POLYCHAETA 



249 



Fig. 185. Partly after Dawydoff, Radial and spiral cleavage, i, Eight-cell 

 stage in a radial type, e.g. Echinoderm. 2, Spiral cleavage, four-cell stage just 

 before cleavage, leading to eight-cell stage. 3, Macromeres stippled. Develop- 

 ment of Polychaeta. Nereis, 4, Diagram of segmenting egg seen from the 

 animal pole, showing the macromeres, the first three quartettes of micromeres 

 and the mesoblast cell ((i*) in the fourth quartette. 5, Later stage, also from 

 animal pole, to show the rosette cells {a^^-d}^), the annelid cross (indicated 

 by stippling), the four groups of prototroch cells (horizontal shading and 

 cilia) and the intermediate girdle cells {a^^-d^~). 6, Vertical section through the 

 same stage as 4, along the line XY. 7, Vertical section through a later stage 

 to illustrate gastrulation : cells derived from d^ (cross-hatched) growing over 

 the macromeres, the mesoblast cell withdrawing into the interior. 



