from the Monoplastides[66] to the Polyplastides, a process which must
have taken place according to Götte. I will for the present omit the
fact that I cannot accept the supposition that the process of
encystment represents death. We may then inquire whether death has
taken the place of encystment among the Polyplastides, or, if this is
not the case, whether any process comparable to encystment exists among
the Polyplastides.
Götte believes that death is always connected with reproduction, and is
a consequence of the latter in both Protozoa and Metazoa. Reproduction
has, in his opinion, a directly ‘fatal effect,’ and the reproducing
individual must die. Thus the may-fly and the butterfly die directly
after laying their eggs, and the male bee dies immediately after
pairing; the Orthonectides expire after expelling their germ-cells,
while _Magosphaera_ resolves itself into germ-cells, and nothing
persists except these elements. It is but a step from this latter
organism to the unicellular animals which transform themselves as a
whole into germ-cells; but in order to achieve this they must undergo
the process of rejuvenescence, which Götte assumes to be the same as
death.
These views contain many fallacies quite apart from the soundness or
unsoundness of their foundation. The process of encystment, as Götte
thinks, represents, in the Monoplastides, true reproduction to which
multiplication by means of division has been secondarily added. This
encystment cannot be dispensed with, for internal causes determine that
it must occasionally interrupt the process of multiplication by simple
division. But, on the other hand, Götte also considers the division of
the contents of the cyst to be a secondary process. The essential
characteristic of encystment is a simple process of rejuvenescence
without multiplication. Hence we are forced to accept a primitive
condition in which simple division as well as the division of the
encysted individual were absent, and in which reproduction consisted
only in an often-repeated process of rejuvenescence among existing
individuals, without any increase in their number. Such a condition is
inconceivable because it would involve a rapid disappearance of the
species, and the whole consideration clearly shows us that division of
un-encysted individuals must have existed from the first, and that
this, and not a vague and mysterious rejuvenescence, has always been
the real and primitive reproduction of the Monoplastides. The fact that
encystment does not always lead to the division of the contents of the
cyst proves, in my opinion, that not reproduction but preservation
against injury from without, was the primitive meaning of encystment.
It is possible that at the present time there are but few Monoplastides
which are able to go through an infinite number of divisions without
the interposition of the resting condition implied by encystment;
although it has not yet been demonstrated for all species[67]. But it
is not right to conclude from this that there is an internal necessity
which leads to encystment, that is to say to identify this process with
rejuvenescence. It is much more probable that encystment is merely an
adaptation to continual changes in the external conditions of life,
such as drought and frost, and perhaps also the want of food which
arises from the over-population of small areas. The same phenomenon is
known in certain low Crustacea—the _Daphnidae_—which possess an
ephippium or protective case for their winter-eggs. This case is only
developed after a certain definite number of generations has been run
through, an event which may happen at any time in the year in species
living in pools which are liable to be often dried-up; but only in the
autumn in such as live in lakes which are never dry. No one ever
doubted that the periodical formation of the ephippium in certain
generations was an adaptation to changes in the external conditions of
life.
Even if the process of rejuvenescence in the Monoplastides were really
equivalent to the death of the higher animals, we could not conclude
from this that it is necessarily associated with reproduction.
Encystment alone is not reproduction, and it first becomes a form of
reproduction when it is associated with the division of the encysted
animal. Simple division was the true and original form of reproduction
in Monoplastides, and even now it is the principal and fundamental form.
Hence we see that among the Monoplastides reproduction is not connected
with death, even if we accept Götte’s view and allow that encystment
represents death. I shall return later on to the relation between death
and reproduction in the Metazoa; but the question first arises whether
encystment, if it is not death, has any analogue in the higher animals,
and further whether death takes that place in their development which
is occupied by encystment in the Monoplastides.
Among the higher Metazoa there can be no doubt as to what we mean by
death, but the precise nature of that which dies is not equally
evident, and the popular conception is not sufficient for us. It is
necessary to distinguish between the mortal and the immortal part of
the individual—the body in its narrower sense (_soma_) and the
germ-cells. Death only affects the former; the germ-cells are
potentially immortal, in so far as they are able, under favourable
circumstances, to develope into a new individual, or, in other words,
to surround themselves with a new body (_soma_)[68].
But how is it with the lowest Polyplastides in which there is no
antithesis between the somatic and germ-cells, and among which each of
the component cells of the multicellular body has retained all the
animal functions of the Monoplastides, even including reproduction?
