To bring together shortly the results of this essay:—the fundamental fact upon which everything else is founded is the fact that _two_ polar bodies are expelled, as a preparation for embryonic development, from all animal eggs which require fertilization, while only _one_ such body is expelled from all parthenogenetic eggs.
This fact in the first place refutes every purely morphological explanation of the process. If it were physiologically valueless, such a phyletic reminiscence of the two successive divisions of the egg-nucleus must have been also retained by the parthenogenetic egg.
In my opinion the expulsion of the first polar body implies the removal of ovogenetic nucleoplasm when it has become superfluous after the maturation of the egg has been completed. The expulsion of the second polar body can only mean the removal of part of the germ-plasm itself, a removal by which the number of ancestral germ-plasms is reduced to one half. This reduction must also take place in the male germ-cells, although we are not able to associate it confidently with any of the histological processes of spermatogenesis which have been hitherto observed.
Parthenogenesis takes place when the whole of the ancestral germ-plasms, inherited from the parents, are retained in the nucleus of the egg-cell. Development by fertilization makes it necessary that half the number of these ancestral germ-plasms must be first expelled from the egg, the original quantity being again restored by the addition of the sperm-nucleus to the remaining half.
In both cases the beginning of embryogenesis depends upon the presence of a certain, and in both cases equal, quantity of germ-plasm. This certain quantity is produced by the addition of the sperm-nucleus to the egg requiring fertilization, and the beginning of embryogenesis immediately follows fertilization. The parthenogenetic egg contains within itself the necessary quantity of germ-plasm, and the latter enters upon active development as soon as the single polar body has removed the ovogenetic nucleoplasm. The question which I have raised on a previous occasion—‘When is the parthenogenetic egg capable of development?’—now admits of the precise answer—‘Immediately after the expulsion of the polar body.’
From the preceding facts and considerations the important conclusion results that the germ-cells of any individual do not contain the same hereditary tendencies, but are all different, in that no two of them contain exactly the same combinations of hereditary tendencies. On this fact the well-known differences between the children of the same parents depend.
But the deeper meaning of this arrangement must doubtless be sought for in the individual variability which is thus continuously kept up and is always being forced into new combinations. Thus sexual reproduction is to be explained as an arrangement which ensures an ever-varying supply of individual differences.
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Footnotes for Essay VI.
Footnote 234:
See Berichten der Naturforschenden Gesellschaft zu Freiburg i. B., Band III. (1887) Heft I, ‘Ueber die Bildung der Richtungskörper bei thierischen Eiern,’ by August Weismann and C. Ischikawa.
Footnote 235:
Vol. I. p. 60.
Footnote 236:
The most recent example of this kind is afforded by the excellent work of O. Schultze, ‘Ueber die Reifung und Befruchtung des Amphibieneies,’ Zeitschr. f. wiss. Zool., Bd. XLV. 1887. Schultze has proved that two polar bodies are expelled from the egg of the Axolotl and of the frog, although all previous observers, including O. Hertwig, had been unable to find them. Thus the latter authority states as the result of an investigation specially directed towards this point, that the nucleus is transformed in a peculiar manner (‘Befruchtung des thierischen Eies,’ III. p. 81).
Footnote 237:
O. Hertwig, ‘Beiträge zur Kenntniss der Bildung, Befruchtung, und Theilung des thierischen Eies,’ Morpholog. Jahrbuch, I, II, and III. 1875-77.
Footnote 238:
H. Fol, ‘Recherches sur la fécondation et le commencement de l’hénogénie chez divers animaux.’ Genève, Bâle, Lyon, 1879.
Footnote 239:
Bütschli, ‘Entwicklungsgeschichtliche Beiträge,’ Zeitschr. f. wiss. Zool. Bd. XXIX. p. 237. 1877.
Footnote 240:
C. S. Minot, ‘Account, etc.’ Proceedings Boston Soc. Nat. Hist., vol. xix. p. 165. 1877.
Footnote 241:
F. M. Balfour, ‘Comparative Embryology.’
Footnote 242:
Nägeli, ‘Mechanisch-physiologische Theorie der Abstammungslehre,’ München und Leipzig, 1884.
Footnote 243:
See the second and fourth Essays in the present volume.
