CYTOKINESIS. 107 



marked eccentricity of the mitotic figure and subsequent inequality of cleavage 

 are intimately associated with processes of differential cell division and the dis- 

 cussion of this subject is postponed to the next section. It need only be said 

 here that there are many evidences that the movement of the entire mitotic figure 

 into its definitive position is a function of the cytoplasm, rather than of the nucleus, 

 centrosome or spindle. This movement does not occur until after the nuclear mem- 

 brane is dissolved at the poles of the spindle and it is probable therefore that the 

 escape of substance from the nucleus acts as a stimulus to the cytoplasm which 

 then moves and behaves in a predetermined manner. 



IV. Some Factors of Differentiation. 



In conclusion a brief summary may be given of the bearing of cytokinesis on 

 problems of differentiation. In the main, differentiation in the early development 

 of an animal consists in the formation of various unlike substances and in their 

 definite localization in different regions of the egg or in different blastomeres. In 

 this localization one of the most important principles is polarity. 



1. Polarity : (a) Unsegmented Egg. — The polarity of the egg, by which 

 is meant the localization of unlike substances and structures with respect to a 

 single axis, the chief axis of the egg, is indicated before maturation by a slight 

 eccentricity of the germinal vesicle ; the latter is, however, entirely surrounded by 

 yolk and the cytoplasm is uniformly distributed throughout the egg. About the 

 time of the entrance of the spermatozoon the wall of the germinal vesicle dissolves 

 and at once movements within the egg substance begin which ultimately lead to 

 the segregation of yolk at one pole and of cytoplasm at the other. There is good 

 evidence that this segregation takes place along preexisting lines of structure, the 

 cytoplasm, mitotic figure and escaped achromatin moving to that pole toward which 

 the germinal vesicle was eccentric, and in all cases this probably corresponds to the 

 free pole of the epithelial cell from which the egg was derived. 



In the gasteropods which I have studied this movement is in no way correlated 

 with nor influenced by gravity, the place of entrance of the spermatozoon, nor, so 

 far as I can see, by any other extrinsic factor. The fact that in some animals the 

 yolk has a greater specific weight than protoplasm has led 0. Hertwig (93, p. 

 215) to assert as a general law that "Polar differentiation consists in this, that the 

 lighter protoplasm collects at one pole and the heavier yolk substance at the other." 

 Khumbler ('99, p. 568), also says " Incontestibly the yolk granules (in telolecithal 

 eggs) are collected in the lower part of the egg through their greater specific weights." 

 Where the yolk is heavier than the protoplasm this may of course be true, but it is 

 by no means generally applicable. In many well known cases among annelids, 

 mollusks and arthropods gravity has no determining influence on the polarity of the 

 egg which is established in a predetermined axis irrespective of the position of this 

 axis with reference to the direction of gravity. 



In the gasteropods both polar bodies are extruded at the same point on the sur- 

 face of the egg, and the animal pole thus established bears an invariable relation to 



