great ingenuity, and as far as possible to support his assumptions by
facts. Moreover, it cannot be denied that there are certain facts which
seem to indicate that the male germ-cell plays a different part from
that taken by the female germ-cell in the formation of a new organism.
For example, it is well known that the offspring of a horse and an ass
is different when the male parent is a horse from what it is when the
male parent is an ass. A stallion and a female ass produce a hinny
which is more like a horse, while a male ass and a mare produce a mule
which is said to be more like an ass[232]. I will refrain from
considering here the opinion of several authors (Darwin, Flourens, and
Bechstein) that the influence of the ass is stronger in both cases,
only predominating to a less extent when the ass is the female parent;
and I will for the sake of brevity accept Brooks’ opinion that in these
cases the influence of the father is greater than that of the mother.
Were this so in all cross-breeding between different species and in all
cases of normal fertilization, we should be compelled to conclude that
the influences of the male and female germ-plasms upon the offspring
differ at any rate in strength. But this is by no means always the
case, for even in horses the reverse may occur. Thus it is stated that
certain female race-horses have always transmitted their own
peculiarities, while others allowed those of the stallion to
preponderate.
In the human species the influence of the mother preponderates quite as
often as that of the father, although in many families most of the
children may take after either parent. There is nevertheless hardly any
large family in which all the children take after the same parent. If
we now try to explain the preponderating influence of one parent by the
supposition of a greater strength in hereditary power, without first
inquiring after some deeper cause, I think the only conclusion
warranted by the facts before us is that this power is rarely or never
equal in both of the conjugating germ-cells, but that even within the
same species, sometimes the male and sometimes the female is the
stronger, and that the strength may even vary in the different
offspring of the same individuals, as we so frequently see in human
families. The egg-cells of the same mother which ripen one after the
other, and also the sperm-cells of the same father, must therefore
present variations in the strength of their hereditary power. It is
then hardly to be wondered at that the relative hereditary power of the
germ-cells in different species should vary, although we cannot as yet
understand why this should be the case.
It would not be very difficult to render these facts intelligible in a
general way by an appeal to physiological principles. The quantity of
germ-plasm contained in a germ-cell is very minute, and together with
the idioplasms of the various ontogenetic stages to which it gives
rise, it must be continually increased by assimilation during the
development of the organism. If now this power of assimilation varied
in intensity, a relatively rapid growth of the idioplasm derived from
one of the parents would ensue, and with it the preponderance of the
hereditary tendencies of the parent in question. Now, it is obvious
that no two cells of the same kind are entirely identical, and hence
there must be differences in their powers of assimilation. Thus the
varying hereditary powers of the egg-cells produced from the same ovary
become explicable, and still more easily the varying powers of the
germ-cells produced in the ovaries or testes of different individuals
of the same species; most easily of all the differences observable in
this respect between the germ-cells of different species.
Of course, this hereditary power is always relative, as may be easily
proved by cross-breeding between different species and races. Thus when
a fantail pigeon is crossed with a laugher, the characters of the
former preponderate, but when crossed with a pouter the characters of
the latter preponderate[233]. The facts afforded by cross-breeding
between hybrids and one of the pure parent species, together with a
consideration of the resulting degree of variability, seem to me to be
even more unfavourable to Brooks’ view. They appear to me to admit of
an interpretation different from that brought forward by him; and when
he proceeds to make use of secondary sexual characters for the purpose
of his theory, I believe that his interpretation of the facts can be
easily controverted. It is hardly possible to conclude that variability
is due to the male parent, because the males in many species of animals
are more variable, or deviate further from the original type, than the
females. It is certainly true that in many species the male sex has
taken the lead in processes of transformation, while the female sex has
followed, but there is no difficulty in finding a better explanation of
the fact than that afforded by the assumption ‘that something within
the animal compels the male to lead and the female to follow in the
evolution of new breeds.’ Brooks has with great ingenuity brought
forward certain instances which cannot be explained with perfect
confidence by Darwin’s theory of sexual selection, but this hardly
justifies us in considering the theory to be generally insufficient,
and in having recourse to a theory of heredity which is as complicated
as it is improbable. The whole idea of the passage of gemmules from the
modified parts of the body into the germ-cells is based upon the
unproved assumption that acquired characters can be transmitted. The
idea that the male germ-cell plays a different part from that of the
female, in the construction of the embryo, seems to me to be untenable,
especially because it conflicts with the simple observation that upon
the whole human children inherit quite as much from the father as from
the mother.
