KARYOKINESIS. 19 



An interfilar substance, which is to all appearances similar to that of the spindle, 

 .surrounds the centrosonies and radiates along the polar fibres, so that in all middle 

 stages of mitosis it is difficult to recognize the jjolar fibres and spindle fibres when 

 once they are surrounded by it. 



In the later stages of mitosis this interfilar substance moves to the poles of the 

 spindle, again allowing the spindle fibres to be seen distinctly ; it also moves inward 

 toward the centrosome, leaving the polar fibres sharph- marked, fig. 22, and thus 

 aggregated, from the spindle and polar rays, forms a sphere with rather indefinite out- 

 lines. This sphere differs notably in character from that which is found in many 

 other animals, e. g., the outer sphere of Unio (Lillie '98) and the cojiche corlicale of 

 Thysanozoon (Yan der Stricht '98). The latter are clear zones of definite outline, with 

 faintl}' staining astral rays running through them ; in Crepidula, on the other hand, 

 this zone is indefinite in outline until the late anaphase or telophase, and is even then 

 not so sharply bounded as Lillie and Van der Stricht have shown it ; further, instead 

 of being a zone which is clearer than surrounding parts, it is denser and more deeply 

 staining. The spindle fibres and polar rays can be traced through this sphere to 

 the centrosome in early stages of mitosis, but in middle stages fewer I'adiating fibres 

 can be seen in it (cf. figs. 3-8a with figs. 11-16). In later stages again polar rays 

 can be traced through it to the centrosome (cf. figs. 22-24 and 34-36). 



The origin of this interfilar substance is difficult to determine. In the aster it 

 seems to be principally derived from hyaloplasm (interalveolar substance) of the cell 

 body, which is aggregated toward the centrosomes, the larger alveoles of the cyto- 

 plasm and the j'olk spherules being crowded out from the centrosome as the inter- 

 alveolar substance moves in toward it. In the spindle, on the other hand, the 

 iiiterfilar substance seems to be formed in large part from achromatic material of the 

 nucleus; such interfilar substance exists before the nuclear membi'ane is broken, 

 though it is at this stage much less dense than in the fully formed spindle. When 

 the nuclear membrane dissolves at the poles this substance escapes into the extra 

 nuclear spindle and spheres ; it is quite possible that at the same time there may be 

 an invasion of the spheres and spindle by hyaloplasm from the cell, this double 

 movement being in the nature of a difl'iision in both directions. The fact tli:it the 

 interfilar substance is denser than either the nuclear sap or hyaloplasm may perhaps 

 indicate that it is a new substance formed by a combination of the two. While tliis 

 suggestion as to the origin of the interfilar substance accoi'ds well with all my obser- 

 vations as to its character and movements, it cannot be considered as more than a 

 suggestion. 



The form and size of the first maturation spindle varies greatly in difi'erent 

 phases. From the prophase to the metaphase it increases in length and diameter, 

 becoming most stout in the metaphase ; in the early anaphase it continues to increase 

 in length, becoming about as long as the radius of the egg, figs. 11-14. ami ;it the 

 same time it grows verv slender; finallv. in the late anai)hase it airain shortens, be- 

 coming stouter, until it is not more than one-half as long as in the metaphase or 

 early anaphase, figs. 15 and 16, and at the same time the chromosomes are pushed 



