No. 2.] THE EGG OF BUFO LENTIGINOSUS. -i^^ 



5. Division of the Segmentatio7t Spindle and Formation 

 of the Daughter Nuclei. 



After the disappearance of the nuclear membrane the chro- 

 matin of the segmentation nucleus becomes arranged at the 

 equator of the spindle in the form of very short rods (PI. XXXI, 

 Fig. 54) which divide in the usual way and migrate to each 

 pole (PI. XXXI, Fig. 55). Here they apparently lose their 

 identity, as they become fused into a rounded mass from which 

 a daughter nucleus develops. At first each daughter nucleus 

 is somewhat irregular in outline and contains a coarse reticulum 

 and several masses of chromatin (PL XXXI, Fig. 52). Later 

 the nucleus rounds up, and the chromatin breaks up into small 

 chromosomes in preparation for the second division. 



During the breaking down of the segmentation nucleus and 

 the arrangement of the chromosomes at the equator of the 

 spindle, the astrospheres increase enormously in volume. Each 

 centrosphere again consists of an irregular reticulum, the fibers 

 of which are directly continuous with the numerous fine rays 

 extending out in every direction. After its complete forma- 

 tion the spindle is barrel-shaped and the ends of its fibers do 

 not converge to any definite points as in the polar spindles, 

 but they run nearly parallel into the reticulum composing the 

 centrospheres. In the anaphase the astrospheres decrease in 

 size, their central reticulum becomes decidedly granular, and 

 the few rays that have not disappeared are again marked by 

 pigment granules (PI. XXXI, Fig. 55). When cell division 

 takes place all the rays have disappeared and the astrospheres 

 appear as rounded granular bodies surrounded by a pigment 

 layer. In preparation for the second division, the astrospheres 

 again show a reticulum composed of fine threads (PI. XXXI, 

 Fig. 52). They divide into two parts and the segmentation 

 spindles are formed as before. 



There is no period when the astrospheres entirely disappear, 

 so they become, in one sense, permanent organs of the cell. 

 Yet it is quite conceivable, as suggested by Kostanecki and 

 Siedlecki ('97), that the substance of which the astrospheres are 

 composed would become equally distributed throughout the 



