exhibit the smallest differences, must really differ in that their
nuclei must contain different kinds of nucleoplasm, which are present
in different proportions. Germ-cells which are to produce a finely
granular, brick-red, winter yolk (_Moina rectirostris_) must possess an
ovogenetic nucleoplasm of a somewhat different molecular structure from
those germ-cells which have only to form a few large blue fat-globules,
as in the summer-eggs of the same species. It is further probable that
different proportions obtain between germ-plasm and ovogenetic
nucleoplasm, in these two kinds of germ-cells; and it would be a very
simple explanation of the otherwise obscure part played by the
food-cells, if we were to suppose that they do not contain any
germ-plasm at all, and on this account do not enter upon embryonic
development, but are arrested after growing to a certain size. Such an
explanation, however, would not by itself show why they subsequently
undergo gradual solution in the surrounding fluids. But since we know
that egg-cells also begin to undergo solution as soon as the parent
Daphnid is poorly nourished, we can hardly help also referring the
solution of the food-cells to insufficient nourishment, occurring as
soon as the egg-cell, after the attainment of a certain size, exercises
a superior power of assimilation. But hitherto we could not in any way
understand why the third out of a group of germ-cells should always
gain this superior power and become an egg-cell. If it could be shown
that its position is more highly favoured in respect of nutrition, we
could understand why it outstrips the other three in development, and
thus prevents them from further growth. But nothing of the kind can be
shown to occur with any degree of probability, as I have previously
mentioned in my works on the subject. At that time, having no better
explanation, I adopted the view in question, although only as a
provisional interpretation. It was not possible for me to seek in the
substance of those four apparently identical cells for the cause of
their different development; but now I am justified in offering the
supposition that during the division of a primitive germ-cell into two,
and afterwards into four germ-cells, an unequal division of the
nucleoplasms takes place, in that one of the four cells receives
germ-plasm as well as ovogenetic nucleoplasm, while the other three
receive the latter alone. Similarly, the fact that the second cell of
the group may occasionally become an egg is also intelligible, although
this fact remained quite inexplicable by my former interpretation. The
fact that true egg-cells, or even the whole ovary with all its
germ-cells, may break up and become absorbed when the animal has been
starved for a certain period of time, seems to me to be no objection to
our present view, any more than the fact that an Infusorian may die
from starvation would be an objection to the supposition of the
immortality of unicellular organisms. The growth of an organism is not
only arrested by its constitution, but also by absolute want of food;
but it would be very foolish to explain the differences in size of the
various species of animals as results of the different conditions of
nutrition to which they were subject. Just as a sparrow, however highly
nourished, could never attain the size or form of an eagle, so a
germ-cell destined to become a summer-egg could never attain the size,
form, or colour of a winter-egg. It is by internal constitutional
causes that the course of development is determined in both these
cases; and in the latter, the cause can hardly be anything more than
the different constitution of the nucleoplasms.
All these considerations depend upon the supposition that the
egg-nucleus contains two kinds of idioplasm, viz. germ-plasm and
ovogenetic nucleoplasm. I have not hitherto brought forward any direct
evidence in favour of this assumption, but I believe that such proofs
can be obtained.
It is well known that there are certain eggs in which the polar bodies
are not expelled until after the entrance of spermatozoa. Brooks[172]
has already made use of this fact as evidence against Minot’s and
Balfour’s theory; for he quite rightly concludes that if the polar
bodies really possess the significance of male cells, we cannot
understand why such eggs are unable to develope without fertilization,
when they still possess the male half of the nucleus necessary for
development. But such eggs (e.g. that of the oyster) do not develope,
but always die if they remain unfertilized.
This argument can only be met by a new hypothesis, the construction of
which I must leave to the defenders of the above-mentioned theory. But
the observation in question seems to me to furnish at the same time a
proof of the co-existence of two different nucleoplasms in the
egg-nucleus. If the nucleoplasm of the polar bodies was also
germ-plasm, we could not understand why such eggs are unable to
develope parthenogenetically, for at least as much germ-plasm is
contained in the unfertilized egg as would have been present after
fertilization.
