DETAILS OF MITOSIS 



8i 



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

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

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

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

 pletion of the spindle (first maturation-division of Thalasscina, Ch(2- 

 topterits), and it is probable that the middle region of the 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 fiat plate completely through 



' t 



/ 



i \ 



\ 



r 



. \ 



B 



X 



sr 



r 



"A 



/ 



r 



C^ 



A 



Fig. 32. — Mitosis in Siypocaulon. 

 A. Early prophase with single aster and centrosome. 



D 



[Swingle.] 



B. Initial formation of intranuclear 

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



still mtact. 



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. Plainer ('86) on Ariou and Lepidop/era, Watase ('91) on Loligo, Braus ('95) on 

 Triton, and Griffin ('96, '99) on Thalasseiiia. Erlanger ('97, 5) endeavours to show that in 

 the mitosis of embryonic cells in the cephalopods {Sepia), where the inpushing of the mem- 

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

 G 



