132 



INVERTEBEATA 



CHAP. 



embryo then begins to rotate within the vitelline membrane, which 

 it soon ruptures, and it then begins its free-swimming existence. 



112 



mac 



mac 



mac 



FIG. 101. Stages in development of the blastula of I'<>l)i<j(>nUus seen in optical 



longitudinal section. 



A, 32-cell stage ; B, 64-cell stage ; C, T<5-cull stage ; D, 116-cell stage. Letters as before. In 

 addition, mar., residual macromeres ; p, polar bodies ; v, \acuoles in cells forming the prototroch ; rit, 

 vitelline membrane ; l<y m , the apical cells ; 1-7112, the mother cells of the Annelidan cross. 



When about 140 cells have been formed the fully-segmented egg 

 constitutes a thick-walled, extremely flattened blastula which is con- 



