54 



PROBLEMS OF FERTILIZATION 



retreats into the interior of the protoplasm and the head 

 of the spermatozoon becomes a narrow chromatin 

 band as it enters through the minute aperture in the 

 vitelline membrane. It is quite obvious that the ini- 

 tiative in the final act of penetration is on the side of the 



FIG. 3. Penetration of the spermatozoon in the egg of Nereis, from 

 sections: a, 37 minutes after insemination; b, c, d, three stages from 

 eggs killed 48^ minutes after insemination; note that the cone sinks into 

 the egg and draws the spermatozoon after it; e, 54 minutes after insemi- 

 nation; the head of the spermatozoon now entirely within the egg is 

 contracting; note that the middle piece remains on the membrane; it 

 does not- enter the egg; the tail also remains outside. 



ovum. The fertilization cone is engulfed by the egg 

 protoplasm and draws the sperm nucleus after it. 

 These events can be understood by assuming that the 

 spermatozoon causes a local diminution of surface 

 tension of the egg in the first place, thus causing an 