Götte believes that the natural death of these organisms (which he
rightly calls Homoplastides) consists in ‘the dissolution of the
cell-colony.’ As an example of such dissolution Götte takes Häckel’s
_Magosphaera planula_, a marine free-swimming organism in the form of a
sphere composed of a single layer of ciliated cells, imbedded in a
jelly. (For figure see below.) This organism cannot however be
‘considered as a genuine perfect Polyplastid, for at a certain time the
component cells part from one another and then continue to live
independently in the condition of Monoplastides.’ These free amoebiform
organisms increase considerably in size, encyst, and finally undergo
numerous divisions—a kind of segmentation within the cyst. The result
of the division is a sphere of ciliated cells similar to that with
which the cycle began. In fact, _Magosphaera_ is not a perfect
Polyplastid, but a transitional form between Polyplastides and
Monoplastides, as the discoverer of the group of animals of which it is
the only representative, indicated, when he named the group
‘Catallacta.’
[Illustration: Development of Magosphaera Planula (after Häckel).
1. Encysted amoeboid form. 2 and 3. Two stages in the division of the
same. 4. Free ciliated sphere, the cells of which are connected by a
gelatinous mass. 5. One of the ciliated cells which has become free
by the breaking up of the sphere. 6. The same in the amoeboid form.
7. The same grown to a larger size.]
According to Götte, the natural death of _Magosphaera_ consists, as in
the undoubted Protozoa, in a process of rejuvenescence by encystment.
The dissolution of the ciliated sphere into single cells ‘cannot be
identical with natural death. For the regular and complete separation
of the _Magosphaera_-cells proves that their individuality has not been
completely subordinated to that of the whole colony, and it proves that
the latter is not completely individualised[69].’
Nothing can be said against this, if we agree in identifying death with
the encystment of the Monoplastides. Now we could, as Götte rightly
remarks, derive the lower forms of Polyplastides from _Magosphaera_ if
‘the connection between the cells of the ciliated sphere were retained
until encystment, viz. until the reproduction of the single cells had
taken place[70].’ After this had been accomplished, Götte considers
that death would consist ‘in the complete separation of the cells from
one another, accompanied in all probability by their simultaneous
change into germ-cells.’ The fallacy in this is evident; if death is
represented in one case by the encystment during which single cells
change into germ-cells, then this must apply to the other case also,
for nothing has changed except the duration of the cell-colony. The
nature of encystment cannot be affected by the fact that the cells
separate from one another a little earlier or a little later. If it is
true that death is represented by encystment among the Monoplastides,
then the same conclusion must also hold for the Polyplastides; or
rather death must be represented in them by the process of
rejuvenescence, which Götte considers to be the essential part of
encystment. Götte ought not to identify death with the dissolution of
the cell-colony of which the lowest and highest Polyplastides are alike
composed; but he should seek it in the process of rejuvenescence which
takes place within the germ-cells. If it is essential to the nature of
reproduction that the cells set apart for that purpose should pass
through a process of rejuvenescence, which is equivalent to death, then
this must be true for the reproductive cells of all organisms. If these
conclusions hold good, there is nothing to prevent us from assuming
that such a process of rejuvenescence actually occurs in the higher
animals. Götte evidently holds this view, as is plainly shown in the
last pages of his essay. He there attempts to bring his views of the
death and rejuvenescence of the germ into harmony with his previously
developed idea of the derivation of death among the Polyplastides from
the dissolution of the cell-colonies. Götte still clings to the view
which he propounded in describing the development of _Bombinator_,
according to which the egg-cell of the higher Metazoa must pass through
a process of rejuvenescence representing death, before it can become a
germ.
According to Götte’s[71] idea ‘the egg of a _Bombinator igneus_ before
fertilization cannot be considered to be a cell either wholly or in
part; and this is equally true of it at its origin and after its
complete development; it is only an essentially homogeneous organic
mass enclosed by a membrane which has been deposited externally.’ This
mass is ‘unorganised and not living[72],’ and ‘during the first
phenomena of its development all vital powers must be excluded.’ In
this way the continuity of life between two successive individuals is
always interrupted; or, as Götte says in his last essay:—‘The
continuity of life between individuals of which one is derived from the
other by means of reproduction, exists neither in the rejuvenescence of
the Monoplastides nor in the condition of the germ among the
Polyplastides—a condition which is derived from the former[73].’
This is quite logical, although in my opinion it is both unproved and
incorrect. But, on the other hand, it is certainly illogical for Götte
to derive the death of the Metazoa in a totally different way, i. e.
from the dissolution of their cell-colonies. It is quite plain that the
death of the Metazoa does not especially concern the reproductive
cells, but the individual which bears them; Götte must therefore seek
for some other origin of death—an origin which will enable it to reach
the body (_soma_)—as opposed to the germ-cells. If there still remained
any doubt about the failure to establish a correspondence between death
and the encystment of the Monoplastides, we have here, at any rate, a
final demonstration of the failure!
But there is yet another great fallacy concealed in this derivation of
the death of the Polyplastides.
