Electron micrograph of a rat egg, showing the head and part of the mid-piece of a sperma- 

 tozoon shortly after passing through the vitelline surface. The head has entered upon the 

 changes that lead -to pronucleus formation. Note that the membrane limiting the egg 

 cytoplasm is folded in around the tip of the sperm head and that the spermatozoon now lacks 

 any evidence of plasma or nuclear membranes, x 20,000. (By courtesy of D. G. Szollosi 

 -and H. Ris.) 



Frontispiece 



