II. i CELL-DIVISION 25 



2. The second type is the so-called spiral form of cleavage 

 (Fig. 13). This is especially characteristic of the eggs of Poly- 

 clads, Nemertines, Molluscs (except Cephalopods), Annelids, and 

 Sipunculoids (Phascotosoma). The peculiarity of this mode of 

 division is that after the four-celled stage the blastomeres 

 usually known as the macromeres give off 'quartettes' of 

 micromeres towards the animal pole, the first quartette being 

 given off dexiotropically (except in cases of reversed cleavage), 



FIG. 13. Diagram of a 'spirally' segmenting egg in the 16-cell stage. 

 2 A-2 D macromeres ; 2 -2 d micromeres of second quartette ; 1 a 1, 1 a 2- 

 1 d 1, 1 (12 micromeres of first quartette. 



the second laeotropically, and so on in regular alternation, until 

 four quartettes have been produced. The cells of each quartette 

 divide meanwhile in conformity with the same law of alterna- 

 tion of direction of cleavage. The direction of division is thus 

 always oblique to the egg-axis, and this obliquity can be 

 observed in the division of the first two blastomeres, the result of 

 which is that of the two sister cells A and B A is nearer to the 

 animal pole than B, while in the other pair C is nearer that pole 

 than D ; A being to the left of B and C to the left of D (to an 

 observer standing in the axis with his head to the animal pole), 

 the division is laeotropic. The arrangement of cells approaches 

 the tetrahedral, especially when, as occurs very frequently, 

 A and C are united by a cross, or polar, furrow above, B and D 

 by a polar furrow at the vegetative pole, as in Nereis, 

 Iclmochiton, Limax, Planorbis, Lepidonolus, Discocelis, and others. 

 In Unio, however, it is B and D that are in contact at 

 the animal, A and C at the vegetative, pole. In other cases 