Among the lowest Polyplastides, where all the cells still remain
similar, and where each cell is also a reproductive cell, the
dissolution of the cell-colony is, according to Götte, to be regarded
as death, inasmuch as ‘the integrity of the mother-individual
absolutely comes to an end’ (l. c., p. 78). The dissolution of a
cell-colony into its component living elements can only be called death
in the most figurative sense, and can have nothing to do with the real
death of the individuals; it only consists in a change from a higher to
a lower stage of individuality. Could we not kill a _Magosphaera_ by
boiling or by some other artificial means, and would not the state
which followed be death? Even if we define death as an arrest of life,
the dissolution of _Magosphaera_ into many single cells which still
live, is not death, for life does not cease in the organic matter of
which the sphere was composed, but expresses itself in another form. It
is mere sophistry to say that life ceases because the cells are no
longer combined into a colony. Life does not in truth cease for a
moment. Nothing concrete dies in the dissolution of _Magosphaera_;
there is no death of a cell-colony, but only of a conception. The
Homoplastides, that is cell-colonies built up of equal cells, have not
yet gained any natural death, because each of their cells is, at the
same time, a somatic as well as a reproductive cell: and they cannot be
subject to natural death, or the species would become extinct.
It is more to the purpose that Götte has sought for an illustration of
death among those remarkable parasites, the Orthonectides, because in
them we do at any rate meet with real death. They are indeed very low
organisms; but nevertheless they stand far above _Magosphaera_, even if
the latter were hypothetically perfected up to the level of a true
Homoplastid, for the cells which compose the body of the Orthonectides
are not all similar, but are so far differentiated that they are even
arranged in the primitive germ-layers, and a form results which has
rightly been compared with that of the Gastrula. It is true they are
not quite so simple as Götte[74] figures them, for they not only
consist of ectoderm and germ-cells, but, according to Julin[75], the
endoderm is arranged in two layers—the germ-cells and a layer which
forms during development a strong muscular coat; and in the second
female form the egg-cells are surrounded by a tolerably thick granular
tissue.
[Illustration: Orthonectides (after Julin).
8. First female form: the cap-like anterior part has become detached
and the egg-cells (_eiz_) are escaping. 9. Second female form: _eiz_
= egg-cells; outside these are the muscular layer (_m_) and the
ectoderm (_ekt_). 10 and 11. Two fragments of such a female broken to
pieces by spontaneous division: the egg-cells are embedded in a
granular mass, and undergo embryonic development in it at a later
period; the whole is surrounded by ciliated cells. 12. Male
discharging the spermatozoa by the breaking up of the ectoderm
(_ekt_); _sp_ spermatozoa; _m_ muscle.]
There is nevertheless no doubt that in the first female form, when
sexually mature, the greater part, not only of the endoderm but of the
whole body, is made up of ova, so that the animal resembles a
thin-walled sac full of eggs. The ova escape by the bursting of the
thin ectoderm, and when they have all escaped, the thin disintegrated
membrane, composed of ciliated cells, is no longer in a condition to
live, and dies at once. This is the course of events as described by
Götte, and he is probably correct in his interpretation. This is the
real death of the Orthonectides, and if we regard them as low primitive
forms (Mesozoa), here for the first time in the ascending series we
meet with natural death. But the causes of this are scarcely so clear
as Götte seems to think when he ascribes it to the effect of
reproduction—an effect which is ‘not only empirically necessary, but
absolutely unavoidable.’ Such a necessity is explained by the fact that
the endoderm consists entirely of germ-cells. Now the life of the
organism, being dependent upon the mutual action of both layers, must
cease as soon as the whole endoderm is extruded during reproduction.
Arguments such as these pass over the presence of a mesoderm; but apart
from this omission, it does not appear to me so self-evident from a
purely physiological standpoint, that the ectodermal sheath with its
muscle layer must die after the extrusion of the germ-cells.
In those females to which Götte refers in this passage, the whole
sheath remains at first quite uninjured, with the exception of a small
cap at the anterior end, which is pushed off to give exit to the ova;
and inasmuch as the sheath continues to swim about in the nutritive
fluids after this has taken place, the proof is at any rate wanting
that it cannot support itself quite as well as before, although it has
lost the germ-cells.
Then why does it die? My answer to this is simple:—because it has lived
its time; because its length of life is limited to a period which
corresponds with the time necessary for complete reproduction. The
physical constitution of the body is so regulated that it remains
capable of living until the extrusion of the reproductive cells, and
then dies, however favourable external conditions may be for its
further support.
The correctness of this explanation is shown by a consideration of the
males and the second form of females; for in these cases the body falls
to pieces, not as a consequence of reproduction, but as a preparation
for it!
