284 W. G. VcmName — Embryology of Eustylochus. 



to apply the term to any visible object it must be the black staining 

 " centralkorn." 



I have already stated that when the centrosome in each aster of 

 the first polar spindle divides, the derived centrosoraes usually sepa- 

 rate in the direction of the axis of the spindle or nearly so (Figs. 5, 

 7, 8, 12 and 13). This applies especially to Eustylochtis ; in Plano- 

 cera we find variations more frequent, and it is not rare to find ^em 

 moving apart in a direction very oblique or even transverse to the 

 spindle axis (Fig. 37), which is rather uncommon in Eustylochus, 

 though not suflSciently so to be abnormal. The result of this is that 

 the second polar spindle when formed has already a radial or nearly 

 radial position, and no rotation of the spindle, or at least only a 

 slight one, is necessary. The outer pole simply moves toward the 

 surface of the egg, the spindle meanwhile lengthening, while the 

 inner pole does not change its position to any great extent. Accord- 

 ing to Lillie (19), a similar position of the spindle is common in 

 Uhio. As the outer pole moves toward the periphery of the egg it 

 often pushes directly through the group of chromosomes, and thus 

 some of the aster rays attach themselves to these and draw them 

 into the equatorial part of the spindle. 



Van der Stricht, on the conti'ary, finds the centrosomes in Thysan- 

 ozoon lying usually in a line perpendicular to the spindle axis, though 

 occasionally parallel or identical with it or in intermediate positions.* 

 Klinckowstrom in Prostheceraeus also finds the second polar spindle 

 in a tangential position during its early stages. 



The question of the relation of the aster rays of the second polar 

 spindle to those of the inner pole of the first, is a most difticult one 

 to answer. This is largely because of the very different appearances 

 which eggs in about the same stage present. 



MacFarland (20) describes the rays of the second polar spindle as 

 arising anew, but before the outer portions of the rays of the old 

 aster have entirely disappeared, so that we have at one time two sets 

 of radiations present, the short new rays, and outside these, sur- 

 rounding the whole spindle, the old aster rays whose central portions 

 have disappeared. 



Some of my preparations suggest such a process. Fig. 37, repre- 

 senting an egg of Planocera nehulosa, shows one of these. The rays 

 of the first polar spindle aster become weak as they approach the 

 centrosphere, Avhich looks as if it were developing new rays. Yet a 

 careful examination of this specimen shows that the old rays may be 

 traced clear to the centrosphere. Such a specimen as that shown in 



