112 CYTOKINESIS. 



The inequality of cell division is most commonly referred to the eccentricity 

 of the mitotic figure, however this may be caused, (see Wilson 1900) ; but it is 

 sometimes associated with a lack of symmetry in the spindle itself. Thus R. 

 Hertwig ('99) finds in Actinosphcerinm that one pole of the spindle \Richtungs- 

 k'orperpol) differs decidedly from the opposite pole, and the new cell wall cuts the 

 connective fibres nearer this pole than the other. In the second maturation also 

 the two centrosomes are unequally developed. Likewise Vejdovsky and Mrazek 

 ('98) find in the late anaphase of the first cleavage of Rhynchelmis that the cen- 

 trosome ( Tochterperiplast) and sphere {Mutterperiplasi) at one pole are much larger 

 than those at the other, corresponding to the fact that the first cleavage is very 

 unequal in this animal. A similar disparity in the size of the two centrosomes and 

 asters has been observed by Wilson ('94) in Nereis, Kostanecki and Wierzejski ('96) 

 in the first maturation of Physa, Lillie ('99) in the first cleavage of Unio; but in all 

 these cases except possibly that of Actinosphcsrium the disparity does not appear 

 until after the spindle has taken an eccentric position in the cell. 



In all the divisions of eggs and blastomeres, among the gasteropods which I 

 have studied, not only the centrosome but every portion of the mitotic figure divides 

 with exact equality, however unequal the approaching cell division may be. The 

 chromosomes not only divide equally, but each half is a mirrored image of the other 

 in shape as well as size. If one may judge by the form of daughter chromosomes, 

 the division is qualitatively as well as quantitatively equal. The centrosomes and 

 asters at the two poles of the spindle are also exactly equal during the earlier 

 stages of division and until the eccentricity of the mitotic figure becomes so great 

 as to limit the size of the peripheral aster [cf. Conklin '94, Wilson '96). Then the 

 centrosome, sphere and aster at the peripheral pole grow smaller than those at the 

 opposite pole and even before the daughter cells are separated they are proportional 

 in size to the volume of cytoplasm in the two cells, figs. 73, 90, text figs. VII— XI. 

 Even after the separation of the daughter cells the nuclei may be entirely equal, 

 but ultimately they also become proportional in size to the volume of cytoplasm in 

 which they lie, and in the next division of these nuclei the chromosomes are pro- 

 portional in size to the nuclei from which they come. 



One may therefore construct a table of the relative sizes of various cell con- 

 stituents, all of which are ultimately reducible to the volume of cytoplasm within 

 the cell. 



Volume of the Centrosome is proportional to that of the Sphere 

 " " Sphere " " " Aster 



" " Aster " " " Cytoplasm 



" " Chromosome " " " Nucleus 



" " Nucleolus " " " Nucleus 



" " Spindle " " " Nucleus 



" " Nucleus " " " Cytoplasm 



The size of all these cell constituents, therefore, is dependent upon the volume 

 of the cytoplasm, and there is the best of evidence that the eccentricity of the 



