Ookinesis in Ccrebratiihis Lacteus. 391 



The nuclear stuff thus exuded, instead of forming the membrane 

 as Rhumbler believes, plays a part in reducing the density of the 

 cytoplasm along the future cleavage plane. The formation of the 

 diastem cannot entirely be attributed to the nuclear fluid, since in 

 some cases the quantity of the fluid is too small to modify the whole 

 cleavage plane. There must be some other factor to })roduce the 

 diastem. Whatever the cause may be, the diastem is of very com- 

 mon occurrence and any explanation of the mechanism of cell divi- 

 sion will not be perfect, unless that solves the problem of its forma- 

 tion (Henneguy, '91, p. 408, Bambeke, '96, p. 34, His, '98, p. 14, 

 Conklin, '02, p. 95, Khumbler, '03, p. 513, Gurwitsch, '04, p. 326). 

 At the same time in certain kinds of cells it cannot be doubted that 

 cytoplasmic currents play an important role in division processes 

 (Conklin, '99, '02, Biitschli, '00). 



Appendix : The explanations of cytodieresis hitherto proposed may 

 be classified under five categories ; viz. 



(a) Explanations based on the contraction of the rays. 



(1) Contraction of permanent rays (Heidenhain, '95, Kosta- 

 necki, '96). 



(2) Contraction of temporary rays (Boveri, Rhumbler et al.). 

 (h) Explanations based on the expansion of rays: Meves "Ex- 

 pansion theory." 



(c) Explanations based on changes of surface tension (those which 

 are based on the change of surface tension alone without discussing 

 its cause are not explanations but mere restatements of fact) : 



(1) Change due to the rays, which represent flows in the cyto- 

 plasm (Biitschli, '76, p. 414, Giardina, '02, pp. 500, 576). 



(2) Change due to the general currents, the rays being not im- 

 portant for cytodieresis (Platner, '86, Loeb, '95 a, b, 

 Conklin, '99, '02, Biitschli, '00). 



(d) Explanations based on the contraction of the peripheral jilasm- 

 layer (rays are of little use) (Flemming, 'YY, Ziegler, '98, pp. 43,- 

 47, '01, p. 126, '04, pp. 552, 553). 



(/) Explanations based on the general contraction of cytoplasm 

 around the centres (rays are of little use) (Ziegler, '95, p. 70, Morgan 

 '99, Gallardo, '02, p. 74, Teichmann, 03, Gurwitsch, '04, Bonnevie, 

 '06). 



