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Fig. 196. Partly after Dawydoff. 1-3, Diagrams of radial and spiral cleavage. 

 4-7, Development of Nereis, i, Eight-cell stage in a radial type, e.g. Echino- 

 derm. 2, Spiral cleavage, four-cell stage just before cleavage, leading to 

 eight-cell stage, sp. spindle. 3, Macromeres stippled. 4, Diagram of seg- 

 menting egg (Nereis), seen from the animal pole, showing the macromeres, 

 the first three quartettes of micromeres and the mesoblast cell (^*) in the 

 fourth quartette. 5, Later stage, also^rom 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. 



