FERTILIZATION OF THE OVUM. 



19 



menstrual cycle of twenty-eight days, — a possibility to be remembered when calcu- 

 lating the probable termination of pregnancy. 



Of the many millions of spermatic elements deposited within the vagina, only 



'^. 



D 



G 



'■*■ ^- 



JV 



J 



K 



M 



Fertilization of the ovum as illustrated by sections of the eggs of the mouse. (Sobotta.) All the figures are 

 magnified 437 diameters except D-G, in which the amplification is 1310 diameters. A-C^ prophases of formation of 

 first polar body (p) ; z, zona pellucida ; 71, nuclear figure; m, head of spermatozoon. D-G, entrance of spermato- 

 zoon (s) into ovum and subsequent changes. H-M, sequence of changes during the formation, approach, and blend- 

 ing of the male (w) and female {f) pronuclei ; />,/>, polar bodies. 



an insignificant number ever reach the vicinity of the ovum. Notwithstanding 

 that probably a number of spermatozoa penetrate the zona pellucida, normal fertili- 

 zation in man and the higher animals is effected by a single seminal element. After 