------------------------------------------------------------------------
Footnotes for Appendices for Essay V.
Footnote 199:
Appendix to page 257.
Footnote 200:
l. c., p. 118.
Footnote 201:
l. c., p. 118.
Footnote 202:
Appendix to page 258.
Footnote 203:
l. c., p. 137.
Footnote 204:
Compare Brücke, ‘Farbenwechsel des Chamäleon.’ Wien. Sitzber. 1851.
Also Leydig, ‘Die in Deutschland lebenden Saurier,’ 1872.
Footnote 205:
‘Philosophical Transactions,’ vol. cxlviii. 1858, pp. 627-644.
Footnote 206:
Adler, ‘Beiträge zur Naturgeschichte der Cynipiden,’ Deutsche entom.
Zeitschr. XXI., 1877, p. 209; and by the same author, ‘Ueber den
Generationswechsel der Eichen-Gallwespen,’ Zeitschr. f. wiss. Zool.,
Bd. XXXV. 1880, p. 151.
Footnote 207:
Beyerinck, ‘Beobachtungen über die ersten Entwicklungsphasen einiger
Cynipidengallen,’ Verhandl. d. Amsterd. Akad. d. Wiss. Bd. XXII. 1883.
Footnote 208:
l. c., p. 144.
Footnote 209:
[It is now known that many such caterpillars are actually modified in
colour by their surroundings, but the process appears to be indirect
and secondarily acquired by the operation of natural selection, like
that of the change of colour in the chamaeleon, frogs, fish, etc.;
although the stimulus of light acts upon the eyes of the latter
animals and upon the skin of the caterpillar. See the seventh Essay
(pp. 394-397) for a more detailed account.—E. B. P.]
Footnote 210:
l. c., p. 150.
Footnote 211:
In order to make the case as simple as possible, I assume that the
insectivorous bird feeds upon a single species of insect, and that
the insect is only attacked by a single species of bird.
Footnote 212:
English Edition, translated by D’Arcy W. Thompson, B.A. London, 1883,
p. 509 et seqq.
Footnote 213:
Appendix to page 260.
Footnote 214:
Ch. Darwin, ‘On the fertilization of Orchids by Insects.’ London,
1877.
Footnote 215:
Compare Hermann Müller, ‘Die Befruchtung der Blumen durch Insekten
und die gegenseitigen Anpassungen beider.’ Leipzig, 1873. See also
many articles by the same author in ‘Kosmos,’ and other periodicals.
These later articles are included in the English translation by
D’Arcy W. Thompson.
Footnote 216:
‘Lectures on the Physiology of Plants,’ translated by H. Marshall
Ward, Oxford, 1887, p. 47.
Footnote 217:
Appendix to page 267.
Footnote 218:
Brown-Séquard, ‘Researches on epilepsy; its artificial production in
animals and its etiology, nature, and treatment.’ Boston, 1857. Also
various papers by the same author in ‘Journal de physiologie de
l’homme,’ Tome I and III, 1858, 1860, and in ‘Archives de physiologie
normale et pathologique,’ Tome I-IV, 1868-1872.
Footnote 219:
‘Oesterreichische medicinische Jahrbücher.’ Jahrgang, 1875, p. 179.
