of _Distoma_ and the Orthonectides, that he is inclined to believe
that the latter are Trematodes, ‘which in spite of sexual maturity
have not developed further than the embryonic condition of the
_Distoma_’ (‘Zur Entwicklungsgeschichte des Leberegels,’ Zool.
Anzeiger, 1881, No. 99). In reference to the Dicyemidae, which
resemble the Orthonectides in their manner of living and in their
structure, Gegenbaur has stated his opinion that they belong to a
‘stage in the development of Platyhelminthes’ (Grundriss d.
vergleich. Anatomie). Giard includes both in the ‘phylum Vermes,’ and
regards them as much degenerated by parasitism; and Whitman—the
latest investigator of the Dicyemids—speaks of them in a similar
manner in his excellent work ‘Contributions to the Life-history and
Classification of Dicyemids’ (Leipzig, 1882).
Footnote 79:
‘Dauer des Lebens;’ translated as the first essay in this volume.
Footnote 80:
See the first essay upon ‘The Duration of Life,’ p. 22 et seq.
Footnote 81:
‘Ursprung des Todes,’ p. 29.
Footnote 82:
l. c., p. 5.
Footnote 83:
See the preceding essay ‘On Heredity.’
Footnote 84:
The problem is very easily solved if we seek assistance from the
principle of panmixia developed in the second essay ‘On Heredity.’ As
soon as natural selection ceases to operate upon any character,
structural or functional, it begins to disappear. As soon, therefore,
as the immortality of somatic cells became useless they would begin
to lose this attribute. The process would take place more quickly, as
the histological differentiation of the somatic cells became more
useful and complete, and thus became less compatible with their
everlasting duration.—A. W. 1888.
Footnote 85:
See the preceding essay ‘On Heredity.’
Footnote 86:
See the first essay on ‘The Duration of Life.’
Footnote 87:
See the first essay on ‘The Duration of Life.’
Footnote 88:
These assumptions can be authenticated among the Infusoria. The
encysted _Colpoda cucullus_, Ehrbg. divides into two, four, eight, or
sixteen parts; _Otostoma Carteri_, into two, four, or eight; _Tillina
magna_, Gruber, into four or five; _Lagynus_ sp. Gruber, into two;
_Amphileptus meleagris_, Ehrbg. into two or four. The last two
species and many others frequently do not divide at all during the
encysted condition. But while any further increase in the number of
divisions within the cyst does not occur in free-swimming Infusoria,
the interesting case of _Ichthyophthirius multifiliis_, Fouquet,
shows that parasitic habits call forth a remarkable increase in the
number of divisions. This animal divides into at least a thousand
daughter individuals.
Footnote 89:
True development also takes place in the above-mentioned
_Ichthyophthirius_. While in other Infusoria the products of fission
exactly resemble the parent, in _Ichthyophthirius_ they have a
different form; the sucking mouth is wanting while provisional
clasping cilia are at first present. In this case therefore the word
germ may be rightly applied, and _Ichthyophthirius_ affords an
interesting example of the phyletic origin of germs among the lower
Flagellata and Gregarines. Cf. Fouquet, ‘Arch. Zool. Expérimentale,’
Tom. V. p. 159. 1876.
Footnote 90:
Bütschli, long ago, doubted the application of the fundamental law of
biogenesis to the Protozoa (cf. ‘Ueber die Entstehung der
Schwärmsprösslings der Podophrya quadripartita,’ Jen. Zeit. f. Med.
u. Naturw. Bd. X. p. 19, Note). Gruber has more recently expressed
similar views, and in fact denies the presence of development in the
Protozoa, and only recognizes growth (‘Dimorpha mutans, Z. f. W. Z.’
Bd. XXXVII. p. 445). This proposition must however be restricted,
inasmuch as a development certainly occurs, although one which is
coenogenetic and not palingenetic.
Footnote 91:
See the first essay on ‘The Duration of Life,’ p. 23 _et seq._
Footnote 92:
See Appendix to the first essay on ‘The Duration of Life,’ pp. 43-46.
Footnote 93:
See the first essay on ‘The Duration of Life,’ p. 21.
------------------------------------------------------------------------
IV.
THE CONTINUITY OF THE GERM-PLASM
AS THE FOUNDATION OF A THEORY OF HEREDITY.
1885.