Götte only mentions the second female form in a note, in which he says,
it appears ‘that in the second female form of these animals the whole
body breaks into many pieces, and the superficial layer gradually
atrophies, so that it dies before the eggs are extruded.’ In Julin’s
account[76], upon which Götte bases his statements, there are, however,
some not unimportant differences. For instance, the eggs are not
extruded at all, but embryonic development takes place within the body
of the mother, which has previously undergone spontaneous division into
several pieces. In this case, the eggs differ from those of the other
female form, inasmuch as they do not constitute the whole of the
endoderm, but are embedded (as was stated above) in a fairly voluminous
granular mass at the expense of which, or at least by means of which,
they are nourished; for they increase considerably in size during their
development. But not only this granular mass, but all the layers of the
body of the mother, even the ectoderm, persist during the embryonic
development of the offspring. Indeed, the ectoderm must continue to
grow during the division of the mother animal, for it gradually covers
in the products of division on all sides, and, by means of its cilia,
causes the animal to swim about in the fluids of its host. After some
time the cilia are lost, and the separate parts into which the mother
animal has divided, fix themselves upon some part of the body-cavity of
the host; the young become free, and the remains of the body of the
mother probably disappear by dissolution and resorption[77]. In this
case the remains of the mother animal seem to be, to some extent,
consumed by the embryos,—a process which sometimes, although very
rarely, happens elsewhere. We can scarcely consider this as a primitive
arrangement, or look upon it as a proof that ‘reproduction’ has a
necessarily fatal effect upon the Polyplastid organism.
In the male, the mass of spermatozoa does not swell out the body to
such an extent that its walls must give way and thus permit an exit,
but the large ectoderm cells atrophy spontaneously at the time of
maturity, and as they fall off, exit is given to the spermatozoa here
and there. In this instance also the dissolution of the body is not a
consequence of reproduction, but reproduction can only take place when
the dissolution of the body has preceded it!
In this remarkable arrangement we cannot discern anything except an
evident adaptation of the life of the body-cells to reproductive
purposes, and this adaptation was rendered possible because, after the
evacuation of the sexual cells, the body ceased to be of any value for
the maintenance of the species.
But even if we assume, that the death of the Orthonectides is, in
Götte’s sense, a consequence of reproduction, inasmuch as, in the two
forms of females as well as in the male, the extrusion of a mass of
developed germ-cells or embryos deprives the organism of the
physiological possibility of living longer, how can we explain the
necessity of death as a direct consequence of reproduction in all
Polyplastides? Is the body—the _soma_—of the Metazoa so imperfectly
developed, as compared with the reproductive cells, that the extrusion
of the latter involves the death of the former? As a matter of fact in
the majority of cases the relations are reversed; the number of
body-cells usually exceeds the germ-cells a hundred- or a
thousand-fold, and the body is, as regards nutrition, so completely
independent of the reproductive cells, that it need not be in the least
disadvantageously affected by their extrusion. And if the
Orthonectid-like ancestors of the Metazoa were compelled to give up
their insignificant somatic part after the extrusion of their
germ-cells, because it could now no longer support itself, does it
therefore follow that the somatic cells had for ever lost the power of
surviving, even when their phyletic descendants were surrounded by more
favourable conditions? Had they to inherit ‘the necessity of death’ for
all time? Whence came this great change in the nature of organisms
which, before the differentiation of Homoplastids into Heteroplastids,
were endowed with the immortality of unicellular beings?
And it must be remembered that it is only an assumption which places
the Orthonectides among the lowest Metazoa (Heteroplastids). I do not
intend to greatly emphasize this point, but the formation of the
Gastrula by embole, and the absence of a mouth and alimentary canal,
shows that these parasites are extremely degenerate, and the same may
be said of almost all endoparasites. The Gastrula, as an independent
organism, was without doubt primitively provided with both mouth and
stomach, and the mass of ova filling the female Orthonectid is an
adaptation to a parasitic life, which on the one side renders the
possession of a stomach a superfluity, and on the other demands the
production of a great number of germ-cells[78]. It is certain that the
Orthonectides, as at present constituted, cannot have lived in the free
condition, and also that their adaptation to parasitism cannot have
arisen at the beginning of the phyletic development of Metazoa, because
they inhabit star-fishes and Nemertines—both relatively highly
developed Metazoa. Hence it is, at any rate, doubtful whether the
Orthonectides can claim to pass as typical forms of the lowest
Heteroplastids, and whether their reproduction can be looked upon ‘as
typical for the unknown ancestors of all Polyplastids’ (l. c., p. 45).
If, however, we accept some organism resembling these Orthonectides as
the most ancient Heteroplastid, being a free-living organism, it must
have had a stomach, and the cells surrounding it must—as a whole or in
part—have possessed the power of digesting; at any rate, they cannot
all have been germ-cells, and therefore it is improbable that death
would be the direct result of the extrusion of the germ-cells.