xxiv GENERAL INTRODUCTION. 



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 (Turbellaria) ; motile, except in Crtistacea 

 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, ^ s - J an ^ 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 Nematodes, 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 r . 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 



