124 



LEECHES 



to proliferate and spread from the animal pole towards the 

 vegetative pole. The germinal bands move before them. At this 

 stage they are seen as two arcs arising from a group of eight cells 

 at the posterior end of the embryo and running on each side to 

 meet anteriorly. As the micromere cap grows and spreads the two 

 germinal bands move towards the vegetative pole. Their anterior 

 and posterior points of union remain almost stationary and the two 

 semi-circular germinal bands rotate about these points and are 

 brought together in the mid-ventral line like the jaws of a trap. The 

 micromeres continue to grow ventrally, finally moving over the top 

 of the germinal bands to meet at the vegetative pole and thus effect 

 the closure of the blastopore. As in the oligochaetes, the two 

 median ventral rows of cells of the germinal bands form the ventral 



Ectodermal 

 micromeres 



Mesoderm 



Cells of 

 germinal 

 band 



Fig. 76. Transverse section through ventral part of a Glossi- 



phonia embryo after the micromeres have covered the germinal 



bands; a, b, c macromeres. After Burger, 1902. 



nerve cord while the others form the circular musculature. The 

 deeper row of mesodermal cells forms the somites. In the Glossi- 

 phoniidae, but not in other families, the micromeres which have 

 spread over the surface of the gastrula form the definitive epi- 

 dermis, the descendants of the first quartette covering the head, 

 those of the second and third the trunk. 



During the next phase of development the main organs of the 

 body are differentiated. The ventral nerve cord, as we have seen, 

 originates from the ventral edges of the germinal bands, but 

 opinions differ about the origin of the cerebral ganglia. Dav^doff 



