188 TORREY 



TEXT-FIGURE 3. 

 A, Somewhat diagrammatic sketch of a four and one-half-hour living embryo 

 to show the ciliation ; a/, apical flagellse ; m, egg membrane ; pr, prototrochal cilia. 

 B, view showing the arrangement of the prototrochal cells ; J>r, prototroch ; d/, 

 blastopore. 



Variations in cleavage. C, view from upper pole of an embryo of about 72 cells 

 showing the normal type of cleavage ; D, same stage in another embryo showing a 

 reversion to a purely radial type of cleavage ; E, an embryo in which the cleavage 

 is almost purely bilateral. In each the cross is bounded by heavy lines. 



