs are not
yet differentiated into male and female, while in some there is a
temporary stage where the outer parts of the organ produce ova and the
inner parts sperms. We have also seen that the ova are cells where
storage is in excess; the sperms are cells in which fission is in
excess. Favourable nutritive conditions may, therefore, not
incomprehensibly lead to the formation of well-stored ova; unfavourable
nutritive conditions, on the other hand, to the formation of highly
subdivided sperms. By correlated variation,[L] the ova-bearing or
sperm-bearing individuals then develop into the often widely different
males and females.
NOTES
[F] Professor Geddes and Mr. J. Arthur Thomson, in their interesting
work on "The Evolution of Sex," regard the ovum in especial, and
the female in general, as preponderatingly anabolic (see note, p.
32); while the sperm in especial, and the male in general, are on
their view preponderatingly katabolic. Regarding, as I do, the
food-yolk as a katabolic product, I cannot altogether follow them.
The differentiation seems to me to have taken place along
divergent lines of katabolism. In the ovum, katabolism has given
rise to storage products; in the sperm, to motor activities
associated with a tendency to fission. The contrast is not between
anabolic and katabolic tendencies, but between storage katabolism
and motor katabolism. Nor do I think that "the essentially
katabolic male-cell brings to the ovum a supply of characteristic
waste products, or katastates, which stimulate the latter to
division" (_l.c._, p. 162). I believe that it brings an inherited
tendency to fission, and thus reintroduces into the fertilized
ovum the tendency which, as ovum, it had renounced in favour of
storage katabolism.
[G] On the other hand, three ova of the crustacean _Apus_ are said to
coalesce to form the single ovum from which one embryo develops.
[H] "The Evolution of Sex," p. 84.
[I] In some forms of life the opening of the cup marks the position of
the future mouth: in others, of the future vent. In yet others it
elongates into a slit, occupying the whole length of the embryo;
the middle part of the slit closes up, and the opening at the far
ends mark the position, the one of the future mouth, the other of
the future vent.
[J] In technical language, the outer layer of cells is called the
_epiblast_, the inner layer the _hypoblast_, and the mid-layer
between them the _mesoblast_.
[K] In technical language, the opening by which the primitive digestive
cavity (or _mesenteron_) communicates with the exterior is called
the _blastopore_. When this closes, the new opening for the mouth
is called the _stomod[oe]um_; that for the vent, the
_proctod[oe]um_.
[L] We have seen that when volume tends to outrun surface, fission may
take place, whereby the same volume has increased surface. But in
unfavourable nutritive conditions, the same surface which had
before been sufficient for nutrition may become, under the less
favourable circumstances, insufficient, and fission may again take
place to give a larger absorbent surface. Hence, possibly, the
connection between insufficient nutriment and highly subdivided
sperms.