RELATION BETWEEN CELLULAR AND NUCLEAR DIVISION 43 



Not only may such changes as are involved in the substitution of 

 amitosis for mitosis, in the reduction of the chromosomes, and in the 

 production of giant nuclei, occur at different periods of development, but 

 also the external conditions may influence the shape of the nucleus and 

 the mode of nuclear division to a greater or less degree. A striking 

 instance of the latter is afforded by the fact that the nucleus of Spirogyra 

 may divide mitotically or amitotically according to the external conditions. 



Thus, according to Nathansohn l , Spirogyra orbicularis and a few other 

 species only divide by amitosis in water containing 0-5 / of ether. Mitotic 

 division recommences in pure water, so that, as in the production of Mucor- 

 yeast, we have an instance of a direct modification of the formative activity 

 by the external conditions. In this case the transmission of the embryonic 

 plasma is possible by amitosis, although in other cases the conditions for 

 amitosis may never occur in particular cells so long as they are capable of 

 division. Amitotic division, however, frequently occurs in dividing-cells, such 

 as those of callus-tissue and of certain animal tissues. 



It has in several cases been noticed that the process of mitosis may be 

 influenced by the external conditions 2 , and it is not inconceivable that it may 

 be found possible to induce the reduction of the chromosomes by artificial means, 

 Or to produce a division of the nucleus in adult cells 3 . 



Huie 4 in fact observed that chemical stimulation of the tentacles of 

 Drosera induced the nuclei of the cells of the head to pass through the 

 commencing stages of karyokinesis. 



It is for general reasons not surprising that Ziegler 5 , and also Boveri 6 , 

 should have observed a division of the centrosome, as well as fission and 

 a partial spindle-formation, in a non-nucleated mass of cytoplasm from the 

 egg of a sea-urchin. 



1 Pfeffer, Bericht d. sachsischen Ges. d. Wissenschaften, 1899, p. 6; Nathansohn, Jahrb. 

 f. wiss. Bot., 1900, Bd. xxxv, p. 48. The remaining literature is quoted in the latter work. 

 V. Hacker, Anatom. Anzeiger, 1900, Bd. xvii, p. 9. 



2 Cf. O. Hertwig, Zellen u. Gewebe, 1893, p. 194; Zimmermann, Morphol. u. Physiol. d. 

 pflanz. Zellkems, 1896, p. 82 ; Nemec, Bot. Centralbl., 1899, Bd. LXXVII, p. 241 ; Flora, 1899, p. 214 ; 

 Hacker, 1. c. 



s See Strasbnrger, Jahrb. f. wiss. Bot. 1897, Bd. xxx, p. 406; Klebs, Biolog. Centralbl., 1899, 

 Bd. xix, p. 220. 



4 Huie, Quart. Journ. of Microscop. Science, 1896, Vol. XXXIX, p. 423; Bot. Centralbl., 1899, 

 Bd. LXXIX, p. 97; O.Rosenberg, Physiolog.-cytolog. Untersuch. liber Drosera rotundifolia, 1899, 



pp. 2, 96. 



5 Ziegler, Archiv f. Entwickelungsmech., 1897, Bd. VI, p. 289. 



6 Boveri, Zur Physiol. d. Kem- u. Zelltheilung, 1897, p. 13. According to R. Hertwig (Abhandl. 

 d. Bayr. Akad., 1898, Bd. xxix, p. 697), Actinosphaerium Eichhorni possesses centrosomes only 

 in certain stages of development. On the occurrence of centrosomes in the vegetable kingdom see 

 Strasburger, Jahrb. f. wiss. Bot., 1897, Bd. xxx, p. 387. 