The only objection which can be raised against this conclusion depends
upon the supposition that the nucleoplasm of the sperm-cell is
qualitatively different from that of the egg-cell. I have already dealt
with this view, but I should wish to refer to it again rather more in
detail. Some years ago I expressed the opinion[173] that the
physiological values of the sperm-cell and of the egg-cell must be
identical; that they stand in the ratio of 1 : 1. But Valaoritis[174]
has brought forward the objection that if we consider the function of a
cell as the measure of its physiological value, it is only necessary to
point to the respective functions of ovum and spermatozoon in order to
show that their physiological values must be different. ‘The egg-cell
alone, by passing more or less completely through the phyletic stages
of the female parent, developes into a similar organism; and although
the presence of the spermatozoon is in most cases required in order to
render possible such a result, the cases of parthenogenesis prove
nevertheless that the egg can do without this stimulus.’ This objection
appeared to be fully justified as long as fertilization was looked upon
as the ‘vitalization of the germ,’ and so long as the sperm-cell was
considered as merely ‘the spark that kindles the gunpowder,’ and
further so long as the germ-substance was believed to be contained in
the cell-body. But now we can hardly give to the body of the egg-cell a
higher significance than that of the common nutritive soil of the two
nuclei which conjugate in fertilization. But these two nuclei ‘are not
different in nature,’ as Strasburger says, and as I fully believe. They
cannot differ in kind, for they both consist of germ-plasm belonging to
the same species of animal or plant; and there cannot be any deeper
contrast between them such as would correspond to the differences
between mature individuals. They cannot, from their essential nature,
exercise any special attraction upon each other, and when we see that
sperm-cell and egg-cell do nevertheless attract each other, as has been
shown in both plants and animals, such a property must have been
secondarily acquired, and has no other significance than to favour the
union of sexual cells—an arrangement which may be compared to the
vibrating flagellum of the spermatozoon or the micropyle of the egg,
but which is not fundamental, and is not based upon the molecular
structure of the germ-plasm. In lower plants, Pfeffer has proved that
certain chemical stimuli emanate from the egg and attract the
spermatozoid; and according to Strasburger, the synergidae in the upper
part of the embryo-sac of Phanerogams secrete a substance which is
capable of directing the growth of the pollen-tube towards the
egg-cell. In animals it is only known as yet that spermatozoa and ova
do attract each other, so that the former find the latter and bore
their way through its membranes. It has also been shown that the
substance of the egg-body moves towards the penetrating spermatozoon
(‘_cones d’exsudation_’ in _Asteridae_: Fol); and that it sometimes
enters upon convulsive movements (_Petromyzon_). Here therefore a
mutual stimulation and attraction must exist; and perhaps we must also
assume that there is an attraction between the two conjugating nuclei,
for we cannot readily understand how the cytoplasm alone could direct
the one to the other, as Strasburger supposes. According to
Strasburger’s hypothesis, we must suppose that part of the specific
cytoplasm of the sperm-cell continues to surround the nucleus after it
has penetrated into the body of the egg. But however this may be, the
assumed attraction between the conjugating nuclei certainly cannot
depend upon the molecular structure of their germ-plasm, which is the
same in both, but it must be due to some accessory circumstance. If it
were possible to introduce the female pronucleus of an egg into another
egg of the same species, immediately after the transformation of the
nucleus of the latter into the female pronucleus, it is very probable
that the two nuclei would conjugate just as if a fertilizing
sperm-nucleus had penetrated. If this were so, the direct proof that
egg-nucleus and sperm-nucleus are identical would be furnished.
Unfortunately the practical difficulties are so great that it is hardly
possible that the experiment can ever be made; but such want of
experimental proof is partially compensated for by the fact,
ascertained by Berthold, that in certain Algae (_Ectocarpus_ and
_Scytosiphon_) there is not only a female, but also a male
parthenogenesis; for he shows that in these species the male germ-cells
may sometimes develope into plants, which however are very weakly[175].
