42 



KARYOKINESIS. 



prophases the fibres of the extra-nuclear spindles are directly continuous with the 

 linin threads of the nucleus, which they closely resemble in every respect, text figs. 

 XVII and XVIII. Like the linin thev branch and anastomose and are studded 

 with oxychromatin granules. This resemblance is so striking that I cannot doubt 

 that the fibres of the extra-nuclear spindles are really derived from the achromatic 

 substance of the nucleus. 



As in the maturation, so also in the cleavage there is an interfilar substance 

 which fills the spaces between the fibres and which constitutes the greater part of 

 the bulk of the amphiaster. This interfilar substance is probably derived in part 

 from the hyaloplasm of the cell body and in part from nuclear sap containing 

 dissolved oxvchi'omatin. 





Fig. XVII. — Propliase of first cleavage of Crepidtda. 



Fig. XVIII. — Prophase of second cleavage of Crepidula. 



Throughout the metaphase the spindle-fibres are to a great extent concealed by 

 this interfilar substance which fills in the whole space between them. In strongly 

 destained specimens, however, the fibres can always be seen in the spindle. After 

 the metaphase, however, no fibres can be seen crossing the dark zone which now 

 surrounds the centrosome ; both polar fibres and spindle-fibres appear to stop at the 

 boundary of this cortical zone, or rather sphere. In the anaphase the structure of 

 the sphere is such that one may be quite sure that neither polar nor spindle-fibres 

 run through it, figs. 58, 59, 60, 66, 67, 68. In both metaphase and anaphase the 

 polar fibres are not always centered on the centi'osome, and if they were continued 

 in a straight line through the s|)here some of them would not touch the centrosome 

 at all, figs. 57, 58, 60, 65, 67. 



Just before the chromosomes reach the boundar}- of the spheres tbe mitotic 

 figure is cylindrical in shape and consists almost entirely of interzonal filaments, 

 figs. 58, 66. As soon as the chromosomes have reached the spheres and are 

 transformed into vesicles, figs. 59, 67, the spindle again becomes wider in the 

 middle than at the ends and contains many fibres which do not reach from 

 pole to pole. 



