KARYOKINESIS AND ITS RELATION TO FERTILIZATION. 187 
equatorial circle (circumference, boundary) of the spindle 
figure, as well as the chromatic loops during the stages of 
the equatorial plate and metakinesis. I have reproduced van 
Beneden’s figure (fig. 18) which brings more clearly into view 
the points here described. 
Very important is van Beneden’s statement (24) that the 
“ sphéres attractives ” are vestigial structures, which share in 
the successive divisions of the segmenting egg, and at first, 
indeed, precede the division of the nucleus. The division of 
the “ sphére attractive’? commences at the central body, both 
parts of which separate a little; then follows the division 
of the sphere, so that both daughter-spheres naturally lie 
close to one another. Imagine (see fig. 13) the first division 
of the egg-cell to be accomplished in its equator; there will 
then be, in the next division, in each of the two first blasto- 
meres about to divide, the two corresponding daughter-spheres 
placed close to one another in the region of the polar circle. 
The cell-axis of each blastomere now passes from the middle of 
the polar circle between the two new central bodies to opposite 
ends of the cell; this axis then has evidently two dissimilar 
poles, since at the one pole lie one of the polar circles and 
two daughter-spheres with two central bodies, whilst at the 
other end of the axis nothing corresponding to this is found. 
Each blastomere is at this time of its existence uniaxial but 
bilaterally symmetrical, since it can be resolved into equal 
halves by a plane passing along the axis mentioned between 
the two spheres. Hach blastomere must show this bilateral 
symmetry, and van Beneden concludes therefore that all cells 
have a bilaterally symmetrical structure, and he would see 
in this the foundation of the bilateral symmetry of the 
organism. 
On account of the above-mentioned behaviour of the spheres 
during division, an important meaning must be ascribed to 
them (‘f nous sommes donc autorisés,” says van Beneden (24), 
“4 penser que la sphere attractive avec son corpuscule central 
constitue un organe permanent, non seulement pour les 
premiéres blastoméres, mais pour toute cellule; qu’elle con- 
