The Development of Phascolosoma. 99 



Forty-eight-cell Stage. 



About seven hours after the beginning of fertilization the fortj^- 

 eight-cell stage is reached (Fig. D), of which thirty-two cells lie in 

 the anterior and equatorial regions (including the primary cells of 

 the prototroch) and sixteen in the posterior. These regions represent 

 respectively the descendants of the first group of (giantj micromeres 

 of the eight-cell stage and of their four sister cells at the vegetative 

 pole. They may be designated roughly as the anterior and posterior 

 hemispheres. The anterior, however, covers more than half of the 

 surface, owing to a flattening and slipping backward of the primary 

 prototroch cells over the blastomeres immediately behind them, and 

 to the sinking of certain cells at the posterior (vegetative) pole. 

 Thus an epibolic invagination begins at this stage. 



30} 



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id"^ 



Fig-. D. 



Cells of the anterior hemisphere in the 48-cell stage in Ph. vulgare. Rosette, 

 dotted; intermediate cells, unshaded; cross cells, barred. Primary prototroch cells 

 shown on the periphery (one lacking in quadrant D). Designation of cells is the 



same as in Fig. C. 



The anterior hemisphere consists of tw^o parts, the apical plate 

 and the prototroch, each consisting of sixteen cells. At the centre 

 of the apical plate is a somewhat diamond-shaped "rosette" of four 

 cells. Two "intermediate" cells extend from each apex of the rosette 

 in a radial direction. These twelve blastomeres are all of about 

 the same size, and form a prominent Greek cross, wiiich stretches 



