KARYOKINESIS AND ITS RELATION TO FERTILIZATION. 191 
the nuclear membrane remains complete during the whole pro- 
cess and towards the end of it tightens, while all the rest of the 
process goes on essentially in the same way as in the objects 
usually studied. Strasburger (191) is, indeed, of opinion that 
this is possible only when no cell-division follows nuclear divi- 
sion, as is the case in the Protozoa. Yet this remark does not 
apply to the objects studied by Schewiakoff, in which this 
process takes place, and a regular cell-division follows upon it.' 
Schewiakoff comes to the conclusion that the invisibility of 
the nuclear membrane does not warrant the supposition that it 
disappears at a certain time during karyokinesis; he is even 
inclined, with Pfitzner, to hold the contrary view, although it 
cannot be put on a firm basis. I, for my part, lay no stress 
on the persistence of the nuclear membrane, but on the main- 
tenance of the nuclear shape, by which I mean that the more 
fluid constituents of the nucleus retain their independence of 
the cell-body (cf. the above-quoted work of Sattler’s). 
In my first memoir on this subject (vide ‘ Deutsche med. 
Wochenschrift,’ 1886, and ‘Arch. fur Anat. und Physiol.,’ 
Physiologische Abtheilung, edited by E. du Bois-Reymond) I 
have spoken in the following way (in favour) of the view of 
the permanency of the nuclear contour during mitosis :—“ I 
would now, after this discovery, put aside altogether the 
barrier between ‘direct’ and ‘indirect’ nuclear division. 
There is only one form of nuclear division, and that, if we 
exclude the nucleolus, is in accordance with Remak’s scheme, 
whereby the nucleus, and later on the cell, becomes con- 
stricted in a definite plane—the plane of division—into two, 
usually, nearly equal halves. We have only now, thanks to 
improved technique, learnt to recognise that a certain con- 
stituent of the nucleus—the so-called nuclear network—under- 
goes certain special transformations, arranging itself in a 
peculiar way and resolving itself into two halves, but all this 
always within the frame of the complete figure, which divides 
in the old manner. When it is insisted that there are many 
cases in which during nuclear division the chromatic figure 
has not been observed—e. g. leucocytes—our reply is that 
VOL, XXX, PART 2,—NEW SER. N 
