THE BIOLOGY OF THE CELL SURFACE 



under which lies the ectoplasm, showing one or two rows of 

 granules. As the germinal vesicle breaks down, membrane 

 and ectoplasm move over the vegetal pole and thus enclose 

 the whole egg which has assumed an ellipsoid form. Usu- 

 ally the spermatozoon enters the egg at the vegetal pole 



o h 

 OcTo 





o O 



O 00 °^0%07 





Fig. 27. — Stages of sperm-penetration, egg of Mytilus (after Meves). 



after this has been covered by the moving ectoplasm. 

 (Fig. 27 from the fertilization-process in the egg of Mytilus 

 shows ectoplasmic changes after fertilization for this class 

 of egg.) 



Entrance of more than one spermatozoon is rare. Within 

 the egg, the spermatozoon after having moved some dis- 

 tance shows an aster with two centrioles; these move apart, 



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