45 CLEAVAGE II 



If, however, exposure to the hypertonic solution is too 

 prolonged, the cy tasters become very numerous, and united 

 by spindles to one another as well as to the cleavage amphi- 

 aster. The chromosomes become scattered not only on the 

 cleavage spindle, but on the other spindles too, and division 

 occurring in all the equators of the multipolar figure, the egg 

 is irregularly divided into several cells at once, as was 

 always the case in the earlier experiments with magnesium 

 chloride. Since division is not restricted to those spindles on 

 which chromosomes are cast, but may occur in their absence, 

 it happens that some of the cells produced are enucleate. 



The centrosomes seen in these ova not only those in the 

 cy tasters, but also those in the cleavage asters are new 

 formations. The foci for the formation of the cytasters may 

 possibly be the chromatic particles in the cytoplasm (remains 

 of the 'yolk-nucleus'), while the development of centrosome 

 from the nucleus is paralleled by the mode of origin of the 

 definitive centrosome in ordinary fertilization. There seems 

 to be no ground for believing that the centrosome of the 

 maturation divisions persists and is revivified. 



In other cases also which have been examined [the 

 Echiuroid Worm Thalassema (Lefevre), the Polychaet Amphi- 

 trite (Scott)] normal cleavage seems to depend on the presence 

 of one amphiaster or cleavage spindle. In Thalassema the 

 two centres with their asters appear simultaneously upon the 

 nuclear membrane. 



It appears therefore that just as the development of a division 

 apparatus in ordinary fertilization is a necessary condition 

 of cleavage, so also in artificial parthenogenesis a bi-polar 

 spindle is a pre-requisite of normal, that is, binary segmenta- 

 tion. The centrosomes about and between which this appa- 

 ratus is produced in the cytoplasm are in the first case of male, 

 in the second of female nuclear origin. 



In conclusion, a word may be said of the part played by 

 this spindle in the division of the nucleus and the cell. In 

 mitosis the chromatic material of the nucleus (which alone is 

 divided) is thrown into the form of chromosomes. These bodies 



