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-oell stage ; B, 64-0611 stage ; C, 76-cell stage ; D, 116-cell stage. Letters as before. In 

 addition, mac, residual macromeres ; j), polar bodies ; v, \acuoles in cells forming the prototroch ; mt, 

 vitelline membrane ; Ig^n, the apical cells ; Iq^'^^, the mother cells of the Annelidau cross. 



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

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



