126 



SCIENCE. 



[Vol. VIII, No- 



bis paper ' on heredity (p. 29 ff .), discusses the rela- 

 tion of nuclei to growth very fully and ably. The 

 great extent of bis learning has enabled him to 

 present the manifold aspects of the question more 

 thoroughly than any other writer. His argumen- 

 tation seems to me so satisfactory that it does not 

 require the weight of his great authority to estab- 

 lish the conclusion that without nuclei there is no 

 growth. Of this, the most faith-compelling evi- 

 dence is offered by the important experiments of 

 Nussbaum and Gruber,'' who found that when 

 unicellular animals are artificially divided, the 

 fragments containing nuclei continue to grow, 

 while pieces without nuclei die off. 3°. The large 

 unicellular Thahophytes, such as Caulerpa and 

 Codium, become multinuclear before they attain 

 their adult size. Further illustrations are given 

 by Kolliker {I. c, pp. 19-20). 4". Perhaps the 

 most striking demonstration of the importance of 

 the nucleus is afforded by the experimental altera- 

 tion of the plane of division of the ovum. Pfitiger ' 

 showed that the plane of the first division of the 

 ovum is altered by tilting the ovum before the 

 division begins, and keeping it in the same position 

 during division ; normally the plane passes through 

 the white pole, but when the ovum is fastened in 

 an oblique position, the plane is not in the axis of 

 the ovum but in the line of gravity. Born " has 

 continued these remarkable experiments, and dis- 

 covered that the nucleus changes its position 

 when the ovum is kept tilted, and that the site of 

 the nucleus determines the plane of division of the 

 ovum. 



Still more pertinent to the theme of this article 

 are the phenomena of the impregnation of the 

 ovum.^ In 1873 Biitschli '^ discovered that two 

 nuclei are present in the fertilized ovum of 

 Rhabditis dolichura, a nematod worm, and 

 that the two nuclei unite, becoming the first 

 nucleus of the embryo. Oscar Hertwig ' proved 



1 '(Die bedeutuDg der zellenkerne fiir die vorgange der 

 vererbung,' in Zeitschr. f. wiss. zool., xlii. pp. 1-4G. 



^ Science, vol. vi. p. 4. See also Nussbaum's later paper 

 in the Archiv filr mikrosJcop. anat., xxvi. p. 485. Nassbaum 

 also cites Pr. Schmitz's experiments on the artificial di- 

 vision of plants. Schmitz's paper I have not seen : it was 

 publibhed in 1879, in the Festschrift der naturforschenden 

 gesellschaft zu Halle. 



3 Pfluger''s Archiv fiir die gesammte physiol., xxxii. 

 pp. 1-80. 



* Breslauer arztlich. zeitschr., 22 Marz. 1884. 1 have not 

 seen the original. There is an abstract in Hofmann und 

 Schwalbe's Jahresbericht for 1884, p. 444. 



!> For a synopsis of recent investigations, the reader is 

 referred to the article ' Impregnation ' by the author in 

 Wood's ' Handbook.' 



8 ' Beitrage zur kenntniss der frellebenden nematoden,' 

 in Nova acta, xxxvi. 1773. 



'■ ' Beitrage zur kenntniss der bildung, befruchtung und 

 theilung des thierischen eies,' in Morphol. jahrbuch, i. 



three years later that the two nuclei, or as they 

 are better called, pronuclei, are derived, one from 

 the nucleus of the ovum, the other probably from 

 the fertilizing male element, the spermatozoon. 

 His observations, which w^ere made at that time on 

 echinoderms, led him to the theory that "impreg- 

 nation depends upon the fusion of two sexually 

 differentiated nuclei." Both Heitwig himself 

 and many others, notably Fol Selenka, Flemming, 

 Platner, and Strassburger, have confirmed this 

 conclusion, so that there is a very strong presump- 

 tion in favor of Hertwig's theory being a true law 

 for all cases of fertilization. Strassburger was 

 for some time ^ an opponent of the exclusive 

 significance of the nuclei, holding the opinion that 

 " there also occurs a copulation between the other 

 equivalent parts of the spermatozoon and ovum," 

 thus making the participation of cell protoplasm 

 essential. But lately " he has acceded to Hertwig's 

 opinion, and has expressed himself in a recent 

 publication '" with great distinctness in favor of 

 the nuclei alone being essential to impregnation. 

 Strassburger observed in some cryptogams the 

 protoplasm of the male element to be so much 

 reduced that hardly more than the nucleus re- 

 mained, and found that in certain phanerogams 

 only the nucleus of the pollen grain reaches the 

 ovum. 



The next point to be brought forward is that 

 the spermatozoon, which forms one of the pro- 

 nuclei, is in many animals developed exclusively 

 from the nucleus. The formation of the sperma- 

 tozoon has been much investigated, and yet very 

 little thoroughly satisfactory work has been pub- 

 lished in result. Although the great majority of 

 the articles report more or less that is valuable, 

 yet they also contain, too often, much that is 

 crude, inaccurate, or even out and out false ; so 

 that it is a difficult task to unsnarl the truth from 

 the mesh of error in which it is ravelled. Kolli- 

 ker," as long ago as 1841, advanced the hypothesis 

 that the spermatozoa of all animals have the 

 significance of nuclei. This is not quite correct, 

 since the seminal corpuscles of nematods have the 

 value of cells, as do probably also those of the 

 higher Crustacea, and possibly of other animals. 

 It still remains true that in the majority of cases 

 the spermatozoa are modified nuclei, and nuclei 

 only. As regards the higher animals, the obser- 



s Ueber befruchtung und zelltheilung, 1878, pp. 75-77. 



' Ueber den bau und das wachsthum der zellhdute, 1882, 

 pp. 250-5J5-3. 



1" Neue xmtersuchungen ilber den befruchtungs-vorgang 

 bei den phanerogamen als grundlage fiir eine theorie der 

 zeugung, Jena, 1884 (see p. 77). 



11 Beitrage zur kenntniss der geschlechtsverhdltnisse 

 und der samenflilssigkeit wirbelloser thiere, nebst einem 

 versuch, etc., Berlin, 1841, 



