120 LEECHES 



any resemblance to the polychaete pattern of spiral cleavage. The 

 aquatic forms are the least modified and in Tubifex four quartettes 

 of micromeres are formed in a fairly typical manner. Instead of 2d 

 giving rise to a plate of irregularly arranged cells it divides in a 

 regular manner to give four mother cells on each side of the 

 embryo. These then proceed to bud off four rows of cells on each 

 side, forming prominent longitudinal bands often called the 

 germinal bands. Beneath these superficial rows of cells are rows 

 of mesodermal cells budded off from the descendants of 4d, much 

 as in polychaetes. The germinal bands are at first situated fairly 

 near to the dorsal side and are separated one from the other only 

 by a small cap of micromeres (Fig. 71) but when the micromeres 



M i cromere 



Germinal 

 band 



acromere 



Fig. 71 . Lateral view of embryo of Tubifex to show one of the 

 two germinal bands. After Penners, 1923. 



begin to multiply the germinal bands are pushed before them 

 towards the ventral surface of the embryo and at the time of closure 

 of the blastopore the germinal bands meet one another along the 

 mid-ventral line. The inner row of cells from each side gives rise 

 to the ventral nerve cord and the other rows form first the circular 

 musculature and later the ectoderm of the adult worm, replacing 

 the embryonic ectoderm formed from the micromere cap. The 

 presence of these regularly arranged longitudinal rows of cells 

 budded off from four mother cells on each side is a feature which 

 distinguishes oligochaete embryos from those of polychaetes and 