Footnote 244:
Hensen, ‘Die Grundlagen der Vererbung,’ Zeitschr. f. wiss. Landwirthschaft. Berlin, 1885, p. 749.
Footnote 245:
O. Hertwig, ‘Lehrbuch der Entwicklungsgeschichte des Menschen und der Wirbelthiere.’ Jena, 1886.
Footnote 246:
Bütschli, ‘Gedanken über die morphologische Bedeutung der sog. Richtungskörperchen,’ Biol. Centralblatt, Bd. VI. p. 5. 1884.
Footnote 247:
This observation was first published as a note at the end of the fourth Essay in the present volume. See p. 249.
Footnote 248:
Weismann, ‘Richtungskörper bei parthenogenetischen Eieren,’ Zool. Anzeiger, 1886, p. 570.
Footnote 249:
Blochmann, ‘Ueber die Richtungskörper bei den Insekteneiern,’ Biolog. Centralblatt., April 15, 1887.
Footnote 250:
F. Stuhlmann, ‘Die Reifung des Arthropodeneies nach Beobachtungen an Insekten, Spinnen, Myriapoden und Peripatus,’ Berichte der naturforschenden Gesellschaft zu Freiburg i. Br., Bd. I. p. 101.
Footnote 251:
In the summer-eggs of Rotifera I have, together with Mr. Ischikawa, observed one polar body, and we were able to establish for certain that a second is not formed. The nuclear spindle had already been observed by Tessin, and Billet had noticed polar bodies in _Philodina_, but without attaching any importance to their number. These latter observations were not conclusive proofs of the formation of polar bodies in parthenogenetic eggs, so long as it was not known whether the summer-eggs of Rotifera may develope parthenogenetically, or whether they can only develope in this way. Knowing now that parthenogenetic eggs expel only one polar body, we may perhaps be permitted to draw the conclusion that the summer-egg of a Rotifer (_Lacinularia_) which expelled only one polar body must have been a parthenogenetic egg. But I may add that we have also succeeded in directly proving the occurrence of parthenogenesis in Rotifera, as will be described in detail in another paper.
Footnote 252:
See Essay IV, Part III. p. 225.
Footnote 253:
E. Bessels, ‘Die Landois’sche Theorie, widerlegt durch das Experiment.’ Zeitschr. f. wiss. Zool. Bd. XVIII. p. 124. 1868.
Footnote 254:
l. c., p. 110.
Footnote 255:
Strasburger, ‘Neue Untersuchungen über den Befruchtungsvorgang bei den Phanerogamen als Grundlage einer Theorie der Zeugung.’ Jena, 1884.
Footnote 256:
Wilhelm Roux, ‘Ueber die Bedeutung der Kerntheilungsfiguren.’ Leipzig, 1884.
Footnote 257:
E. van Beneden, ‘Recherches sur la maturation de l’œuf, la fécondation et la division cellulaire.’ Gand et Leipzig, Paris, 1883.
Footnote 258:
J. B. Carnoy, ‘La Cytodiérèse de l’œuf, la vésicule germinative et les globules polaires de l’Ascaris megalocephala.’ Louvain, Gand, Lierre, 1886.
Footnote 259:
See p. 364.
Footnote 260:
Wilhelm Roux, ‘Beiträge zur Entwicklungsmechanik des Embryo,’ No. 3, Breslauer ärztliche Zeitschrift, 1885, p. 45.
Footnote 261:
Carnoy, ‘La Cytodiérèse chez les Arthropodes.’ Louvain, Gand, Lierre, 1885.
Footnote 262:
Flemming, ‘Neue Beiträge zur Kenntniss der Zelle.’ Arch. f. mikr. Anat. Bd. XXIX, 1887.
Footnote 263:
Carnoy, ‘La Cytodiérèse de l’œuf; la vésicule germinative et les globules polaires chez quelques Nématodes.’ Louvain, Gand, Lierre. 1886.
Footnote 264:
Hensen, ‘Die Grundlagen der Vererbung nach dem gegenwärtigen Wissenskreis,’ Zeitschr. f. wissenschaftl. Landwirthschaft, Berlin, 1885, p. 731.
Footnote 265:
See the preceding Essay on ‘The Significance of Sexual Reproduction in the theory of Natural Selection.’
Footnote 266:
E. van Beneden and Julin, ‘La Spermatogénèse chez l’Ascaride mégalocéphale.’ Brussels, 1884.
Footnote 267:
Carnoy, ‘La Cytodiérèse chez les Arthropodes.’
Footnote 268:
Gustav Platner, ‘Die Karyokinese bei den Lepidopteren als Grundlage für eine Theorie der Zelltheilung.’ Internation. Monatsschrift f. Anatomie und Histologie, Bd. III. Heft 10. Leipzig, 1886.
Footnote 269:
La Valette St. George, ‘Ueber die Genese der Samenkörper.’ Fünfte Mittheilung. Die Spermatogenese bei den Säugethieren und dem Menschen,’ Archiv f. mikrosk. Anat. Bd. XV. 1878.
Footnote 270:
Weismann, ‘Studien zur Descendenztheorie,’ ii. p. 306, Leipzig, 1876, translated by Meldola; see ‘Studies in the Theory of Descent,’ p. 680.
Footnote 271:
l. c., p. 92.
Footnote 272:
[The similar conclusion that identical ova lead to the appearance of identical individuals was drawn from the same data by Francis Galton in 1875. See ‘The history of the Twins, as a criterion of the relative powers of Nature and Nurture,’ by Francis Galton, F.R.S., Journal of the Anthropological Institute, 1875, p. 391; also by the same author, ‘Short Notes on Heredity, etc. in Twins,’ in the same Journal, 1875, p. 325.
The author investigated about eighty cases of close similarity between twins, and was able to obtain instructive details in thirty-five of these. Of the latter there were no less than seven cases ‘in which both twins suffered from some special ailment or had some exceptional peculiarity;’ in nine cases it appeared that ‘both twins are apt to sicken at the same time;’ in eleven cases there was evidence for a remarkable association of ideas; in sixteen cases the tastes and dispositions were described as closely similar. These points of identity are given in addition to the more superficial indications presented by the failure of strangers or even parents to distinguish between the twins. A very interesting part of the investigation was concerned with the after-lives of the thirty-five twins. ‘In some cases the resemblance of body and mind had continued unaltered up to old age, notwithstanding very different conditions of life,’ in the other cases ‘the parents ascribed such dissimilarity as there was, wholly, or almost wholly, to some form of illness.’
The conclusions of the author are as follows: ‘Twins who closely resembled each other in childhood and early youth, and were reared under not very dissimilar conditions, either grow unlike through the development of natural characteristics which had lain dormant at first, or else they continue their lives, keeping time like two watches, hardly to be thrown out of accord except by some physical jar. Nature is far stronger than nurture within the limited range that I have been careful to assign to the latter.’ And again, ‘where the maladies of twins are continually alike, the clocks of their two lives move regularly on, and at the same rate, governed by their internal mechanism. Necessitarians may derive new arguments from the life histories of twins.’
The above facts and conclusions held for twins of the same sex, of which at any rate the majority are shown by Kleinwächter’s observations to have been enclosed in the same embryonic membranes, and therefore presumably to have been derived from a single ovum; but in rarer cases the twins, although also invariably of the same sex, were marked by remarkable differences, greater than those which usually distinguish children of the same family. Mr. Galton met with twenty of these cases. In such twins the conditions of training, etc. had been as similar as possible, so that the evidence of the power of nature over nurture is strongly confirmed. Mr. Galton writes, ‘I have not a single case in which my correspondents speak of originally dissimilar characters having become assimilated through identity of nurture. The impression that all this evidence leaves on the mind is one of wonder whether nurture can do anything at all, beyond giving instruction and professional training.’
The fact that twins produced from a single ovum seem to be invariably of the same sex is in itself extremely interesting, for it proves that the sex of the individual is predetermined in the fertilized ovum.—E. B. P.]
Footnote 273:
Fol, Recherches sur la fécondation et le commencement de l’hénogénie: Genève, Bâle, Lyon. 1879.
Footnote 274:
Born, ‘Ueber Doppelbildungen beim Frosch und deren Entstehung.’ Breslauer ärztl. Zeitschrift, 1882.
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