which directs the development of the individual, and a propagative
half, which reaches the germ-cells and there remains inactive, and
later gives rise to the succeeding generation, constitutes _the theory
of the continuity of the germ-plasm_, which I first stated in a work
which appeared in the year 1885. Its fundamental idea had already been
expressed much earlier by Francis Galton (1872), without however being
fully appreciated at the time or having any influence on the course
of science, and the same is true with the later theoretical views of
Jäger, Rauber, and Nussbaum, all of whom reached the same idea quite
independently of each other, and sought to elaborate it more or less
fully.
The hypothesis does not depend for support merely on a recognition of
its theoretical necessity; on the contrary, there is a whole series of
facts which may be adduced as strongly in its favour.
Thus, even the familiar fact that the excision of the reproductive
organs in all animals produces sterility proves that no other cells
of the body are able to give rise to germ-cells; germ-plasm cannot be
produced _de novo_. An unmistakable corroboration of this, it seems
to me, is to be found in the conditions of germ-cell formation in
the medusoids and hydroid polyps, for here it is apparent that the
birthplace of the germs, that is, the place at which the germ-cells
of the animal are formed, has been shifted backwards in the course
of phylogenetic evolution, that is, has been moved nearer to the
starting-point of development. This shifting has exactly followed
the 'germ-tracks.' as we shall see, although in some cases it would
have been more advantageous if the birthplace of the germ-cells could
have lain outside of these. Obviously, then, it is only the existing
cell-generations of the germ-track which were able to give rise to
germ-cells, or, in other words, they alone contained the indispensable
germ-plasm. With the help of Figs. 94 and 95 I hope to be able to make
this matter clear.
[Illustration: FIG. 94. Diagram to illustrate the phylogenetic shifting
back of the origins of the germ-cells in medusoids and hydroids. A
composite picture. _A_, branch of a polyp colony. _P_, polyp-head with
mouth (_m_) and tentacles. _St_, stalk of the polyp. _M_, medusoid-bud
with the bell (_Gl_). _T_, marginal tentacle. _m_, mouth. _Mst_,
manubrium. _GphK_, a gonophore-bud. _GH_, gastric cavity. _ekt_,
ectoderm. _ent_, endoderm. _st_, supporting lamella. The germ-cells
(_kz_) arise in the medusoid in the ectoderm of the manubrium--first
phyletic stage--where they also attain maturity. In the gonophore-bud
(_GphK_) they arise in the ectoderm (_kz´_), or further down in the
stalk of the polyp at _kz´´_--third phyletic stage, or in the ectoderm
of the branch from which the polyp has arisen, at _kz´´´_--fourth
phyletic stage of the shunting of the originative area of the
germ-cells. In the two last cases the germ-cells migrate until they
reach their primitive place of origination in the medusoid, or in the
corresponding layer of the medusoid gonophore, as may be more clearly
seen in Fig. 95. Drawn from my sketch by Dr. Petrunkewitsch.]
In the hydroid polyps and their medusoids the germ-cells always
arise in the ectoderm; in species which produce sexual medusoids by
budding, the germ-cells arise in the ectoderm of the manubrium of these
medusoids (Fig. 94, _M_, _kz_). But in many species these sexual stages
have degenerated in the course of phylogeny into so-called gonophores,
that is, to medusoids which still exhibit more or less complete bells,
but neither mouth (_m_) nor marginal tentacles (_T_), and which no
longer break away from the colony to swim freely about, to feed
independently, and to produce and ripen germ-cells. The degeneration
of the 'gonophores' often goes even further; in many the medusoid bell
is represented only by a thin layer of cells, and in some even this
token of descent from medusoid ancestry is absent, and they are mere
single-layered closed brood-sacs (Fig. 95, _Gph_).
The adherence of the sexual animal to the hydroid colony has,
however, made a more rapid ripening of the germ-cells possible, and
nature has taken advantage of this possibility in all the cases
known to me, for the germ-cells no longer arise in the manubrium
of the mature degenerate medusoid, that is, of the gonophore, but
_earlier_, before the bud which becomes a gonophore possesses a
manubrium. The birthplace of the germ-cells is thus shifted back from
the manubrium of the medusoid to the young gonophore-bud (Fig. 94,
_M_, _kz_). The same thing occurs in species in which the medusoids
are liberated, but live only for a short time, for instance, in the
genus _Podocoryne_. Although perfect medusoids are formed, these have
their germ-cells fully developed at the time of their liberation from
the hydroid colony. But in species in which the medusoid-buds have
really degenerated and are no longer liberated, the birthplace of the
germ-cells is shifted _even further back_, and in the first place into
the stalk (_St_, _kz´´_) of the polyp from the gonophore-buds. This
is the case in the genus _Hydractinia_. In the further course of the
process the birthplace of the germ-cells has shifted as far back as to
the branch from which the polyp has grown out (Fig. 94, _A_, _kz´´´_);
and finally, in the cases in which the medusoid has degenerated to
a mere brood-sac (Fig. 95, _Gph_), even to the generation of polyps
immediately before, that is, into the polyp-stem from which the branch
arises that bears the polyps producing the gonophore-bud (Fig. 95,
_kz´´´_). Then we find the birthplace of the germ-cells _still_ further
back (Fig. 95, _kz´´´´_), for the egg and sperm-cells arise in the stem
of the principal polyps (the main stem of the colony). The advantage of
this arrangement is easily seen, for the principal polyp is present
earlier than those of the secondary branches, and these again earlier
than the polyp which bears the sexual buds, and this, finally, earlier
than the sexual bud which it bears. Thus this shunting backwards of the
birthplace of the germ-cells means an earlier origin of the primordium
(_Anlage_) of the germ-cells, and consequently an earlier maturing of
these.