Furthermore the process of conjugation may be considered as a proof
that this view as to the secondary importance of sexual differentiation
is the true one. At the present time there can hardly be any hesitation
in accepting the view that conjugation is the sexual reproduction of
unicellular organisms. In these the two conjugating cells are almost
always identical in appearance, and there is no evidence in favour of
the assumption that they are not also identical in molecular structure,
at least so far as one individual of the same species may be identical
with another. But there are also forms in which the conjugating cells
are distinctly differentiated into male and female, and these are
connected with the former by a gradual transition: thus in _Pandorina_,
a genus of _Volvocineae_, we are unable to make out any differences
between the conjugating cells, while large egg-cells and minute
sperm-cells exist in the closely allied _Volvox_. If we must suppose
that the conjugation of two entirely identical Infusoria has the same
physiological effect as the union of two sexual cells in higher animals
and plants, we cannot escape the conclusion that the process is
essentially the same throughout: and that therefore the differences,
which are perhaps already indicated in _Pandorina_ and are very
distinct in _Volvox_ and in all higher organisms, have nothing to do
with the nature of the process, but are of quite secondary importance.
If we further take into account the extremely different constitution of
the two kinds of sexual cells in size, appearance, membranes, motile
power, and finally in number, no doubt remains that these differences
are only adaptations which secure the meeting of the two kinds of
conjugating cells: that in each species they are adaptations to the
peculiar conditions under which fertilization takes place.
NOTE.
It is of considerable importance for the proper appreciation of the
views advanced in the present essay, to ascertain whether a polar body
is or is not expelled from eggs which develope parthenogenetically. I
wish therefore to briefly state that I have recently succeeded in
proving the formation of a polar body of distinctly cellular structure
in the summer-eggs of _Daphnidae_. I propose to publish a more detailed
account in a future paper.
A. W.
_June 22, 1885._
------------------------------------------------------------------------
Footnotes for Essay IV.
Footnote 94:
Häckel, ‘Ueber die Wellenzeugung der Lebenstheilchen etc.,’ Berlin,
1876.
Footnote 95:
Darwin, ‘The Variation of Animals and Plants under Domestication,’
vol. ii. 1875, chap. xxvii. pp. 344-399.
Footnote 96:
His, ‘Unsre Körperform etc.,’ Leipzig, 1875.
Footnote 97:
Brooks, ‘The Law of Heredity,’ Baltimore, 1883.
Footnote 98:
Galton’s experiments on transfusion in Rabbits have in the mean time
really proved that Darwin’s gemmules do not exist. Roth indeed states
that Darwin has never maintained that his gemmules make use of the
circulation as a medium, but while on the one hand it cannot be shown
why they should fail to take the favourable opportunities afforded by
such a medium, inasmuch as they are said to be constantly circulating
through the body; so on the other hand we cannot understand how the
gemmules could contrive to avoid the circulation. Darwin has acted
very wisely in avoiding any explanation of the exact course in which
his gemmules circulate. He offered his hypothesis as a formal and not
as a real explanation.
Professor Meldola points out to me that Darwin did not admit that
Galton’s experiments disproved pangenesis (‘Nature,’ April 27, 1871,
p. 502), and Galton also admitted this in the next number of ‘Nature’
(May 4, 1871, p. 5).—A. W. 1889.
Footnote 99:
Weismann, ‘Ueber die Vererbung.’ Jena, 1883; translated in the
present volume as the second essay ‘On Heredity.’
Footnote 100:
E. Roth, ‘Die Thatsachen der Vererbung.’ 2. Aufl., Berlin, 1885, p.
14.
Footnote 101:
Jäger, ‘Lehrbuch der allgemeinen Zoologie,’ Bd. II. Leipzig, 1878.
Footnote 102:
M. Nussbaum, ‘Die Differenzirung des Geschlechts im Thierreich,’
Arch. f. Mikrosk. Anat., Bd. XVIII. 1880.
