128 



LEECHES 



right sides respectively, while the large micromere 2d lies pos- 

 teriorly (Fig. 80). 2d divides three times to form eight cells, four 

 on each side in the postero-dorsal position, and these are the 

 mother cells destined to bud off the longitudinal rows of cells 



(c) 



Fig. so. Segmentation in the egg of Erpobdella. The mother 

 cells of the germinal bands are dotted, the mesoblasts hatched. 

 The arrow points to the animal pole, (a) 9-cell stage viewed 

 from the animal pole; (b) 16-cell stage from the D-quadrant; 

 (c) 23 -cell stage in dorsal view, (a) after Sukatschoff, 1903; 

 (b) and (c) after Dimpker, 1917. 



comprising the germinal bands. 4d divides once to form a pair 

 of mesoblasts and these eventually bud off rows of cells for the 

 segmental mesoderm. 



Thus in a gnathobdellid, as in a glossiphoniid leech, the germ 

 bands are derived from 2d and the mesoblasts are derived from a 

 macromere of the D quadrant. The peculiar feature of the 

 development of a gnathobdellid leech is that the macromeres A^ B 

 and C become inactive after cutting off one or two micromeres. 

 They do not form the endoderm for this is derived from the 

 micromeres 2c^ 3d and 4d which pass into the blastocoel at an early 

 stage. Almost the whole of the rest of the adult leech is derived 

 from the cells 2d and 4D, so that with the exception of the 

 micromere 2c in the endoderm, the whole of the adult animal is 

 derived from the D quadrant. 



Another difference between gnathobdellids and glossiphoniids 

 is that whereas the glossiphoniids completely lack a larval stage, 

 the gnathobdellids form larval organs which are discarded at 



