l6o H. W. BEAMS. 



second polar division. In each cycle the rythmic disappearance 

 and reappearance of the rays and successive division of the 

 chromosomes take place. In Fig. 19 the rays are extended and 

 the chromosomes are scattered in an arc midway the length of 

 the rays. The chromosomes are dividing and will later be recol- 

 lected at the nucleus where the astral rays are focused. Fig. 20 

 shows the completion of a monocentric cycle in which the astral 

 rays have been lost and the double number of chromosomes are 

 resting free in the cytoplasm. I have observed eggs where the 

 chromosome count showed they had passed through three mono- 

 centric cycles, which were comparable to the first two polar and 

 the first cleavage divisions. In Fig. 21 the first polar division has 

 been carried through normally and the first polar body extruded 

 in the normal way. The second maturation aster has failed to 

 divide and is going through monocentric cycles. The astral rays 

 are receding and the chromosomes have been collected at the 

 nucleus. The astral rays are soon lost to view entirely and the 

 deeply stained zone with a central body persists for a short time ; 

 but it, too, is soon lost. 



Some Unusual Types. Many very abnormal conditions of 

 nuclear mitosis arise in eggs that have been over-exposed to the 

 activating agent. Fig. 22 shows an early phase of a tripolar con- 

 dition in which the asters have taken a triangular position around 

 the chromatin material. This figure is the result of a simultaneous 

 appearance of two centers, such as described by Mead ('98) for 

 the first polar amphiaster in Chatoptcrns. One of the centers has 

 divided and migrated to a position where the force is at equili- 

 brium between the three centers. The other aster has apparently 

 divided, but the central bodies remain close together, forming a 

 wide focus for the astral rays. In some cases the two primary 

 polar centers divide and move apart to form a typical tetraster, 

 as described by Boveri for dispermatic eggs. In such abnormal 

 conditions normal development never follows which is probably 

 due to the abnormal combinations of the chromosomes. 



The primary maturation monaster has been observed to divide 

 into two, three, or more centers, giving rise to triasters, tetraster, 

 or multiple asters. These various abnormal astral figures may 

 give rise to several nuclei within the egg. This type of abnormal 



