290 W. G. VanNmne — Embryology of Eustylochus. 



20). These have for a time a suggestion of their former arrange- 

 ment in radial lines (Fig. 19a). 



This may have practically disappeared by the time the pronuclei 

 have their final position, but there remains at least an area free from 

 yolk the position of which (central to the female pronucleus) is 

 between the pronuclei when they approacli each other, and in it the 

 cleavage spindle appears. 



Some of my specimens (Figs. 22 and 24 for example) plainly show 

 that the spindle is at first short but afterwards increases in length. 

 This indicates that the asters arise by the division of a single one. 

 If we are ready to believe that the cleavage-centrosomes are derived 

 from the spermatozoon, this would su2:)port Van der Stricht's theory 

 that the separation of the sperm-centrosome from the sperm-nucleus, 

 and consequently its division into two, is often delayed until this 

 stage. 



On the other hand, ray preparations of Planocera indicate that the 

 sperm-centrosome separates from the nucleus very early and that it 

 divides during the maturation stages. I have not observed a case 

 where the cleavage aster is still single. Fig. 22 shows the nearest 

 approach to this condition that I have found. It also shows the first 

 appearance of the cleavage centrosomes. 



From their first appearance (Fig. ^2) the asters are connected by a 

 central spindle and the centrosomes are surrounded by centrospheres 

 which become more definite in outline in the later stages. Beaded 

 threads of chromatin, staining more deeply than the remainder of 

 the network, appear within the pronuclei. The aster rays begin to 

 jjenetrate the pronuclei, whose membranes begin to dissolve away 

 in the vicinity of the centrosomes. The threads of chromatin break 

 up into segments, the chromosomes of the cleavage spindle, which 

 continue to shorten and thicken and finally assume the form of a 

 rather short rod with smooth outline bent into the shape of a U 

 or a V with the point rounded. Ten are formed from each pro- 

 nucleus. 



The spindle continues to grow, its aster rays and fibres increasing 

 in number. If it did not at first lie transverse to the main axis of 

 the Q^'g, it rotates suflicientl}' to take such a direction. It usually 

 lies almost in the center of the Qgg. The aster-fibres attach them- 

 selves to the chromosomes and draw them into the equator of the 

 spindle, where they take a position with the free ends of the loops 

 directed away from the axis of the spindle. They lie near the 

 outer part or circumference of the equatorial plane of the spindle. 



