132 



INVEETEBEATA 



CHAP. 



embryo then begins to rotate within the vitelline membrane, which 

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



mac 



mac 



mac 



FIG. 101. Stages in development of the blastula of Polygordius seen in optical 



longitudinal section. 



A, 32-cell stage ; B, 64-cell stage ; C, 76-cell stage ; D, 116-cell stage. Letters as before. In 

 addition, mac, residual macromeres f<p, polar bodies ; v, \acuoles in cells forming the prototroch ; vit, 

 vitelline membrane ; 1</ 111 , the apical cells ; Is 112 , 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- 



