304 LOCOMOTORY AND PROTOPLASMIC MOVEMENTS 



division might present certain advantages. The latest researches seem, however, to 

 show that the nucleus is absent from, or at least not yet differentiated in, certain lower 

 organisms l . It is still possible that the chromatin elements may be present, but 

 diffusely distributed 2 , although it is to be remembered that the structures to which 

 this general term is given cannot be precisely identical in all organisms, but must 

 exhibit greater or smaller dissimilarities capable of hereditary transmission. 



Protoplasmic fusion. Contact must naturally precede fusion, but does not 

 necessarily produce it, as for instance when similar or dissimilar organs of the cell, 

 or dissimilar protoplasts, come into contact. Thus the closest contact does not 

 produce fusion between the plasmodia of different species of Myxomycetes 3 , whereas 

 plasmodia of the same species readily unite. Even when a fragment of a foreign 

 plasmodium is ingested by another species no fusion occurs between them 4 . In the 

 case of the swarm-cells of Aethalium the capacity for fusion only appears at a certain 

 stage of development, and hence it is possible under suitable conditions to permanently 

 prevent the appearance of the fusion stage so that no plasmodium is formed 5 . Actual 

 fusion does not occur in Dictyoslelium and other Acrasiae, although the amoebae 

 come into close contact and form an aggregate plasmodium 6 . Similarly, sperms do 

 not fuse with one another, but readily unite with appropriate ova, in which the fusion 

 of male and female pro-nuclei ultimately occurs. It is worthy of note that immediately 

 after the entry into the ovum changes take place at the surface which prevent the 

 penetration of additional sperms. Probably it was owing to the suppression of these 

 changes by the agency of chloral hydrate that Hertwig was able to cause the entry of 

 a number of spermatozoids into the egg of a sea-urchin 7 . The production of hybrids 

 shows that the protoplasts of dissimilar species may unite, and it is possible that 

 successful grafting involves the fusion of the interprotoplasmic connexions in the 

 neighbouring cells of scion and stock 8 . 



In addition to intimate contact at some point or other, fusion involves the 

 rupture of the intervening surface-tension films. This occurs naturally when the 

 whole of the intervening medium is displaced at any one point, for the existence of 

 the surface-tension film is dependent upon contact with a dissimilar non-wetting 

 medium. Hence the presence of impurities on the surface of drops of mercury hinders 

 their fusion greatly, and the same result will be attained whenever a thin layer of the 

 surrounding medium is maintained between two drops of similar liquid 9 . It is owing 



1 See especially in regard to bacteria, Hinze, Ber. d. hot. Ges., 1901, p. 369; Unters. ii. d. Bau 

 von Beggiatoa mirabilis, 1902; Schaudinn, Archiv f. Protistenkunde, 1902, Bd. I, p. 335; Ernst, 

 Centralbl. f. Bact., 1902, Bd. vin, Abth. ii, p. i ; Biitschli, Protozoen, 1880, p. 107. 



2 R. Hertwig, 1. c., 1902, p. 6. 



3 Cienkowski, Jahrb. f. wiss. Bot., 1863, Bd. Ill, p. 337. 



4 Celakovsky, Flora, 1892, Ergzbd., p. 215. 



5 Klebs, Jahrb. f. wiss. Bot., 1900, Bd. xxxv, p. 196. 



6 Cf. Zopf in Schenk's Handbnch d. Botanik, 1887, Bd. Ill, Abth. ii, p. 22 ; Potts, Flora, 

 1902, Ergzbd., p. 281. 



7 O. Hertwig, Zelle u. Gewebe, 1893, p. 93. 



8 Strasburger, Jahrb. f. wiss. Bot., 1901, Bd. xxxvi, pp. 586, 592; Meyer, Bot. Ztg., 1902, 



P- 173- 



9 Quincke, Pfliiger's Archiv f. Physiol., 1879, Bd. xix, p. 129 ; Berthold, Protoplasmamechanik, 

 1886, p. 107; Rhumbler, Biol. Centralbl., 1898, Bd. xvm, p. 115. 



