DETAILS OF MITOSIS 8 1 



origin, since the ends of the spindle begin to form before dissolution 

 of thejiuclear membrane, and the latter is pushed inwards in folds 

 by the ingrowing fibres (Figs. 25, C, 99). 1 In some cases, however, 

 it seems -certain that the nuclear membrane fades away before com- 

 pletion of the spindle (first maturation-division of Thalassema, Ckce- 

 topterus\ and it is probable that the middle region of_tk spindle is 

 here formed from the linin-network. In most, if not all, mitoses of 

 the second type the chromosomes do not form a ring about the 

 equator of the spindle, but extend in a flat plate completely through 



D 



Fig. 32. Mitosis in Stypocaulon. [SWINGLE.] 



A. Early prophase with single aster and centrosome. B. Initial formation ot intranuclear 

 spindle. C. Divergence of the daughter-centrosomes. D. Early anaphase ; nuclear membrane 

 still intact. 



its substance. Here, therefore, it is impossible to speak of a " cen- 

 tral spindle." It is nevertheless probable that the spindle-fibres are 

 of two kinds, viz. continuous fibres, which form the interzonal fibres 

 seen during the anaphases, and half-spindle fibres, extending only 

 from the poles to the chromosomes. It is possible that these two 

 kinds of fibres, though having the same origin, respectively corre- 



1 Cf. Platner ('86) on Arion and I.epiiloptera* Watase ('91) on 7 oligo, Eraus ('95) on 

 Triton, and Griffin ('96, '99) on Thalassema. Erlanger ('97, 5) endeavours t<> show that in 

 the mitosis of embryonic cells in the cephalopocls (Seflia), where the inpushing of the mem- 

 brane was previously shown by Watase, the entire spindle arises from the nucleus. 



