KARYOKINESIS AND ITS RELATION TO FERTILIZATION. 205 
kraft auf die Theilung der Zellen?” Jena, 1884—rejects this, 
and suggests that the nucleus always endeavours to take 
up that position in the cell which corresponds to the centre 
of its sphere of action. Ina perfectly spherical protoplasmic 
egg-cell this will be the centre; ina meroblastic egg, where 
the protoplasm, representing the sphere of action of the 
nucleus, rests like a cap on the yolk, it will lie in the geometric 
centre of this cap, and must extend itself in the spindle figure 
in the longitudinal direction of the protoplasm. Now since 
the division of the nucleus always takes place at right angles 
to the axis of the nuclear spindle, the position of the nucleus 
will determine, according to Hertwig’s law, the direction of 
the division-plane in segmentation. Oskar Schultze (182, 183) 
has endeavoured to explain the peculiar position of the spindle 
figure during the formation of the directive corpuscles. I here 
refer the reader to his interesting description. 
Finally, the re-establishment of the nucleoli in the daughter- 
nuclei, as well as the actual division of the cell, must be men- 
tioned. As regards the former point, Strasburger (191) saw in 
the chromatic threads of the daughter-nuclei, and lying close to 
them, a number of small bodies make their appearance, which 
later on fuse together to form one or two larger nucleoli (S piro- 
gyra). Meunier (186), in agreement with his previously pub- 
lished discovery, states that all the chromatin (nuclein) of 
the daughter-nucleus unites to form a nucleolus, which has a 
special membrane. I omit other accounts, only very scattered 
and poor, which exist in the literature as to nucleoli and their 
formation in karyokinesis, since they are not sufficient for us to 
form even an approximately definite opinion on the subject. 
I mentioned above that cell-division usually occurs in the 
phase of the formation of the daughter-skein. As for the details 
about the phenomenon of cell-division, it may here be briefly 
stated that in animal cells, according to Flemming’s description 
(58), a constriction of the cell-protoplasm occurs at the level of 
the equator of the spindle figure. At this time, as Rabl (165) has 
especially pointed out, the protoplasm distinctly exhibits two 
zones, an outer darker and an inner lighter one (see fig. 12). 
