38 



TISSUES 



FIG. 9 



FIG. 10 



FIG. 12 



Stages in the fertilization of Physa fontinalis. 

 Wierzejski.) 



(After Kostanecki and 



Fia. 9. Mother-star stage passing into metakinesis for the formation of 

 the first polar body. The spermatozoon is enclosed in the egg in toto. 



FIG. 10. Formation of first polar body; centrosome divided. 



FIG. 11. First polar body formed. Monaster stage for the formation of 

 the second polar body. Sperm radiation is separated from the sperm nucleus. 



FIG. 12. Formation of the second polar body. Sperm radiation with two 

 centrosomes near the vesicular sperm nucleus. 