Footnote 220:
A direct transmission of the germs of disease through the
reproductive cells has lately been rendered probable in the case of
tuberculosis, for the bacilli have been found in tubercles in the
lungs of an eight-months’ fœtal calf, the mother being affected at
the time with acute tuberculosis. However it is not impossible that
infection may have arisen through the placenta. See ‘Fortschritte der
Medicin,’ Bd. III, 1885, p. 198.
Footnote 221:
Compare Unvericht, ‘Experimentelle und klinische Untersuchungen über
die Epilepsie.’ Berlin, 1883. With regard to the question of
hereditary transmission, the part of the brain in which the epileptic
centre is placed is of no importance.
Footnote 222:
Compare Ziemssen’s Handbuch der spec. Pathologie und Therapie.’ Bd.
XII. 2. Hälfte; Artikel ‘Epilepsie und Eklampsie.’ Leipzig, 1877.
Footnote 223:
l. c., p. 269.
Footnote 224:
It is generally known that the earlier physiologists believed in what
was called the ‘evolutionary theory,’ or the ‘theory of
preformation.’ This assumes that the germ contains, in a minute form,
the whole of the fully-developed animal. All the parts of the adult
are preformed in the germ, and development only consists in the
growth of these parts and their more perfect arrangement. This theory
was generally accepted until the middle of the last century, when
Kaspar Friedrich Wolff brought forward the theory of ‘epigenesis,’
which since that time has been the dominant one. This assumes that no
special parts of the germ are preformations of certain parts of the
fully-developed animal, and that these latter arise by a series of
changes in the germ, which gradually gives rise to them. In modern
times the theory of preformation has been revived in a less crude
form, as is shown by the ideas of Nägeli, and by Darwin’s
‘pangenesis.’—A. W., 1888.
Footnote 225:
Nägeli, l. c. p. 110.
Footnote 226:
See Darwin, ‘The Variation of Animals and Plants under
Domestication.’ 1875. Vol. I. p. 311.
Footnote 227:
Appendix to page 290.
Footnote 228:
Weismann, ‘Naturgeschichte der Daphnoiden,’ Zeitschrift f. wiss.
Zool. XXIII. 1879.
Footnote 229:
Appendix to page 277.
Footnote 230:
Compare W. K. Brooks, ‘The Law of Heredity, a Study of the Cause of
Variation, and the Origin of living Organisms.’ Baltimore, 1883.
Footnote 231:
l. c., p. 82.
Footnote 232:
This seems to be the general opinion (see the quotation from Huxley
in Brooks’ ‘Heredity,’ p. 127); but I rather doubt whether there is
such a constant difference between mules and hinnies. Furthermore, I
cannot accept the opinion that mules always resemble the ass more
than the horse. I have seen many mules which bore a much stronger
likeness to the latter. I believe that it is at present impossible to
decide whether there is a constant difference between mules and
hinnies, because the latter are very rarely seen, and because mules
are extremely variable. I attempted to decide the question last
winter by a careful study of the Italian mules, but I could not come
across a single hinny. These hybrids are very rarely produced,
because it is believed that they are extremely obstinate and
bad-tempered. I afterwards saw two true hinnies at Professor Kühn’s
Agricultural Institute at Halle. These hinnies by no means answered
to the popular opinion, for they were quite tractable and
good-tempered. They looked rather more like horses than asses,
although they resembled the latter in size. In this case it was quite
certain that one parent was a stallion and the other a female ass.—A.
W. 1889.
Footnote 233:
Darwin, ‘Variation of Animals and Plants under Domestication,’ 1875,
Vol. II. p. 41.
------------------------------------------------------------------------
VI.
ON THE NUMBER OF POLAR BODIES AND
THEIR SIGNIFICANCE IN HEREDITY.
1887.
------------------------------------------------------------------------
ON THE NUMBER OF POLAR BODIES AND THEIR SIGNIFICANCE IN HEREDITY.
PREFACE.
The following paper stands in close relation to a series of short
essays which I have published from time to time since the year 1881.