------------------------------------------------------------------------
CONTINUITY OF THE GERM-PLASM, &c.
PREFACE.
The ideas developed in this essay were first expressed during the past
winter in a lecture delivered to the students of this University
(Freiburg), and they were shortly afterwards—in February and the
beginning of March—written in their present form. I mention this,
because I might otherwise be reproached for a somewhat partial use of
the most recent publications on related subjects. Thus I did not
receive Oscar Hertwig’s paper—‘Contributions to the Theory of Heredity’
(Zur Theorie der Vererbung), until after I had finished writing my
essay, and I could not therefore make as much use of it as I should
otherwise have done. Furthermore, the paper by Kölliker on ‘The
Significance of the Nucleus in the Phenomena of Heredity’ (Die
Bedeutung der Zellkerne für die Vorgänge der Vererbung), did not appear
until after the completion of my manuscript. The essential treatment of
the subject would not, however, have been altered if I had received the
papers at an earlier date, for as far as the most important point—the
significance of the nucleus—is concerned, my views are in accordance
with those of both the above-named investigators; while the points upon
which our views do not coincide had already received attention in the
manuscript.
A. W.
Freiburg I. Breisgau,
June 16, 1885.
------------------------------------------------------------------------
CONTINUITY OF THE GERM-PLASM, &c.
CONTENTS.
Introduction 165
I. The Germ-Plasm 174
1. Historical development of the theory as to the 174
localization of the germ-plasm in the nucleus
2. Nägeli’s ‘idioplasm’ is not identical with Weismann’s 180
‘germ-plasm’
3. A retransformation of somatic idioplasm into germ- 183
idioplasm does not take place
4. Confirmation of the theory as to the significance of the 185
nuclear substance afforded by Nussbaum’s and Gruber’s
experiments on regeneration in Infusoria
5. The nucleoplasm changes during ontogeny according to a 186
certain law
6. The identity of the daughter-nuclei produced by indirect 187
nuclear division, as assumed by Strasburger, is not
necessary for my theory
7. The gradual decrease in complexity of the structure of 190
the nucleus during ontogeny
8. Nägeli’s view on the germs (‘Anlagen’) in the idioplasm 192
9. The manner in which germ-cells arise from somatic cells 194
10. ’Embryonic’ cells in the mature organism 196
11. The rule of probability is against a retransformation of 198
somatic idioplasm into germ-plasm
12. The regular cyclical development of the idioplasm 199
founded upon phylogeny by Nägeli
13. It follows from phyletic considerations that the germ- 201
cells have not arisen at the end of ontogeny
14. They originally arose at the beginning of ontogeny, but 202
at a later period the time of their origin was displaced
15. A continuity of the germ-cells does not now exist in 205
most cases
16. But there is a continuity of the germ-plasm 205
17. Strasburger’s objection to my supposition that the germ- 209
plasm passes along distinct routes
18. The cell-body may remain unchanged when the nucleus is 210
changed
19. It is conceivable that all somatic nuclei may contain 211
some germ-plasm
II. The Significance of the Polar Bodies 212
1. The egg-cell contains two kinds of idioplasm; germ-plasm 213
and histogenetic nucleoplasm
2. The expulsion of the polar bodies signifies the removal 214
of the histogenetic nucleoplasm
3. Other theories as to the significance of the polar 214
bodies
4. The modes of occurrence of polar bodies 217
5. Their possible occurrence in male germ-cells 219
6. There are two kinds of nucleoplasm in the male germ- 219
cells
7. Polar bodies in plants 222
8. Morphological origin of polar bodies 223
III. On the Nature of Parthenogenesis 225
1. The phenomena exhibited in the maturation of the egg are 225
identical in parthenogenetic and sexual development
2. The difference between parthenogenetic and sexual cells 226
must be of a quantitative nature
3. The quantity of the germ-plasm determines development 227
4. The expulsion of polar bodies depends upon the 230
antagonism between germ-plasm and ovogenetic nucleoplasm
5. Fertilization does not act dynamically 231
6. An insufficient quantity of germ-plasm arrests 232
development
7. Relation of the nucleus to the cell 234
8. The case of the bee does not constitute any objection to 234
my theory
9. Strasburger’s views upon parthenogenesis 237
10. Parthenogenesis does not depend upon abundant nutrition 239
11. The indirect causes of sexual and parthenogenetic 241
reproduction
12. The direct causes 242
13. Explanation of the formation of nutritive cells 243
14. Identity of the germ-plasm in male and female germ-cells 246
Note 249
------------------------------------------------------------------------
IV.
