KARYOKINESIS. 41 



(2) the origin of the new centrosomes and central spindle from this reticulum; in 

 some cases at least the cortical zone also arises within the old centrosome, so that 

 the entire initial spindle of one cell generation arises within the centrosome of the 

 preceding generation. 



b. Central Spindles. In the case of the first cleavage the central spindle is 

 formed after the centrosomes have taken their definitive positions at the poles of 

 the germ nuclei, figs. 55 and 56. It first appears as a few fibres running from 

 centrosome to centrosome in the line of contact between the egg and sperm nuclei. 

 These fibres run independently from pole to pole and do not branch and show cross 

 anastomoses with one another, so far as I have been able to observe ; there are no 

 varicosities or granules on them, as is the case in the later cleavages. The central 

 spindles for the second cleavage are shown in surface view in fig. 82 and text fig. 

 VIII, running from centrosome to centrosome over the surface of the nuclei and in 

 the groove between the nuclear halves. In a section of a somewhat earlier stage 

 (fig. 63), I have been unable to detect the fibres of the central spindle, though there 

 is a clear area free from granules lying immediately over the nucleus and between 

 the centrosomes in the position of the central spindle. 



In the later cleavages the origin of the central spindle within the mother centro- 

 somes can be plainly observed figs. 70, 74, 75, 76. The central spindle is in these 

 cases a long drawn out reticulum with granules at its nodes. These granules gradu- 

 ally disappear as the spindle elongates and their substance is evidently transformed 

 into the central spindle fibres. 



In Crepidula, then, there appear to be two methods of origin of the central 

 spindle : in the first cleavage the spindle arises in the cytoplasm between two inde- 

 pendent centrosomes ; in all the other cleavages the centrosomes and central spindle 

 arises as a unit structure within the mother centrosome ; in the former case the 

 fibres arise de novo between the centrosomes, in the later they arise as a centro- 

 desmus (second maturation) or from the centrosomal reticulum (later cleavages). 



3. Polar Rays and Spindle Fibres. — When first visible the polar rays are 

 extremely short and delicate fibres and their presence is to be recognized rather by 

 the clear area ("cortical zone") surrounding the centrosome than by the recognition 

 of individual fibres, figs. 70, 76. Soon these fibres become larger and longer and 

 are plainly visible, figs. 52, 53, 63. Those directed toward the nucleus become 

 stouter and more numerous than the others, and the nuclear membrane is fre- 

 quently indented where they come into contact with it, figs. 53, 54, 71. In some 

 cases, however, the nuclear membrane is not indented, but is drawn out into 

 a cone, the apex of which lies near the centrosome. Whether the membrane 

 is invaginated or evaginated, there is in both cases an escape of achromatic 

 nuclear substance at the poles, and it is clue to this substance that the extra-nuclear 

 fibres grow stouter and become covered with oxy chromatin granules, text figs. 

 XVII-XIX. In the first maturation division, not only the fibres of the extra- 

 nuclear spindle, but also all the polar fibres are studded with these granules ; in 

 the cleavage, however, I have not observed them on the polar fibres. In early 



6 JOURN. A. N. S. PHILA., VOL. XII. 



