II. i CELL-DIVISION 31 



and in the division of the pole cells of Annelids (Wilson and 

 Vejdovsky). It is doubtful, however, whether it is not the 

 inequality in the cells that is responsible for the inequality of 

 the asters, there being more room in a large cell for the out- 

 growth of the astral rays. At any rate, there are many cases 

 of unequal cleavage in polar body formation where the asters 

 are of the same size. Until evidence is brought forward of 

 a difference in the size of the centrosomes the hypothesis is no 

 more than a conjecture. 



Before quitting this subject we should refer to a rule which 

 Ztir Strassen has found to hold good for the rate of segmentation 

 of Ascaris megalocepliala. The cells do not all divide at the 

 same rate, but in certain groups of cells division is found to 

 occur simultaneously. These cells are related, descended from 

 some one cell, and the more nearly related the cells are, the more 

 nearly together do they divide. Coincidence in time of division 

 depends therefore on the degree of cell-relationship. 



The direction of division of the cell depends upon that of the 

 nucleus, since, speaking generally, the division occurs in the 

 equatorial plane of the spindle, or, in other words, the plane 

 of division is at right angles to the direction of elongation of 

 the spindle or separation of the centrosomes. The latter again 

 depends on the relation between the nuclear spindle and centro- 

 somes on the one hand, and on the other the cytoplasm and its 

 contents, more particularly the yolk. The relation between 

 the (resting) nucleus and the cytoplasm has been expressed by 

 O. Hertwig in the following empirical rule: 'The nucleus always 

 seeks to place itself in the centre of its sphere of activity/ 

 The sphere of its activity being not the inert yolk but the 

 cytoplasm, we find, in accordance with this rule, that the nucleus 

 places itself in the centre of the egg where the yolk is uniformly 

 distributed (isolecithal), nearer the animal pole but still in the 

 axis where the yolk is on one side (telolecithal). Examples of 

 the former condition are to be found in Echinoids (the fertiliza- 

 tion nucleus is nearly, but not quite, central) and large-yolked 

 Arthropod ova, of the latter in the eggs of Vertebrates, Molluscs, 

 and many others. 



The nucleus, however, may wander from this position, as occurs, 