THE CONTINUITY OF THE GERM-PLASM AS THE
FOUNDATION OF A THEORY OF HEREDITY.
Introduction.
When we see that, in the higher organisms, the smallest structural
details, and the most minute peculiarities of bodily and mental
disposition, are transmitted from one generation to another; when we
find in all species of plants and animals a thousand characteristic
peculiarities of structure continued unchanged through long series of
generations; when we even see them in many cases unchanged throughout
whole geological periods; we very naturally ask for the causes of such
a striking phenomenon: and enquire how it is that such facts become
possible, how it is that the individual is able to transmit its
structural features to its offspring with such precision. And the
immediate answer to such a question must be given in the following
terms:—‘A single cell out of the millions of diversely differentiated
cells which compose the body, becomes specialized as a sexual cell; it
is thrown off from the organism and is capable of reproducing all the
peculiarities of the parent body, in the new individual which springs
from it by cell-division and the complex process of differentiation.’
Then the more precise question follows: ‘How is it that such a single
cell can reproduce the _tout ensemble_ of the parent with all the
faithfulness of a portrait?’
The answer is extremely difficult; and no one of the many attempts to
solve the problem can be looked upon as satisfactory; no one of them
can be regarded as even the beginning of a solution or as a secure
foundation from which a complete solution may be expected in the
future. Neither Häckel’s[94], ‘Perigenesis of the Plastidule,’ nor
Darwin’s[95] ‘Pangenesis,’ can be regarded as such a beginning. The
former hypothesis does not really treat of that part of the problem
which is here placed in the foreground, viz. the explanation of the
fact that the tendencies of heredity are present in single cells, but
it is rather concerned with the question as to the manner in which it
is possible to conceive the transmission of a certain tendency of
development into the sexual cell, and ultimately into the organism
arising from it. The same may be said of the hypothesis of His[96],
who, like Häckel, regards heredity as the transmission of certain kinds
of motion. On the other hand, it must be conceded that Darwin’s
hypothesis goes to the very root of the question, but he is content to
give, as it were, a provisional or purely formal solution, which, as he
himself says, does not claim to afford insight into the real phenomena,
but only to give us the opportunity of looking at all the facts of
heredity from a common standpoint. It has achieved this end, and I
believe it has unconsciously done more, in that the thoroughly logical
application of its principles has shown that the real causes of
heredity cannot lie in the formation of gemmules or in any allied
phenomena. The improbabilities to which any such theory would lead are
so great that we can affirm with certainty that its details cannot
accord with existing facts. Furthermore, Brooks’[97] well-considered
and brilliant attempt to modify the theory of Pangenesis, cannot escape
the reproach that it is based upon possibilities, which one might
certainly describe as improbabilities. But although I am of opinion
that the whole foundation of the theory of Pangenesis, however it may
be modified, must be abandoned, I think, nevertheless, its author
deserves great credit, and that its production has been one of those
indirect roads along which science has been compelled to travel in
order to arrive at the truth. Pangenesis is a modern revival of the
oldest theory of heredity, that of Democritus, according to which the
sperm is secreted from all parts of the body of both sexes during
copulation, and is animated by a bodily force; according to this theory
also, the sperm from each part of the body reproduces the same part[98].
If, according to the received physiological and morphological ideas of
the day, it is impossible to imagine that gemmules produced by each
cell of the organism are at all times to be found in all parts of the
body, and furthermore that these gemmules are collected in the sexual
cells, which are then able to again reproduce in a certain order each
separate cell of the organism, so that each sexual cell is capable of
developing into the likeness of the parent body; if all this is
inconceivable, we must enquire for some other way in which we can
arrive at a foundation for the true understanding of heredity. My
present task is not to deal with the whole question of heredity, but
only with the single although fundamental question—‘How is it that a
single cell of the body can contain within itself all the hereditary
tendencies of the whole organism?’ I am here leaving out of account the
further question as to the forces and the mechanism by which these
tendencies are developed in the building-up of the organism. On this
account I abstain from considering at present the views of Nägeli, for
as will be shown later on, they only slightly touch this fundamental
question, although they may certainly claim to be of the highest
importance with respect to the further question alluded to above.