Footnote 103:
I have since learnt that Professor Rauber of Dorpat also expressed
similar views in 1880; and Professor Herdman of Liverpool informs me
that Mr. Francis Galton had brought forward in 1876 a theory of
heredity of which the fundamental idea in some ways approached that
of the continuity of the germ-plasm (‘Journal of the Anthropological
Institute,’ vol. v; London, 1876).—A. W., 1888.
[A less complete theory was brought forward by Galton at an earlier
date, in 1872 (see Proc. Roy. Soc. No. 136, p. 394). In this paper he
proposed the idea that heredity chiefly depends upon the development
of the offspring from elements directly derived from the fertilized
ovum which had produced the parent. Galton speaks of the fact that
‘each individual may properly be conceived as consisting of two
parts, one of which is latent and only known to us by its effects on
his posterity, while the other is patent, and constitutes the person
manifest to our senses. The adjacent and, in a broad sense, separate
lines of growth in which the patent and latent elements are situated,
diverge from a common group and converge to a common contribution,
because they were both evolved out of elements contained in a
structureless ovum, and they, jointly, contribute the elements which
form the structureless ova of their offspring.’ The following diagram
shows clearly ‘that the span of each of the links in the general
chain of heredity extends from one structureless stage to another,
and not from person to person:—
Structureless elements {...Adult Father... } structureless elements
in Father {...Latent in Father...} in Offspring.’
Again Galton states—‘Out of the structureless ovum the embryonic
elements are taken ... and these are developed (_a_) into the visible
adult individual; on the other hand ..., after the embryonic elements
have been segregated, the large residue is developed (_b_) into the
latent elements contained in the adult individual.’ The above quoted
sentences and diagram indicate that Galton does not derive the whole
of the hereditary tendencies from the latent elements, but that he
believes some effect is also produced by the patent elements. When
however he contrasts the relative power of these two influences, he
attaches comparatively little importance to the patent elements. Thus
if any character be fixed upon, Galton states that it ‘may be
conceived (1) as purely personal, without the concurrence of any
latent equivalents, (2) as personal but conjoined with latent
equivalents, and (3) as existent wholly in a latent form.’ He argues
that the hereditary power in the first case is exceedingly feeble,
because ‘the effects of the use and disuse of limbs, and those of
habit, are transmitted to posterity in only a very slight degree.’ He
also argues that many instances of the supposed transmission of
personal characters are really due to latent equivalents. ‘The
personal manifestation is, on the average, though it need not be so
in every case, a certain proof of the existence of latent elements.’
Having argued that the strength of the latter in heredity is further
supported by the facts of reversion, Galton considers it is safe to
conclude ‘that the contribution from the patent elements is very much
less than from the latent ones.’ In the later development of his
theory, Galton adheres to the conception of ‘gemmules’ and accepts
Darwin’s views, although ‘with considerable modification.’ Together
with pangenesis itself, Galton’s theory must be looked upon as
_preformational_, and so far it is in opposition to Weismann’s theory
which is _epigenetic_. See Appendix IV. to the next Essay (V.), pp.
316-319.—E. B. P.]
Footnote 104:
Nägeli, ‘Mechanisch-physiologische Theorie der Abstammungslehre.’
München u. Leipzig, 1884.
Footnote 105:
O. Hertwig, ‘Beiträge zur Kenntniss der Bildung, Befruchtung und
Theilung des thierischen Eies.’ Leipzig, 1876.
Footnote 106:
Fol, ‘Recherches sur la fécondation, etc.’ Genève, 1879.
Footnote 107:
Kölliker formerly stated, and has again repeated in his most recent
publication, that the spermatozoa (‘Samenfäden’) are mere nuclei. At
the same time he recognizes the existence of sperm-cells in certain
species. But proofs of the former assertion ought to be much stronger
in order to be sufficient to support so improbable a hypothesis as
that the elements of fertilization may possess a varying
morphological value. Compare Zeitschr. f. wiss. Zool., Bd. XLII.
Footnote 108:
F. M. Balfour, ‘Comparative Embryology,’ vol. i. p. 69.
Footnote 109:
Arch. f. mikr. Anat., Bd. 23. p. 182, 1884.