The first of these treated of ‘The Duration of Life,’ and the last of
‘The Significance of Sexual Reproduction.’ The present essay is most
intimately connected with that upon ‘The Continuity of the Germ-plasm,’
and has, in fact, grown out of the explanation of the meaning of polar
bodies in the animal egg, brought forward in that essay. The
explanation rested upon a trustworthy and solid foundation, as I am now
able to maintain with even greater confidence than at that time. It
rested upon the idea that in the egg-cell, a cell with a high degree of
histological differentiation, two different kinds of nuclear substance
exert their influence, one after the other. But continued investigation
has shown me that the explanation built upon this idea is only correct
in part, and that it does not exhaust the full meaning of the formation
of polar bodies. In the present essay I hope to complete the
explanation by the addition of essential elements, and I trust that, at
the same time, I shall succeed in throwing new light upon the
mysterious problems of sexual reproduction and parthenogenesis.
It is obvious that this essay can only contain an attempt at an
explanation, an hypothesis, and not a solution which is above
criticism, like the results of mathematical calculation. But no
biological theory of the present day can escape a similar fate, for the
mathematical key which opens the door leading to the secrets of life
has not yet been found, and a considerable period of time must elapse
before its discovery. But although I can only offer an hypothesis, I
hope to be able to show that it has not been rashly adopted, but that
it has grown in a natural manner from the secure foundation of
ascertained facts.
Nothing impresses the stamp of truth upon an hypothesis more than the
fact that its light renders intelligible not only those facts for the
explanation of which it has been framed, but also other and more
distantly related groups of phenomena. This seems to me to be the case
with my hypothesis, since the interpretation of polar bodies and the
ideas derived from it unite from very different points of view, the
facts of reproduction, heredity and even the transformation of species,
into a comprehensive system, which although by no means complete, is
nevertheless harmonious, and therefore satisfactory.
Only the most essential elements of the new facts which form the
foundation of the views developed in this essay will be briefly
mentioned. My object is to show all the theoretical bearings of these
new facts, not to describe them in technical detail. Such a description
accompanied by the necessary figures will shortly be given in another
place[234].
A. W.
Freiburg I. Br., _May 30, 1887_.
------------------------------------------------------------------------
ON THE NUMBER OF POLAR BODIES, &c.
CONTENTS.
PAGE
I. PARTHENOGENETIC AND SEXUAL EGG 339
The process of the formation of polar bodies very 339
widely distributed
The significance of polar bodies according to 340
Minot, Balfour, and van Beneden
My hypothesis of the removal of the histogenetic 341
part of the nucleus
Confirmation by the discovery of polar bodies in 345
parthenogenetic eggs
Parthenogenetic eggs form only _one_ polar body, 346
while eggs requiring fertilization form _two_
Parthenogenesis depends upon the fact that the 348
part of the nucleus which is expelled from sexual
eggs in the second polar body, remains in the egg
History of this discovery 349
II. SIGNIFICANCE OF THE SECOND POLAR BODY 352
Refutation of Minot’s theory 353
The second division of the nuclear spindle 355
involves a reduction of the ancestral germ-plasms
The theoretical necessity for such reduction 356
Phyletic origin of the germ-plasms in existing 357
species
The necessary reduction takes place by a special 358
form of nuclear division
The division which causes this reduction has 360
probably been already observed
Van Beneden’s and Carnoy’s observations 360
Two different physiological effects of 364
karyokinesis
Significance of direct nuclear division 365
Arguments in support of the view that the division 367
of the egg-nucleus which causes reduction must
occur at the end of ovogenetic development
Such nuclear division is to be found in the 368
formation of the second polar body
History of the origin of this view 368
III. THE FOREGOING CONSIDERATIONS APPLIED TO THE MALE 370
GERM-CELLS
The male germ-cells also require division in order 370
to reduce the ancestral germ-plasms
The germ-plasms of the parents must be contained 370
in the germ-plasm of the offspring
Advantages which the egg gains by the late 371
occurrence of the ‘reducing division’
The causes of unequal division in the formation of 373
polar bodies
These causes do not apply to the sperm-cell 373
Different kinds of nuclear division occur in 375
spermatogenesis
Some of these may be interpreted as ‘reducing 375
divisions’
The paranucleus (‘Nebenkern’) of spermatogenesis 376
probably contains the histogenetic nucleoplasm
IV. THE FOREGOING CONSIDERATIONS APPLIED TO PLANTS 377
V. CONCLUSIONS AS REGARDS HEREDITY 378
The germ-cell of an individual contains an unequal 378
combination of hereditary tendencies
Dissimilarity between the offspring of the same 379
parents
Identity of twins produced from a single egg 380
VI. RECAPITULATION 383
------------------------------------------------------------------------
VI.
