II CLEAVAGE 51 



cleavage, and for the divisions of the micromeres of the various 

 quartettes. It is, as a matter of fact, only a special case of 

 a rule which is seen in segmentations of the other types also, 

 namely, that successive divisions are at right angles to one 

 another. It is known as Sachs' rule, since this botanist first 

 formulated it for plant cell-divisions, and depends, in part at 

 least, on the fact that the centrosomes,in preparation for the next 

 division, divide at right angles to the previous spindle-axis. 

 The pattern of cleavage is conditioned obviously by the 



r B 



2. A 



2 O 



FIG. 11. Diagram of a spirally segmenting egg in the 16-cell stage. 

 2 A-2D macromeres; 2 a-2 d micromeres of second quartette ; lal, 12- 

 Idl, 1 d2 micromeres of first quartette. 



shapes, sizes, and arrangement of the cells, and these in turn 

 on the rate and direction of division, and the movements of the 

 cells on one another ; and these again on the relation between 

 the dividing nucleus with its centrosomes and the cytoplasm, 

 and on the properties of that plasma and its inclusions. 



These relations are expressed in certain rules, one of which, 

 that of Sach's, has just been referred to. Another is Balfour's 

 rule that yolk impedes division, hence the more rapid division 

 and smaller size of animal cells in telolecithal eggs. Hertwig's 

 rules state that the nucleus lies in the centre of the cytoplasm, 

 and that the dividing nucleus or spindle elongates in the direc- 

 tion of greatest protoplasmic mass or, as Pnuger worded it, the 

 direction of least resistance, resistance being offered by the yolk. 

 These rules explain a good many of the features of division in 

 the various types. The properties of the cytoplasm which play 



E 2 



