REPRODUCTION AND DEVELOPMENT 



123 



As in oligochaetes these rows of cells grow forward from a postero- 

 dorsal position forming prominent ridges at the junction of the 

 micromere cap and the macromeres. At the same time two rows 

 of mesoderm cells are formed beneath them. They are developed 

 from a pair of mesoblasts which in this instance result from the 

 division of 3D and are therefore 4D and 4d. This state of affairs 

 is different from anything found in either polychaetes or oligo- 

 chaetes, for in these groups it is 4d which gives rise to the right 

 and left mesoblasts, while 4D contributes to the endoderm. In the 

 Glossiphoniidae there is no contribution to the endoderm from 

 any cell of the D quadrant. 



At this stage we have a blastula which consists of a mass of 

 micromeres at the animal pole, three macromeres at the vegetative 

 pole and between them on each side a ridge, comprising four 

 superficial and one deep row of cells, the germinal band and its 

 underlying mesoderm. The micromeres of the fourth quartette 

 sink in and join the macromeres, which are destined to form the 

 endoderm, but the micromeres of the first three quartettes begin 



Pharynx 



Mocromere A 

 under covering 

 of micromeres 



Germinal band 



Fig. 75. Gastrulation in Glossiphonia. (a) the micromere cap 

 is beginning to spread towards the vegetative pole while the 

 germinal bands move before it; (b) a more advanced stage; 

 (c) the two germinal bands have almost met ventrally. All viewed 

 from the anterior end. After Whitman, 1878, modified. 