ON THE NUMBER OF POLAR BODIES AND THEIR
SIGNIFICANCE IN HEREDITY.
I. Parthenogenetic and Sexual Egg.
Hitherto no value has been attached to the question whether an animal
egg produces one or two polar bodies. Several observers have found two
such bodies in many different groups of animals, both high and low in
the scale of organization. In certain species only one has been
observed, in others again three, four, or five (e. g. Bischoff, in the
rabbit). Many observers did not even record the number of polar bodies
found by them, and simply spoke of ‘polar bodies.’ As long as their
formation was looked upon as a process of secondary physiological
importance—as an ‘excretion,’ or a ‘process of purification,’ or even
as the ‘excreta’ (!) of the egg, as a ‘rejuvenescence of the nucleus,’
or of mere historical interest as a reminiscence of ancestral
processes, without any present physiological meaning—so long was it
unnecessary to attach any importance to the number of these bodies, or
to pay special attention to them. Of all the above-mentioned views, the
one which explained polar bodies as a mere reminiscence of ancestral
processes seemed to be especially well founded. Ten years ago we were
far from being able to prove that polar bodies occurred in all animal
eggs, and even in 1880, Balfour said in his excellent ‘Comparative
Embryology,’ ‘It is very possible, not to say probable, that such
changes [the formation of polar bodies] are universal in the animal
kingdom, but the present state of our knowledge does not justify us in
saying so[235].’
Even at the present day we are not, strictly speaking, justified in
making this assertion, for polar bodies have not yet been proved to
occur in certain groups of animals, such as reptiles and birds; but
they have been detected in the great majority of the large groups of
the animal kingdom, and wherever they have been looked for with the aid
of our modern highly efficient appliances, they have been found[236].
A deeper insight into the process of fertilization has above all led to
a closer study of antecedent phenomena.
O. Hertwig[237] and Fol[238] showed that the formation of polar bodies
was connected with a division of the nuclear substance of the egg.
Hertwig and Bütschli[239] then proved that the body expelled from the
egg possessed the nature of a cell, and thus led the way to the view
that the formation of polar bodies is a process of cell-division,
although a very unequal one. Even then there was no reason for
attaching any special importance to the number of these bodies; nor
should we have such a reason if we agreed with Minot[240],
Balfour[241], and van Beneden in ascribing a high physiological
significance to this process, and assumed that the expelled polar body
is the male part of the previously hermaphrodite egg-cell. We should
not know in what proportion the quantities of the ‘male’ and ‘female’
parts were present, and it would therefore be impossible to decide, _a
priori_, whether the ‘male’ part had to be removed from the body of the
egg-cell in one, two, or more portions.
Even after the view that the nuclear substance is the essential element
in fertilization had gained ground—a view chiefly due to Strasburger’s
investigations on the process of fertilization in Phanerogams—and after
Hertwig’s opinion had been confirmed, that the process of fertilization
is essentially the conjugation of nuclei, even then there appeared to
be no reason why the number of divisions undergone by the nucleus of
the mature egg should be looked upon as an essential feature.