[Illustration: FIG. 95. Diagram to illustrate the migration of the
germ-cells in hydro-medusæ from their remotely shunted place of origin
to their primitive place of origin in the gonophore, in which they
attain to maturity. The state of affairs in Eudendrium is taken as the
basis of the diagram. _HP_, one of the principal polyps. _mu_, mouth.
_ma_, gut-cavity. _t_, tentacle. _Sta_, its stem. _A_, a branch of the
polyp colony. _SP_, lateral polyp. _Gph_, a medusoid-bud completely
degenerated into a mere gonophore. _Ei_, ovum. _GH_, gastric cavity.
_st_, supporting lamella. The originative area of the germ-cells
lies in the stem of the principal polyp at _kz´´´´_, whence the
germ-cells first migrate into the endoderm of the branch (_A_) at
_kz´´´_, creeping within which they reach _kz´´_ in the lateral polyp
(blastostyle), finally reaching the gonophore (_kz_) and passing again
into the ectoderm. Drawn from my sketch by Dr. Petrunkewitsch.]
But none of all these germ-cells come to maturity in the birthplace to
which they have been shifted, for they migrate independently from it to
the place at which they primitively arose, namely, into the manubrium
of the medusoid, which is still present even when great degeneration
has occurred, or even--in the most extreme cases of degeneration--into
the ectoderm of the brood-sac. This is the case in the genus
_Eudendrium_, of which Fig. 95 gives a diagrammatic representation.
The most interesting feature of this migration of the germ-cells is
that the cells invariably arise in the ectoderm (_kz´´´´_), then pierce
through the supporting lamella (_st_) into the endoderm (_kz´´´_), and
then creep along it to their maturing-place. Once there they break
through again to the outer layer of cells, the ectoderm (_kz_), and
come to maturity (_Ei_). That they make their way through the endoderm
is probably to be explained by the fact that they are there in direct
proximity to the food-stream which flows through the colony (_GH_ =
gastric cavity), and they are thus more richly nourished there than
in the ectoderm. But although this is the case, they never arise in
the endoderm; in no single case is the birthplace of the germ-cells to
be found in the endoderm, but always in the ectoderm, no matter how
far back it may have been shunted. Even when the germ-cells migrate
through the endoderm, their first recognizable appearance is invariably
in the ectoderm, as, for instance, in _Podocoryne_ and _Hydractinia_.
The course of affairs is thus exactly what it would necessarily be if
our supposition were correct, that only definite cell-generations--in
this case the ectoderm-cells--contain the complete germ-plasm. If the
endoderm-cells also contained germ-plasm it would be hard to understand
why the germ-cells never arise from them, since their situation offers
much better conditions for their further development than that of
the ectoderm-cells. It would also be hard to understand why such a
circuitous route was chosen as that exhibited by the migration of
the young germ-cells into the endoderm. Something must be lacking in
the endoderm that is necessary to make a cell into a germ-cell: that
something is the germ-plasm.
If we accept the theory of the continuity of the germ-plasm as in the
main correct, it appears that higher animals and plants are constructed
of two kinds of elements, the somatic cells and the germ-cells; both
owe their being to the germ-plasm of the ovum, but the former do not
contain it complete but only in individual determinants[24], and
therefore can never give rise again to the rank of germ-cells; the
others contain the latent germ-plasm intact, and can therefore produce
not only cells like themselves for a certain time by division, but
have also the power, when they are mature and the necessary conditions
have been fulfilled, of bringing forth a new individual of the same
species. The former have only a limited length of life, they die--they
must necessarily die--when the life of the individual to which they
belong is at an end; the latter are potentially immortal, like the
unicellular organisms, that is, they can in favourable circumstances
give rise to the germ-cells of a new individual, and so on for all
time, as far as we can see. The germ-plasm of a species is thus never
formed _de novo_, but it grows and increases ceaselessly; it is handed
on from one generation to another like a long root creeping through
the earth, from which at regular distances shoots grow up and become
plants, the individuals of the successive generations. If these
conditions be considered from the point of view of reproduction, the
germ-cells appear the most important part of the individual, for they
alone maintain the species, and the body sinks down almost to the
level of a mere cradle for the germ-cells, a place in which they are
formed, and under favourable conditions are nourished, multiply, and
attain to maturity. But the matter can also be looked at in an opposite
light, and then the endless root of the germ-plasm, with its germ-cells
ever forming new individuals, may be regarded as the means by which
alone nature was able to create multicellular organisms, individuals
of higher and higher differentiation and capacity, able to adapt
themselves to all possible conditions, and to make the fullest use of
all the possibilities of life.
[24] Boveri has recently made an observation upon the thread-worm of
the horse, which points to the correctness of the conception of the
germ-plasm. The two first segmentation-cells both receive the four
chromosomes of the species, but, in one of the two, a portion of the
chromatin breaks off and degenerates, or dissolves, at least as far
as can be seen. The other cell retains the whole mass of chromatin,
and from this there arise later the primitive genital-cells. In the
germ-track, therefore--so we must interpret it--the whole of the
germ-plasm is retained, while a part of it is withdrawn from the soma.
I have only partly described the process, and I do not wish to enter in
detail on an interpretation of it, since it seems to me obscure and to
require further observations before an interpretation can be attempted
with any confidence.
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Transcriber's Notes:
On page 104, the link for 'Elymnias lais' has been corrected
from PlII to PlIII.
Illustrations have been moved out of mid-paragraph.
Variations in spelling and hyphenation are retained.
Punctuation has been retained as published.
Bold type is shown as =strong=.
Italics are shown thus: _sloping_.
Small capitals have been capitalised.