Footnote 110:
Born, ‘Biologische Untersuchungen,’ I, Arch. Mikr. Anat., Bd. XXIV.
Footnote 111:
Roux, ‘Beiträge zum Entwicklungsmechanismus des Embryo,’ 1884.
Footnote 112:
O. Hertwig, ‘Welchen Einfluss übt die Schwerkraft,’ etc. Jena, 1884.
Footnote 113:
[Our present knowledge of the development of vegetable ova (including
the position of the parts of the embryo) is also in favour of the
view that it is not influenced by external causes, such as
gravitation and light. It takes place in a manner characteristic of
the genus or species, and essentially depends on other causes which
are fixed by heredity, see Heinricher ‘Beeinflusst das Licht die
Organanlage am Farnembryo?’ in Mittheilungen aus dem Botanischen
Institute zu Graz, II. Jena, 1888.—S. S.]
Footnote 114:
E. van Beneden, ‘Recherches sur la maturation de l’œuf,’ etc., 1883.
Footnote 115:
M. Nussbaum, ‘Ueber die Veränderung der Geschlechtsprodukte bis zur
Eifurchung,’ Arch. Mikr. Anat., 1884.
Footnote 116:
Eduard Strasburger, ‘Neue Untersuchungen über den Befruchtungsvorgang
bei den Phanerogamen als Grundlage für eine Theorie der Zeugung.’
Jena, 1884.
[It is now generally admitted that, in the Vascular Cryptogams, as
also in Mosses and Liverworts, the bodies of the spermatozoids are
formed by the nuclei of the cells from which they arise. Only the
cilia which they possess, and which obviously merely serve as
locomotive organs, are said to arise from the surrounding cytoplasm.
It is therefore in these plants also the nucleus of the male cell
which effects the fertilization of the ovum. See Göbel, ‘Outlines of
Classification and Special Morphology,’ translated by H. E. F.
Garnsey, edited by I. B. Balfour, Oxford, 1887, p. 203, and Douglas
H. Campbell, ‘Zur Entwicklungsgeschichte der Spermatozoiden,’ in
Berichte d. deutschen bot. Gesellschaft, vol. v (1887), p. 120.—S. S.]
Footnote 117:
O. Hertwig, ‘Das Problem der Befruchtung und der Isotropie des
Eies.’ Jena, 1885.
Footnote 118:
This opinion was first expressed in my lecture, ‘Ueber die Dauer des
Lebens,’ Jena, 1882, translated as the first essay in the present
volume.
Footnote 119:
M. Nussbaum, ‘Sitzungber. der Niederrheinischen Gesellschaft fur
Natur- und Heilkunde.’ Dec. 15, 1884.
Footnote 120:
A. Gruber, ‘Biologisches Centralblatt,’ Bd. IV. No. 23, and V. No. 5.
Footnote 121:
According to the observations of Nussbaum and van Beneden, the egg of
_Ascaris_ departs from the ordinary type, but I think that the latter
observer goes too far when he concludes from the form of the nuclear
spindle (of which the two halves are inclined to each other at an
angle) that we have before us a process entirely different from that
of ordinary nuclear division.
Footnote 122:
Trinchese, ‘I primi momenti dell’ evoluzione nei molluschi,’ Atti
Acad. Lyncei (3) vii. 1879, Roma.
Footnote 123:
M. Nussbaum, ‘Archiv für Mikroskopische Anatomie,’ Bd. XVIII und
XXIII.
Footnote 124:
Valaoritis, ‘Die Genesis des Thier-Eies.’ Leipzig, 1882.
Footnote 125:
Kölliker, ‘Die Bedeutung der Zellkerne,’ etc.; Zeitschr. f. wiss.
Zool. Bd. XLII.
Footnote 126:
‘Compt. rend.’ Tom. LIV. p. 150.
Footnote 127:
‘Entwicklung der Dipteren.’ Leipzig, 1864.
Footnote 128:
‘Zeitschr. f. wiss. Zool.’ Bd. XVI. p. 389 (1866).
Footnote 129:
‘Compt. rend.’ Nov. 13, 1882.
