18 KARYOKINESIS. 



The infolding of the nuclear wall mentioned above, must, of course, be accom- 

 panied by the escape of some substance from the nucleus. Coincidently with this 

 infolding of the nuclear membrane the polar fibres and extra-nuclear spindle fibres 

 become longer, stouter and granular. At the same time the spindle and the sphere 

 surrounding the centrosome stain more deeply, owing to the presence of an interfilar 

 substance, which stains like nuclear sap. 1 The nuclear membrane then completely 

 disappears, but the nuclear contents preserve for some time the outline of the 

 nucleus and can easily be distinguished from the surrounding cytoplasm because of 

 greater affinity for stains, figs. 6 and 7. The entire mitotic figure, with the excep- 

 tion of the polar systems, lies within this granular area and the enormous growth 

 of the spindle is at the expense of this intra-nuclear substance. Not all of the 

 achromatic substance of the germinal vesicle is confined to the spindle and the polar 

 systems, a large part of it passes directly into the cytoplasm, which is increased in 

 quantity after the nuclear membrane dissolves. 



Both the aster and the spindle are plainly composed, during the early stages 

 of mitosis, of two constituents, viz., fibres with their attached granules and an inter- 

 filar substance. Between and around the spindle fibres, both in the first matur- 

 ation and in all subsequent divisions, there is a homogeneous interfilar substance 

 which colors deeply with plasma stains. This substance is sharply delimited from 

 the surrounding cytoplasm, as is shown in figs. 12 and 12a and also in later stages 

 of both the maturation and cleavage. 



A cross section through the equator of the spindle in the metaphase, fig. 12a, 

 shows the interfilar substance of the spindle as a homogeneous mass, staining deeply 

 with orange or eosin, and with stellate radiations running out into the cytoplasm in 

 all directions. These radiations around the equator are shorter, blunter and more 

 irregular than those at the poles. The}' are probably caused as follows : in the forma- 

 tion of the definitive spindle there is a general elongation of the linin reticulum in the 

 direction of the spindle axis and a contraction of the reticulum at right angles to that 

 axis ; at the same time there is a condensation of the interfilar substance, the more 

 fluid karyolymph being squeezed out of the spindle. In this equatorial shrinkage 

 some of the linin threads probably remain attached peripherally and thus cause the 

 stellate radiations. 2 The chromosomes lie within this interfilar substance, though 

 occasionally one is found just outside of it, fig. 12a, and the}' are scattered through 

 the entire thickness of the spindle, and not merely in a wreath around the periphery. 

 There is, therefore, strictly speaking, no "central spindle" in Crepidula as contrasted 

 with a -''peripheral spindle," but the fibres of the two must be intimately com- 

 mingled. 



1 Driiner calls attention to the fact that the nuclear membrane is dissolved at points opposite cen- 

 trosomes and that coincidently the rays grow stronger; and R. Hertwig ('99) has observed in Ac-tino- 

 sphcerium that nuclear material probably escapes into the plasma cones, since the latter stain more 

 deeply about the same time that the nucleus shrinks in size. 



2 Similar radiations around the equator of the spindle have been figured by Korschelt ('95, figs. 

 131, 139), Mead ('98, fig. 18), and Gardiner ('98, figs. 28, 34). 



