122 



LEECHES 



two unequal blastomeres, AB being smaller than CD and subse- 

 quent divisions in CD precede those in AB. Blastomere D is 

 larger than C and contains most of the pole plasm. The first set 

 of micromeres is offset in a clockwise direction when viewed from 

 the animal pole (Fig. 73) and the second set in an anticlockwise 

 direction. The cleavage of the egg of Theromyzon is particularly 

 interesting because the pattern of spiral cleavage is so like that of 



(a) (b) 



Fig. 73. (a) early stage in the spiral cleavage of an egg of 



Glossiphonia complanata viewed from animal pole ; (b) later stage, 



showing cap of micromeres, mother cells of germinal bands 



(dotted) and mesoblast (hatched). After Muller, 1932. 



polychaetes that Schmidt (1917) was able to recognize the cells 

 of the " apical rosette " and the " annehd cross ". This is unusual 

 because the formation of such a regular pattern is normally 

 prevented by the precocious cleavage of the D blastomere but it is 

 usually possible in the eggs of Glossiphoniidae to follow the 

 formation of four complete sets of micromeres much as in Tubifex. 

 The micromere 2d stands out from the others because it is usually 

 as large as the macromere 2D. 2d divides several times, cutting off 

 a number of small cells destined to join the micromere cap, and 

 then forms eight equal sized cells. These are the mother cells of 

 eight rows which are budded off to form germinal bands (Fig. 74). 



Germinal band 



Fig. 74. Late cleavage stage of Glossiphonia showing germinal 

 bands. After Whitman, 1878, modified. 