Now if it is impossible for the germ-cell to be, as it were, an extract
of the whole body, and for all the cells of the organism to despatch
small particles to the germ-cells, from which the latter derive their
power of heredity; then there remain, as it seems to me, only two other
possible, physiologically conceivable, theories as to the origin of
germ-cells, manifesting such powers as we know they possess. Either the
substance of the parent germ-cell is capable of undergoing a series of
changes which, after the building-up of a new individual, leads back
again to identical germ-cells; or the germ-cells are not derived at
all, as far as their essential and characteristic substance is
concerned, from the body of the individual, but they are derived
directly from the parent germ-cell.
I believe that the latter view is the true one: I have expounded it for
a number of years, and have attempted to defend it, and to work out its
further details in various publications. I propose to call it the
theory of ‘The Continuity of the Germ-plasm,’ for it is founded upon
the idea that heredity is brought about by the transference from one
generation to another, of a substance with a definite chemical, and
above all, molecular constitution. I have called this substance
‘germ-plasm,’ and have assumed that it possesses a highly complex
structure, conferring upon it the power of developing into a complex
organism. I have attempted to explain heredity by supposing that in
each ontogeny, a part of the specific germ-plasm contained in the
parent egg-cell is not used up in the construction of the body of the
offspring, but is reserved unchanged for the formation of the
germ-cells of the following generation.
It is clear that this view of the origin of germ-cells explains the
phenomena of heredity very simply, inasmuch as heredity becomes thus a
question of growth and of assimilation,—the most fundamental of all
vital phenomena. If the germ-cells of successive generations are
directly continuous, and thus only form, as it were, different parts of
the same substance, it follows that these cells must, or at any rate
may, possess the same molecular constitution, and that they would
therefore pass through exactly the same stages under certain conditions
of development, and would form the same final product. The hypothesis
of the continuity of the germ-plasm gives an identical starting-point
to each successive generation, and thus explains how it is that an
identical product arises from all of them. In other words, the
hypothesis explains heredity as part of the underlying problems of
assimilation and of the causes which act directly during ontogeny: it
therefore builds a foundation from which the explanation of these
phenomena can be attempted.
It is true that this theory also meets with difficulties, for it seems
to be unable to do justice to a certain class of phenomena, viz. the
transmission of so-called acquired characters. I therefore gave
immediate and special attention to this point in my first publication
on heredity[99], and I believe that I have shown that the hypothesis of
the transmission of acquired characters—up to that time generally
accepted—is, to say the least, very far from being proved, and that
entire classes of facts which have been interpreted under this
hypothesis may be quite as well interpreted otherwise, while in many
cases they must be explained differently. I have shown that there is no
ascertained fact, which, at least up to the present time, remains in
irrevocable conflict with the hypothesis of the continuity of the
germ-plasm; and I do not know any reason why I should modify this
opinion to-day, for I have not heard of any objection which appears to
be feasible. E. Roth[100] has objected that in pathology we everywhere
meet with the fact that acquired local disease may be transmitted to
the offspring as a predisposition; but all such cases are exposed to
the serious criticism that the very point that first needs to be placed
on a secure footing is incapable of proof, viz. the hypothesis that the
causes which in each particular case led to the predisposition, were
really acquired. It is not my intention, on the present occasion, to
enter fully into the question of acquired characters; I hope to be able
to consider the subject in greater detail at a future date. But in the
meantime I should wish to point out that we ought, above all, to be
clear as to what we really mean by the expression ‘acquired character.’
An organism cannot acquire anything unless it already possesses the
predisposition to acquire it: acquired characters are therefore no more
than local or sometimes general variations which arise under the
stimulus provided by certain external influences. If by the
long-continued handling of a rifle, the so-called ‘Exercierknochen’ (a
bony growth caused by the pressure of the weapon in drilling) is
developed, such a result depends upon the fact that the bone in
question, like every other bone, contains within itself a
predisposition to react upon certain mechanical stimuli, by growth in a
certain direction and to a certain extent. The predisposition towards
an ‘Exercierknochen’ is therefore already present, or else the growth
could not be formed; and the same reasoning applies to all other
‘acquired characters.’