Footnote 130:
Grobben, ‘Arbeiten d. Wien. Zool. Instituts,’ Bd. II. p. 203.
Footnote 131:
Bütschli, ‘Zeitschrift f. wiss. Zool.’ Bd. XXIII. p. 409.
Footnote 132:
‘Science,’ vol. iv. No. 90, 1884.
Footnote 133:
Among unicellular organisms, encysted individuals are often called
germs. They sometimes differ from the adult organism in their smaller
size and simpler structure (_Gregarinidae_), but they represent the
same morphological stage of individuality.
Footnote 134:
Compare Bütschli in Bronn’s ‘Klassen und Ordnungen des Thierreichs,’
Bd. I. p. 777.
Footnote 135:
Gustav Jäger, ‘Lehrbuch der Allgemeinen Zoologie,’ Leipzig, 1878; II.
Abtheilung. Probably on account of the extravagant and superficial
speculations of the author, the valuable ideas contained in his book
have been generally overlooked. It is only lately that I have become
aware of Jäger’s above-mentioned hypothesis. M. Nussbaum seems to
have also arrived at the same conclusion quite independently of
Jäger. The latter has not attempted to work out his hypothesis with
any degree of completeness. The above-mentioned observations are
followed immediately by quite valueless considerations, as, for
instance, that the ontogenetic and phyletic groups are in concentric
ratio! The author might as well speak of a quadrangular or triangular
ratio!
Footnote 136:
[Facts of the same kind are also known in the Vascular Cryptogams,
Muscineae, Characeae, Florideae, etc.—S. S.]
Footnote 137:
Weismann, ‘Die Entstehung der Sexualzellen bei den Hydromedusen.’
Jena, 1883.
Footnote 138:
[I adopt this term, suggested by E. Ray Lankester and G. C. Bourne,
as the name of the supporting lamina of Coelenterata. See ‘Quart.
Journ. Microsc. Sci.’ Jan. 1887, p. 28.—E. B. P.]
Footnote 139:
Dr. Clemens Hartlaub, ‘Ueber die Entstehung der Sexualzellen bei
Obelia.’ Freiburg, Inaugural Dissertation: see also ‘Zeitschrift für
wissenschaftliche Zoologie.’ Bd. XLI. 1884.
Footnote 140:
English translation, by H. Marshall Ward. Oxford, 1887, Clarendon
Press.
Footnote 141:
[Such gland-cells are known in both animals and plants. See W.
Gardiner and Tokutaro Ito, On the structure of the mucilage-secreting
cells of _Blechnum occidentale_ L., and _Osmunda regalis_ L., ‘Annals
of Botany,’ vol. i. p. 49.—S. S.]
Footnote 142:
Thus in 1877 Bütschli thought that ‘the chief significance of the
formation of polar bodies lies in the removal of part of the nucleus
of the egg, whether this removal is effected by simple expulsion or
by the budding of the egg-cell.’ ‘Entwicklungsgeschichtliche
Beiträge;’ Zeitschrift für wissenschaftliche Zoologie, Bd. XXIX. p.
237, footnote.
Footnote 143:
C. S. Minot, ‘Account, etc.;’ Proc. Boston Soc. Nat. Hist. vol. xix.
p. 165, 1877.
Footnote 144:
E. van Beneden and Boveri have recently, quite independently of each
other, made a more exact study of these ‘Polkörperchen’
(‘Centrosoma,’ Boveri). They show that nuclear division starts from
these bodies, although the mode of origin of the latter is not yet
quite clear.—A. W., 1888.
Footnote 145:
The existence of polar bodies in sponges has been recently proved by
Fiedler: Zool., Anzeiger., Nov. 28, 1887.—A. W., 1888.
Footnote 146:
They have now been observed in many species, so that their general
occurrence in insects is tolerably certain. Compare bibliography
given in Weismann and Ischikawa, ‘Weitere Untersuchungen zum
Zahlengesetz der Richtungskörper,’ ‘Zoolog. Jahrbücher,’ vol. iii.
1888, p. 593.—A. W., 1888.
Footnote 147: