Feb. 6, 1890] 



NATURE 



321 



only one to hold it. Waldeyer (" Ueber Karyokinese und 

 ihre Beziehung zu den Befruchtungs-vorgange," Arch, 

 mikr. Anat., xxxii., 1888) has considered the observed 

 facts insufficient to justify the regarding of the nuclear 

 loops as idioplasm ; Whitman (" The Seat of Formative 

 and Regenerative Energy," Boston, 1888) among zoologists 

 has expressed himself against this view, and the same occurs 

 in the recent book of Geddes and Thomson (" The Evolu- 

 tion of Sex," London, 1889). The facts which led me to 

 the idea that the nuclear threads were the real carriers of 

 heredity — were, in fact, the idioplasma— are enumerated 

 in Essay IV. ; they were primarily the observations of E. 

 van Beneden on the phenomena of fertilization in the 

 ovum of Ascarta megalocephala, ihost of Strasburger on 

 fertilization in the Phanerogams by a mere nucleus, and 

 the researches of Nussbaum and Gruber on division in 

 the Infusoria. One may further cite as of essential im- 

 portance the facts of karyokinesis/i?r se, and the circum- 

 stance that, only on the supposition that the nucleus 

 contains the idioplasma can the extrusion of polar bodies 

 from the animal ovum be rendered comprehensible. The 

 latter process divides the nuclear substance of the ovum 

 into two quantitatively equal halves, but the body of the 

 ovum into two unequal halves, the size of which is different 

 in every species. The essential part of the process must 

 therefore be the division of the nuclear substance, not 

 that of the cell-mass. These facts on reflection so com- 

 pletely convinced me that the nucleus alone acts as carrier 

 of hereditary tendencies, that the theory of the physio- 

 logical equality of the nuclei of the sexual elements which 

 I had propounded ten years before (1873) struck me as a 

 certainty ; and I then advanced the theory of fertilization 

 which is contained on p. 246 of Essay IV. I believe 

 that till recently Strasburger and I alone had expressed 

 similar views of the essence of fertilization, at least so far 

 as relates to the homodynamy of the sexual nuclei. That 

 most distinguished observer, E. van Beneden, who has 

 won such renown in the investigation of the process of 

 fertilization, took his stand with regard to its theoretical 

 significance on the platform of the older view, which re- 

 garded it as the union of two elements intrinsically and 

 essentially the opposite of each other. He could not free 

 himself from that dominant and deeply rooted idea, that 

 the difference between the sexes is something fundamental, 

 an essential principle of existence. The fertilized oosperm 

 is in his eyes a hermaphrodite object, uniting in itself 

 both male and female essences, an idea in which many 

 other observers (cf. Kolliker, " Die Bedeutung der 

 Zellenkerne fiir die Vorgange der Vererbung," Zeii. wiss. 

 ZooL, xlii., 1885) have followed him, and of which the 

 logical sequence is that all the cells of the body are to be 

 regarded as hermaphrodite ! 



Van Beneden was also influenced by the idea which 

 sways the naturalists of so many countries, that fertiliza- 

 tion is a process of rejuvenescence, in the sense that 

 without it life cannot be prolonged to the end. Many 

 still hold to this idea ; Maupas (" Recherches expdr. sur 

 la multiplication des infusoires cilids," Arch. zool. exp. 

 gen., (2) vi. p. 165) very recently believed that he had 

 found a proof of its correctness, and attempted to show 

 that Infusoria, for a continuance of existence, must from 

 time to time enter into conjugation, or die from internal 

 causes if this conjugation be prevented. Even were his 

 observations correct, they would still fall short of proving 

 his conclusions ; they would prove nothing against the 

 immortality of the Protozoa, or for a rejuvenescence in 

 the sense here intended ; they would rather state the 

 platitude that ovum and spermatozoon must die, if the 

 condition of their continued existence, namely fusion, 

 inevitable in most species of plants and animals, be 

 prohibited ; but this is an accidental, not a natural, 

 death. Richard Hertwig (" Ueber die Conjugation der 

 Infusorien," Miinchen, 1889) has also briefly shown that 

 the facts, on which Maupas bases his inference, are not 



universally true ; that Infusoria hindered from conjuga- 

 tion do not die, but increase by division, and may pro- 

 duce whole colonies of animals — nay, that they are 

 generally thus rendered abnormally prolific. 



I am distinctly opposed to the rejuvenescence theory, 

 whether applied to unicellular or multicellular organisms ; 

 my view is expressed in Essay IV., and may be sum- 

 marized in this position — we should no longer speak of 

 the conjugating nuclei of the sexual elements as male 

 and female, but as paternal and maternal, there is no 

 opposition of the one to the other, they are essentially 

 alike, and differ only so far as one individual differs from 

 another of the same species. Fertilization is no process 

 of rejuvenescence, but merely a union of the hereditary 

 tendencies of two individuals ; tendencies which are 

 bound up with the matter of the nuclear loops ; the cell- 

 body of the ovum and spermatozoon is indifferent in this 

 connection, and plays merely the part of a nutritive 

 matter which is modified and shaped by the dominant 

 idioplasm of the nucleus in a definite way, as clay in the 

 sculptor's hand. The different appearance and function 

 of ovum and spermatozoon, and their mutual attraction, 

 rest on secondary adaptations, qualified to ensure that 

 they shall meet and that their idioplasmata shall come 

 into contact, &c. ; and as with the cells, so the differentia- 

 tion oi persons into male and female is also secondary ; 

 all the numerous differences of form and function which 

 characterize sex in the higher animals, the so-called 

 " secondary sexual characters,'' which reach even into 

 the highest spiritual regions of mankind, are nothing but 

 adaptations to ensure the union of the hereditary ten- 

 dencies of two individuals. 



These are briefly the views of fertilization which I 

 have indicated since 1873, but have only published in a 

 finished and definite shape since the discovery by van 

 Beneden of the morphological processes in the fertiliza- 

 tion of the ovum of Ascaris (Essay IV., 1885). I con- 

 cluded then with these words : — " If it were possible to 

 introduce the female pro-nucleus of an tgg into another 

 egg of the same species, immediately after the transforma- 

 tion of the latter into the female pro-nucleus, it is very 

 probable that the two nuclei would conjugate just as if a 

 fertilizing sperm-nucleus had penetrated [the ovum]. 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 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 develop 

 into plants, which however are very weakly." 



I have since attempted to fertilize one frog's &%g with 

 the nucleus of another ; the experiment was, as one 

 would expect, not successful, owing to the enormous 

 havoc caused by introducing a cannula into the egg ; but 

 Boveri (" Ein geschlechtlich erzeugter Organismus ohne 

 miitterliche Eigenschaften," Ges. Morph. Physiol. Miin- 

 chen, 16 Jul!, 1889) was more fortunate, in finding an object 

 which allowed of the converse experiment to mine ; follow- 

 ing Hertwig's example, he removed the nucleus from an 

 Echinoid ovum by agitation, and brought such denucleated 

 ova to develop by introducing spermatozoa. From the 

 spermatozoan nucleus was formed a regular segmentation- 

 nucleus, the embryogenesis pursued its regular course, 

 and there was formed a complete though small free-swim- 

 ming larva, which lived for a week. From this experiment 

 alone it follows that the views of Strasburger and myself 

 on fertilization are correct, viz. that the sperm-nucleus can 

 play the part of ovum-nucleus and vice versd, and the 

 older view, to which Prof. Vines (" Lectures on the Physio- 

 logy of Plants," Cambridge, 1886, pp. 638-681) has also 

 sworn allegiance, must be given up. 



