xxiv GENERAL INTRODUCTION. 



r 



nucleus or nuclear moiety they contain pass through mitotic changes 1 . 

 In some Rotifera, Crustacea, and Insecta one polar body only is formed, 

 and the ovum then proceeds to segment. Such ova are termed partheno- 

 genetic, and the process parthenogenesis. But in the vast majority of multi- 

 cellular animals it is necessary for the ovum to be impregnated, i.e. it must 

 fuse or conjugate with another cell, the spermatozoon, just as in some 

 unicellular animals a temporary or permanent conjugation between two 

 individuals is requisite from time to time to perpetuate the race. The 

 spermatozoon is typically a flagellate cell, with or without the addition of 

 a vibratile membrane ; it is rarely amoeboid ; sometimes of very various 

 shapes even in the same class (Turbellarid) ; motile, except in Crustacea 

 and a few other Arthropods 2 . It is produced by the repeated fission of 

 a cell, or of part of a cell, belonging to a testis, an organ homologous 

 with the ovary. During its evolution, a process analogous to the formation 

 of polar bodies in the ovum, or homologous with it, is supposed to occur 

 very generally 3 . The testis may co-exist with the ovary in the same 



1 The formation of polar globules is certainly due to cell-fission ; the fact that the nucleus under- 

 goes mitotic changes during their appearance is sufficient to prove the point. They may themselves 

 divide again, and their nuclear fragment show mitosis : see especially Trinchesi, ' Evoluzione nei 

 Molluschi,' Atti Acad. Lyncei, (3), vii. 1879, P^ 5 - J anc ^ 8. But they are commonly degenerate in 

 structure. For recent observations see A. Thomson, 'Recent Researches on Oogenesis,' Q. J. M. 

 xxvi. 1886, p. 591, with lit. p. 605. Their significance is a difficult question. If a spermatozoon 

 enters the ovum, e. g. in an Asterias before they are formed, no union between the male and female 

 pronuclei takes place until the process is completed. A single polar body is found in partheno- 

 genetic ova (Weismann, SB. Natf. Ges. Freiburg, i. B., iii. (i), 1887), a fact that disproves the 

 view that the bodies are a male element which must be got rid of before impregnation, or a safeguard 

 against self-fertilisation. See the views of Weismann, Nature, xxxvi., p. 607, and Minot, American 

 Naturalist, xiv. 1880, p. 106 ; cf. Balfour, Comp. Embryology, i. pp. 61-4, and E. van Beneden, 

 'Recherches sur la fecondation,' Arch, de Biologic, iv. 1883, especially pp. 482, 527, 603 et seqq. 

 Compare with the account given by the last-named, Carnoy, on the germinal vesicle and polar 

 globules in Ascaris megalocephala, ' La Cellule,' Lierre,ii. (i), 1886 ; in sundry Nemat odes, Id. op. cit. 

 iii. 1887, and ibid, in the appendix to the 'conference.' For views connected with the import of the 

 nucleus, &c. to the doctrine of Heredity, see Weismann, ' Die Continuitat des Keimplasma's,' Jena, 

 1885, summarised by Moseley, Nature, xxxiii. 1885-6, p. 154; Id. 'Die Bedeutung der sexuellen 

 Fortpflanzung,' &c., Jena, 1886, similarly summarised, Nature, xxxiv. 1886, p. 629 ; also Kolliker, 

 ' Die Bedeutung der Zellkerne,' &c., Z. W. Z. xlii. 1885, summarised in the American Naturalist, xix. 

 1885, p. 1222; Id. 'Das Karyoplasma,' &c., Z. W T . Z. xliv. 1886; O. Hertwig, 'Das Problem der 

 Befruchtung,' &c., J. Z. xviii. 1885, and the chapters on the reproduction of plants in Vines, Phy- 

 siology of Plants, Cambridge, 1886, or the corresponding lectures in Sachs, 'Physiology of Plants,' 

 transl. by Marshall Ward, Clarendon Press, 1887; also Geddes, 'Theory of Growth,' &c., Proc. Roy. 

 Soc. Edinburgh, 1886. 



2 The immobility of the spermatozoa of Crustacea and some other Arthropoda is somewhat 

 doubtful. The spermatozoon of Cypris becomes active when transferred to the female ; and that of 

 the Cladoceran Polyphemus shows amoeboid motion (Zaccharias, Z. W. Z. xli. 1885). 



3 For the accessory globule of the spermatozoa, see E. van Beneden and Julin, Bull. Acad. Roy. 

 Belg. (3), vii. 1884, p. 322 ; Brown, Q. J. M. xxv. 1885, pp. 350-1, 357 ; and A. Thomson, 'Recent 

 Researches on Oogenesis,' Q. J. M. xxvi. 1886, pp. 596-8. The accessory globule has been supposed 

 to get rid of a female element in the developing spermatozoon and thus to be homologous with a 



